A NEW PREDICTION AND PREVENTION APPROACH FOR TRENCH
EXCAVATION CAVE-INS
By
Krupesh S.Kakkente

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
MASTER OF SCIENCE
Construction Management
2012

ABSTRACT
A NEW PREDICTION AND PREVENTION APPROACH FOR TRENCH
EXCAVATION CAVE-INS
By
Krupesh S.Kakkente
Existing safety protocols imposed by OSHA are supposed to reduce the accidents; however there are high risk factors associated with trench excavations. The thesis explains the
reasons for existence of hazards and injuries due to the lack of a predictive and preventive
application in the field of trench excavation. The research aims to explore trench cave-in
patterns and understand how to prevent accidents by implementing finite element method.
Trench cave-ins are simulated and the patterns of failure are seen. A new protocol for trench
excavation is proposed and a proof of concept using systems dynamics approach is also developed to evaluate the impact of implementing a new safety program within a construction
company.
Keywords: Trench Excavation Cave-Ins, Finite Element Method, Systems Dynamics, Safety
Program.

To
most important of them all, Mom, Dad, and little Sis.

iii

ACKNOWLEDGMENTS

Truth be told, one could hardly ever ask for a friendlier and mentor of a supervisor than Dr.
Armagan Korkmaz. The relentless effort put into the thesis by him supported me through
the very beginning concept of this project until the very end, made it a project that was
both very dear to him and myself. I would like to thank Dr. Sincem Korkmaz for having a
nurturing demeanor towards my ideas and introducing me to Dr. Armagan Korkmaz who
shared my excitement for Finite element sciences. She pushed me to refine and perform
research in a disciplined manner, for which I will always be grateful.
I would like to sincerely thank Dr. Tariq Abdelhamid for inviting me to the Construction
Industry Research and Education Center (CIREC) at MSU. I am thankful for all the support
which helped me through tough times personally. Special thanks to Ankur Desai, Naveen
Nerwal and Don Schaffer for helping focus on my vision through the writing of this thesis
and helping make the time at CIREC a wonderful one. I am thankful to god for giving me
the most loving family, who have helped me to stand where I am today. My loving mother
and father who stopped at nothing to give me the best there is, while my adorable little
sister who supported and encouraged me to work hard and live happily at every stage of my
life. Great friends who stepped in at all the times both good and bad for me, and inspired
me to be sucsseful- Rahul and Kavitha . And finally a shout out to Guarang Shrikhande for
helping through the inception of the idea till the very end in helping with editing the thesis.
Thank You.

iv

TABLE OF CONTENTS

LIST of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
List OF TABLES

vii

List OF FIGURES
LIST of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
1 Introduction
Chapter 1 Introduction
1.1 Overview . . . . . . .
1.2 Research Rationale .
1.3 Research Objectives .
1.4 Thesis Overview . .

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1
1
3
5
6

Chapter 2 Literature Review
2 Literature Review
2.1 Safety in the Construction Industry Focusing on Trench Excavations . . . . .
2.1.1 Case study 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.2 Case study 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1.3 Case study 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Finite Element Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Case studies of application of FEM . . . . . . . . . . . . . . . . . . .
2.2.2 Modeling in ABAQUS . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Development of FEM model to determine earth pressure in trench
rescue shoring systems . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Guidance and Training Program for Trench Excavation Related Industry Professionals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Case study 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2 Case study 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4 System Dyanmics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.1 Case study 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.2 Case study 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4.3 Case study 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

24
24
26
28
29
30
31

3 Research Methodology
Chapter 3 Research Methodology
3.1 Traditional Concepts of Soil Mechanics . . . . .
3.2 Methodology . . . . . . . . . . . . . . . . . . .
3.2.1 Trench with homogenous soil conditions
3.2.2 3D edge crack in trench . . . . . . . . .
3.2.3 Trench simulation with 2 layer soil . . .

34
35
38
40
45
50

v

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14
19
22
22

4 Analysis of FEM Simulations
Chapter 4 Analysis of FEM Simulations
4.1 Analysis of Results of ABAQUS FEM Models Discussed in Research Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Comparison of The Results to a Safe Practice of Trench Excavation . . . . .
4.2.1 Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . .

54

5 Guidelines to Build The FEM Model
Chapter 5 Guidelines to Build The FEM Model
5.1 Creating the Un-cracked Domain . . . . . . .
5.2 Creating The Cracked Domain . . . . . . . . .
5.3 Creating the Boundary Conditions and Loads
5.4 Solving the System of Equations . . . . . . . .
5.5 E-Leanring . . . . . . . . . . . . . . . . . . . .

70
71
75
77
80
81

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66
69

6 Systems Dynamics Model: To To Help PredictCompany Behavior Towards
Chapter 6 Systems Dynamics Model: Help Predict Company Behavior Towards
New Safety Protocol
New Safety Protocol
6.1 Model Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.2 Model Equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6.3 Analysis of The Results of FEM Simulation Model . . . . . . . . . . . . . . .
6.4 Limitations of Safety Program Simulator . . . . . . . . . . . . . . . . . . . .

82
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84
86
87

7 Conclusions
Chapter 7 Conclusions

91

vi

LIST OF TABLES

Table 1.1

Deaths Due to excavations and trench cave-ins (BLS, 2008) . . . . .

2

Table 3.1

Summary of varios soil properties (Robert, 1990) . . . . . . . . . . .

34

Table 6.1

U.S Excavation and trenching injuries and fatalities (Rocheleau, 2007) 83

Table 6.2

Equation variables and the description . . . . . . . . . . . . . . . . .

vii

85

LIST OF FIGURES

Figure 1.1

Trench Soil Cave-in ”For interpretation of the references to color in
this and all other figures, the reader is referred to the electronic version of this thesis.” . . . . . . . . . . . . . . . . . . . . . . . . . . .

2

Figure 1.2

Thesis Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7

Figure 2.1

Frame-work for Literature Review . . . . . . . . . . . . . . . . . . .

9

Figure 2.2

Continuous System and Finite Member . . . . . . . . . . . . . . . .

15

Figure 2.3

Stress states in 3D and 2D . . . . . . . . . . . . . . . . . . . . . . .

16

Figure 2.4

The modified Von-Mises Failure criterion . . . . . . . . . . . . . . .

17

Figure 2.5

2x2x2 integration point scheme in hexahedral elements . . . . . . . .

18

Figure 3.1

Typical Stress Strain curve for soils . . . . . . . . . . . . . . . . . .

35

Figure 3.2

Mohr-Coulomb Soil Modeling . . . . . . . . . . . . . . . . . . . . . .

36

Figure 3.3

Research Methodology . . . . . . . . . . . . . . . . . . . . . . . . .

38

Figure 3.4

Research Methodology . . . . . . . . . . . . . . . . . . . . . . . . .

39

Figure 3.5

Geometry of Trench Section . . . . . . . . . . . . . . . . . . . . . .

40

Figure 3.6

3D Extruded Trench Section . . . . . . . . . . . . . . . . . . . . . .

41

Figure 3.7

Boundary Conditions and Cell Partition . . . . . . . . . . . . . . . .

42

Figure 3.8

Meshed Trench . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

43

Figure 3.9

Mesh Deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . .

44

viii

Figure 3.10

Mesh Deformation Contour . . . . . . . . . . . . . . . . . . . . . . .

44

Figure 3.11

Geometry of Trench . . . . . . . . . . . . . . . . . . . . . . . . . . .

45

Figure 3.12

Trench Extrude and Meshing Results . . . . . . . . . . . . . . . . .

46

Figure 3.13

Crack Placement in Trench Domain . . . . . . . . . . . . . . . . . .

47

Figure 3.14

Loads Applied to Top Surfaces . . . . . . . . . . . . . . . . . . . . .

48

Figure 3.15

Boundary Conditions . . . . . . . . . . . . . . . . . . . . . . . . . .

48

Figure 3.16

Resulting Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

49

Figure 3.17

Geometry for The Two Layered Soil Trench . . . . . . . . . . . . . .

50

Figure 3.18

Boundary Layer Conditions

. . . . . . . . . . . . . . . . . . . . . .

51

Figure 3.19

Loading Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . .

52

Figure 3.20

Meshed Multi Soil Layer Trench . . . . . . . . . . . . . . . . . . . .

52

Figure 3.21

Results of Soil failure . . . . . . . . . . . . . . . . . . . . . . . . . .

53

Figure 4.1

X Values Definition . . . . . . . . . . . . . . . . . . . . . . . . . . .

56

Figure 4.2

Result of Stresses in Trench With Homogenous Soil . . . . . . . . .

56

Figure 4.3

Path Selected for Stress Plots . . . . . . . . . . . . . . . . . . . . . .

57

Figure 4.4

Stress Versus Distance along Path . . . . . . . . . . . . . . . . . . .

58

Figure 4.5

Trench Stress Results in Crack Trench Simulation . . . . . . . . . .

59

Figure 4.6

Selected paths for Stress Plotting . . . . . . . . . . . . . . . . . . .

60

Figure 4.7

Results of Stress Plots . . . . . . . . . . . . . . . . . . . . . . . . . .

61

Figure 4.8

Nodes Path Selected and Results of Stress Plotting . . . . . . . . . .

63

ix

Figure 4.9

Fatigue In Trench with Two Layers of soil . . . . . . . . . . . . . . .

64

Figure 4.10

Nodes Path Selected and Results of Stress Plotting . . . . . . . . . .

65

Figure 4.11

Geometry of Slope and Meshed Part . . . . . . . . . . . . . . . . . .

66

Figure 4.12

Boundary Conditions and Loading Conditions . . . . . . . . . . . .

67

Figure 4.13

Fatigue Results in Trench Section . . . . . . . . . . . . . . . . . . .

67

Figure 4.14

Nodes Path Selected and Results of Stress Plotting . . . . . . . . . .

68

Figure 5.1

Outline of ABAQUS Model Building . . . . . . . . . . . . . . . . . .

71

Figure 5.2

Geometry of Trench . . . . . . . . . . . . . . . . . . . . . . . . . . .

72

Figure 5.3

Trench Extrude and Meshing Results . . . . . . . . . . . . . . . . .

73

Figure 5.4

Trench Extrude and Meshing Results . . . . . . . . . . . . . . . . .

74

Figure 5.5

Cranck Instance . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

75

Figure 5.6

Crack Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . .

76

Figure 5.7

Loading Condtions . . . . . . . . . . . . . . . . . . . . . . . . . . .

77

Figure 5.8

Boundary Conddtions . . . . . . . . . . . . . . . . . . . . . . . . . .

79

Figure 5.9

Results of Crack Growth in Trench

. . . . . . . . . . . . . . . . . .

80

Figure 6.1

Trench Safety Program Simulator . . . . . . . . . . . . . . . . . . .

88

Figure 6.2

SD Model With Equations . . . . . . . . . . . . . . . . . . . . . . .

89

Figure 6.3

Prevented Accidents versus Accidents Caused . . . . . . . . . . . . .

90

Figure 6.4

Trench hazard Condition . . . . . . . . . . . . . . . . . . . . . . . .

90

x

Chapter 1
Introduction

1.1

Overview

The construction industry, with 7.2 million wage and salary jobs and 1.8 million selfemployed, was one of the Nation’s largest industries as reported by the United States Department of Labor (BLS, 2008). Earnings in construction are higher than the average for
all industries. Construction also offers the opportunities for individuals who want to be self
employed. Construction is one of the most lucrative and prospering industries worldwide;
however it is also one of the most hazardous in existence. As reported by the European
Union Statistical Information Instititution construction is the most land based work sector
after finishing industry (Eurostat, 2010). Data from the U.S. BLS show that many construction trades workers experienced a work-related injury and illness rate that was higher than
the national average. Table 1.1 shows the rate of accidents that have been caused only due
to excavations and trench cave-ins (BLS, 2008).

1

Table 1.1: Deaths Due to excavations and trench cave-ins (BLS, 2008)
Year
2001
2002
2003
2004
2005
2006

Deaths
36
34
48
41
44
28

Occupational Safety and Health Administration (OSHA) identifies trenches as one of the
most hazardous construction work conditions. The usual conditions under which the person
dies as determined by the investigations is mainly due the soil pressure that bears over the
chest of the construction worker, which prevents the chest from expanding and contracting,
resulting in death due to suffocation. 1.5 m3 of soil can weigh between 18kN to 27kN; Soil
burying a person under these pressures can kill the worker in less than 3 minutes (Landscape
Management, 2009).

Figure 1.1: Trench Soil Cave-in ”For interpretation of the references to color in this and all
other figures, the reader is referred to the electronic version of this thesis.”

2

The costs arising against a contractor in the event of such hazards are tremendous.
Generally trench excavation contracts are subcontracted by small size landscape service or
utility firms who can run out of business with any such occurrence of fatalities to their
workers. In the occurrence of such an accident, the contractor will have to bear the costs of
OSHA investigation and legal fees; in addition he should be faced with increased insurance
premiums, loss of time, increased costs of repair to the construction, the need to hire new
workers and train them and potential loss of customers. OSHA has safety programs for
proper execution of trenching work; however it is found that in many instances injury and
death still occur in spite of such programs. The need for a methodology to predict such
hazards and help save lives and help small size construction companies to grow has risen.

1.2

Research Rationale

The lack of effectiveness of current safety programs is identified within the thesis. Most of
the trenchc excavation cave-ins have been identified to occur due to lack of implementation of
required types of shoring methods to prevent soil cave-ins. Since the contractors performing
excavation jobs are often tied up critically to their limited funds and low bids; they avoid the
implementation of required safety conditions and shoring systems to maintain profit margins.
The goal of the research is to recognize a tool which can foster safety in trench excavations
by predicting the trends of trench failures and developing a guideline to make it easy for
adoption by the construction crews. The thesis helps to realize a relatively easy-use tool
which can help predict soil strata behavior during the execution of a trench excavation and
to prove the usefullness and ease of implementing finite element method as a an effective tool
for safety programs in trench excavation. A benefit of using the finite element based safety
3

program is that it can provide for protection against legal claims in occurence of accidents.
FEM is used in geotechnical analysis and predicting soil behaviors. FEM has been
extensively applied to the tunnel construction fields. FEM is also accurate in simulating
in situ stress states, fluctuations of shield pressures, changes in construction process, and
deformations of ground surface due to tunneling exercises (Ongsuksun et al., 2009). To
provide an average excavation contractor with a relatively easy method to perform static
soil prediction simulations of subsurface conditions in a trench is the ultimate goal of the
current research. The result will be a process which can involve a preliminary investigation
of the site, transferring the results of the site investigation into a finite element prediction
tool. The tool enables prediction of excavation cave ins that may occur due to the trenching
operations, live weight soil pile placed around the trench, the vibrations occurring onsite due
traffic of trains and also due to changed conditions of rain storms and existence of high water
table at site. The thesis focuses on understanding causes of trench excavation soil failures
and methods predicting these failures for prevention.
Systems dynamics (SD) simulation was the solution first invented in 1950 by Forrester
to solve the problems of dynamic complexity (Davis and O’Donnel, 1997). The concept of
dynamic complexity can be explained as the unintended results that occur as a result of
simple interactions or decisions but have the most counterintuitive results. Most companies
and managerial teams have fallen prey to the occurrence of dynamic complexity as very
though out decisions have yielded promising results in the short term, but over the long run
the same system yielded to failure. Dynamic complexity arises not from the factors in the
system that affects the end user but they way in which the factors related to each other
(Davis and O’Donnel, 1997).
4

The reason for opting SD in the thesis is to understand how the implementation of new
safety protocol can affect the change in working of a construction firm related to majorly
trench excavations. Organizational safety culture, especially its definitions, dimensions, and
enablers, as well as the development of tools for assessing and monitoring its health, have
been give due importance in the recent years in order to identify areas for safety performance
improvement (Choudhary et al., 2007). The lack of importance to such factors have caused
sufficient risk and fatalities to human lives. Sherif and Chinda (2011) designed a causal
model to simulate the interactions and relationships among Construction Safety Culture
(CSC) enablers, as well as the potential impact of each enabler on safety goals over a period
of time. The model with the help of other systems dynamics models could be tailored to
suit the needs of the research.

1.3

Research Objectives

The present thesis aims to provide the following as the results for the construction industry:

1) 1) Realize the Computer program which can be used to predict trends in trench
excavation cave-ins.
2) Organize a methodology to help the easy adoption of the prediction strategy in the
industry by the construction crews for trench excavation safety.
3) Analyze the risks associated with implementing a new safety protocol and understanding the implications for the construction firms.
5

1.4

Thesis Overview

The thesis involves 7 chapters. In the first Chapter problem statement is laid out. The
research rationale helps further the transition of the problem into understanding briefly how
the problem is reallly justified; and how it is aimed to help the construction industry become
a safer place.
The literature review covers the grounds on aspects of the thesis. Related research works
are divided into modules for easier flow and transition of the paper.
In Chapter 3, the approach to solve the problems as outlined in the thesis are detailed
in here. The steps taken to model a simple trench cave-in under homogenous conditions are
also included within the chapter
In Chapter 4, FEM simulations which predict the failure of soils when not being safely
executed are compared with Benched Trenches in FEM- ABAQUS models for verification.
The results prove the effectiveness of simulating trench conditions.
In Chapter 5, implementation of FEM approach in safety program is a need for safety
programs implemented on a regular basis in the construction industry. The change need to
be brought up requires the various team players associated with trench excavation.
Chapter 6 helps to understand the impact of the behavioral change in an organization
that could occur with the implementation of a new safety program.
Chapter 7 summaries the goal and objectives and achieved results of the thesis. The
future recommendations are also given here.

6

Thesis Overview

Chapter 1

Introduction: Problem statement, goals and objectives,
Research rationale

Chapter 2

Literature Review: Safety intrench excavation, FEM
modeling and background, guidance and training,
systems dynamics background

Chapter 3

Research Methodology: Traditional geo-tech and FEM
theories, simulations in ABAQUS

Chapter 4

Analysis of FEM simulations: Analysis and results
discussions, Comparison to safer excavation
standards.

Chapter 5

Guidelines to Build FEM model: Creating Various
domains. Results, E-learning

Chapter 6

Systems Dynamics: Model Development, assumptions,
Results and discussions, Limitations

Chapter 7

Conclusions: New safety program, proposed future
research
Figure 1.2: Thesis Overview

7

Chapter 2
Literature Review
Chapter 2 outlines the literature review conducted in order to identify the various factors that
owe to unsafe conditions in the trench excavation sequences of construction. In the chapter,
the finite element method, its applications and backgrounds are provided. Implementation of
new strategies or policies that can change organizational behavior is discussed. In the thesis,
current practices and existing knowledge are the foundation upon which new FEM based
approach is developed. The traditional geotechincal conspets and FEM concepts which have
been used in the research are included in the chapter three.

8

Literature Review

Trench Excavation
Safety

Guidelines and
Training Programs

- Causes for incident
- Concepts of FEM
occurrences in
- Various FEM
construction
programs
- OSHA Web Based
- Implementation of
Programs
FEM in ABAQUS
- Case studies on OSHA -Case Studies of
citations
implementation of
FEM
- Thesis,
- Web Articles, OSHA
Dissertations,
website, Proquest
Proquest Search,
Search,
Books, Dassault
Systemes manual

Systems Dynamics

- Development of
Training Modules
- Case Studies
identifying the best
possible ways to
develop training
- E- Learning

-Definition of SD
-Use of SD
- Case studies of
implementing SD
in
construction and
safety.

- Thesis, Proquest
Search, Books,
Dassault
Systemes manual,

Finite Element
Method

- Thesis,
Proquest
Search, Books,
Dissertations

Figure 2.1: Frame-work for Literature Review

2.1

Safety in the Construction Industry Focusing on
Trench Excavations

Construction safety researchers have postulated several accident causation models which
cite the reasons for hazards and their occurrences. Some of the root causes, as identified by
Abdelhamid and Everett (2000) are management deficiencies, workers’ attitude and training.
The element of unpredictability of the subsurface conditions also mounts to a significant
amount risk to the construction work. For example the work that is expedited in the onset
of a rainy day can lead to various unpredictable subsurface conditions that can change due to
the increased rate of excavation and piling up of large amount of soil along the trench lines.
9

The use can be easily given as an example to cause a cave in and can endanger the lives of
the workers inside the trenches. Another potential threat of cave-ins has been observed in
case of deep trenches being excavated along road pavements and along railway tracks (GIT,
2010).
OSHA has designed a web based Training Program. NIOSH Trench Safety Awareness
(2010) illustrates the four types trench collapse. Although many of these programs allow
to understand the reasons and safety precautions to follow in the event of a cave in; none
of them can help predict any of the types of soil collapses on the site. Death toll has far
exceeded the numbers it should have ever reached as of today.
Research conducted by Arboleda and Abraham (Carlos and Abraham, 2004)found that
”When cave-ins occurred, 49% of the fatalities occurred in projects with costs under $250,000
and 73% of the fatalities in projects with costs under $1,000,000. Also, 31% of the accidents occurred in projects with fewer than 10 workers and costs under $250,000, i.e., when
the project is smaller, the likelihood of a trench cave-in is higher”. The research provides
sufficient evidence as to why most excavation contractors fail to follow the OSHA safety
requirements while performing trench excavations and it can be inferred that an inexpensive
tool to predict such hazards of cave-ins during excavation can prove to significantly reduce
the everlasting death toll.
U.S. OSHA has stated that ”Without training a person cannot possibly be qualified for
to supervise trench and excavation work” (Prentiss, 2004). It falls to the management to get
the appropriately trained personnel to maintain a safe working site. OSHA also requires a
supervisory member of the excavation team to have an 8 hour course that may vary based on
experience. The training consists of modules that inform the personnel of hazards associated,
10

the probabilities of being buried alive and the causes due to which trench cave-ins occur.
However they are not well equipped with a method to predict such hazards in spite of the
training. The training and safety protocol if followed in detail may in fact help avoid a large
number of such incidents; but, it is because of becoming complacent in a work environment
which leads to instances of lack of needed attention to detail at trench examinations for
cracks.
The human tendency to become comfortable at a work environment is a very dangerous
reason for many construction accidents. The situation needs a major behavior modification
in the work force, which is not a logical attainable solution. Hence the alternative is to have
computers come to the aid of mankind and help monitor such situations and predict them.
OSHA Reported Incidents and Implications are complied below as three case studies.

2.1.1

Case study 1

The event of an incident OSHA takes up to six months for its investigations, levying fines
of up to $7,000 for hazards an employer should have been aware of or $70,000 for willful
disregard of or indifference to regulations. The agency’s inspectors are also told, while on
route, to various locations on the job to stop and check out any work sites they pass with
signs of trench work that lacks proper shoring (Rotstein and Gary, 2004). Another Heavy
violation was cited and fined $510,750 for inadequate trench safety. Such incidents afore
mentioned can easily put construction companies of mid size completely out of business
therefore reinforcing the requirement of an innovative and cheap means of predicting trench
cave-ins.
11

2.1.2

Case study 2

July 27, 2006 a trench cave-in occurred at Christiansburg, Virginia when two workers who
were working for construction company got into a trench where no safety precautions were
taken. The company said the workers were preparing to connect a new sewer line to an
existing one. The existing line was busted, however, and water began filling the trench.
One of them who has been doing site work for 30 years, jumped into the 3.5 m deep trench
to try to set a pump to get the water out, company declared that the employees just got
excited (Shawna, 2006). This statement simply shows the unnerving implications of worker’s
complacency on jobsite can have for the human safety.

2.1.3

Case study 3

OSHA’s national statistics report an average of 30 to 50 cave-in deaths annually. it was
maintained that the numbers are hardly a fraction of the actual incidents that occur every
year; while these numbers could very well be three or four times higher (Shawna, 2006).
In owner of a concrete company was operating a backhoe and managing the excavation
for a new sewer line. Two employees were laying sewer pipe in an unprotected excavation
approximately 2.5m deep and 1.5m wide. The sides of the excavation were nearly vertical.
As the owner was digging the trench, he placed the spoils along the north edge of the
excavation. Prior to the fatal collapse, two employees were in the trench when soil broke
loose from the north side and they had to run toward the east end of the excavation to avoid
being buried by a cave-in. They narrowly escaped being buried in the soil. They cleared up
the soil and continued to work and nearly 10 minutes after resuming the work another cave-in
occurred burying the two workers. One of the employees was killed in the accident and the
12

other worker was seriously injured. The company received a combined total of three alleged
willful violations with a proposed penalty of $99,400 and two alleged serious violations with
a proposed penalty of $4,200, for a total proposed penalty of $103,600 (US Fed News, 2011).

13

2.2

Finite Element Method

The literature presented below shows the potential of implementing FEM approaches to
predict and prevent trench cave-ins. One of the previous researches conducted has been to
predict subsurface soil behaviors for tunneling projects that led development in numerical
techniques which have been successful in simulating the sub-surface conditions in 3-D. Successful verification of the FEM simulations is carried out in a centrifuge model (Meguid et al.,
2008). The numerical method that has been proved to be most effective in such simulation
and predictions is FEM.
One such application of FEM has been used to predict slope stability (Swan and Seo,
1999) for heavy geotechnical construction related problems and to prevent hazards that
could occur due to heavy downpours and changing water table conditions that could put
roads and transportation related projects into jeopardy. Different types of soils and moisture
conditions prevailing in these soils have different stress-strain properties. Hence it becomes
highly difficult to model all these different types of soil behaviors and help predict their
failures during a trenching exercise (Adhikary Guo, 2009).
ANSYS (2011) integrated civil FEM module is used in the research studies for predicting
soil behaviors and the performance of trench excavations (Xiujuan et al., 2008). The studies
use the soil’s modulus of elasticity and Poisson’s ratio of the proposed site in order to model
the settlement that could occur on the event of construction of the oil tank and also the
settlement that could occur due to filling up of the tank by using ANSYS civil FEM. Once
the soil deformation was studied, the simulation offered results on how to design and build
the tank. Another research conducted by Xi cites the use of ANSYS to simulate the seepage
flow or subsurface flow of water through soils (Xi et al., 2008). Xi’s study serves as a good
14

resource of information to predict the movement of ground water and its effects on soil
properties.
The finite element method is used to obtain values of stresses developed within the
structure, on the nodes of each FE. In FEM, the system is meshed in finite members instead
of driving equations. The meshed members are solved considering the entire system. The
accuracy of the stress output is related to the FE meshing and the density of the realized
grid. The boundary conditions of members are superposed to form the equations in the
matrices in the system. In Figure 2.2, the continuous system can be seen (Bathe, 1967;
Zienkiewicz and Taylor, 1967).

Joints
Meshed Member

Figure 2.2: Continuous System and Finite Member

Calculation of the effect of the eliminated principal stress on the remaining two was made
using the Hooke’s law. In Figure the principal stresses are shown for a tri-axial stress state,
while in Figure 2.3 its equivalent biaxial stress state is presented after elimination of the
principal stress (Syrmakezis et al., 2006).

15

1T
3T
2T
2T
3T
1T
((a)) 3-D
1T

2T

2T

1T
((b)) 2-D
Figure 2.3: Stress states in 3D and 2D

For a three-dimensional stress state, the strains on plane (1, 2) are equal to(Syrmakezis
et al., 2006):

T
T
T
εT = σ2 − ν · (σ1 + σ3 ) /E
2

B
T
T
σ1 = σ1 − ν · σ3 /(1 − ν)
16

(2.2)

B
T
T
σ2 = σ2 − ν · σ3 /(1 − ν)

B
B
εB = σ1 − ν · σ2 /E
1

(2.1)

B
B
εB = σ2 − ν · σ1 /E
2

T
T
T
εT = σ1 − ν · (σ2 + σ3 ) /E
1

(2.3)

T
T
B
σ2 = σ2 − ν · σ1 /(1 − ν)

T
T
B
σ3 = σ3 − ν · σ1 /(1 − ν)

(2.4)

T
T
B
σ1 = σ1 − ν · σ2 /(1 − ν)

T
T
B
σ3 = σ3 − ν · σ2 /(1 − ν)

(2.5)

fwt
S3

fwc
S45

Tension
S2
1
S1

fwc

Compression
Figure 2.4: The modified Von-Mises Failure criterion

The modified criterion can be expressed in a three-dimensional system, where, plane of
direct stresses is perpendicular to the third axis and represents applied shear stresses on the
node, resulting in the semi-empirical failure surface, presented in Figure 2.4, for zero shear
stress level (Syrmakezis et al., 2006).

17

Some of the element types used in FE analysis are eight-node brick element (C3D8
and F3D8) C3D8R and F3D8R, Twenty-node brick element (C3D20 and F3D20), Fournode tetrahedral element (C3D4 and F3D4) etc. (Dhondt, 2010). The most common and
more accurate of these elements for sweep meshing techniques is C3D20R, a quadratic brick
element with reduced integration (2x2x2 integration points). The Figure XX shows the
element of C3D20R which is chosen as an ideal element choice for the simulations in the
research because of its adaptability to various geometry.

7

8

5

6

4

4
2

1

Figure 2.5: 2x2x2 integration point scheme in hexahedral elements

18

2.2.1

Case studies of application of FEM

There are considerable number of studies available for analysing the danger associated with
deep excavations using FEA; however the same implementation in the shallow excavations
lacks effort. Bentler et al. (1998) worked one developing adjustments to Sage, a FEM program developed at Virginia Tech for analysis of soil interaction problems. In the process to
develop and understand the performance of deep excavation they studied files performances
of the excavations for two main reasons. One of which was gain insight into the experiences
gained during design and construction of real excavations. The researchers learned that a
great deal could be learned about the behavior of deep excavations when careful observations
of construction activities, site conditions were made. The second reason was for reviewing
field performance variables was to learn about factors that couldn’t be modeled analytically. Such findings bring in the understanding of how to approach the modeling of shallow
excavation projects.
The literature review conducted brought to light detailed case histories of trench excavation cave-ins of how and why they occurred. Such data provides the insight of how
the construction work was being executed prior to the fatalities or unfortunate incidents
occurred. The data is critical to understand why the contractors were neglecting the safer
practices. Most of the times even though shoring systems are onsite they are not in place
where they have to be. Such practices could be taken into account for negligence and model
the trenches in FEM as unprotected cave-ins.
Tamano et al. (1991) conducted studies to understand the mechanical behavior of ground
heave due to 21m deep excavation in Osaka. The research team developed 2-D FEM models
and compared the heave values of the model to the base heave values. The results of the
19

study showed astounding comparability. Significant heave was found to have occurred 50m
below the 21m excavation.
Finno et al. (1992) observed the ground movements adjacent to deep excavations made
through soft Chicago clay. The research team studied the effects of deep foundation installation, cycles of excavation and bracing, and strut removal on observed movements. The
researchers concluded that construction activities can change the stress in the ground and
should be taken into account while estimating ground movements. Their findings primarily
translated to the understanding that finite element analyses to predict excavation performance are limited by the ability to model construction activities.
Lee et al. (1998) monitored the corner effects on the behavior of a strutted excavation
for the immigration building in Singapore. The researchers compared the 2-D and 3-D finite
element wall movements and settlements of the excavation site to the field measurements
of the same variables and found astounding comparison between the models and the real
world measurements. Lee et al found that 3-D analyses seemed to agree more with the field
measurements than 2-D analyses.
Finite element analyses may become imperative in conditions where in high-rise buildings
exist next to a proposed excavation site. Such a case study was reported by Brassinga et
al.(1991) where the team studied the response of a high-rise building next to a braced
excavation in soft soils in Rotterdam, Netherland. Significant consolidation was found to
have occurred after continuing settlements with the progress in time of excavation process.
The researchers concluded that major reason for the consolidation was due to dewatering.
A similar instance can easily occur in steel shored or braced trench wall and the case
could potentially mean shift in the braced trench laterally. Although the incident may not
20

cause fatal threats to the workers, however it can impose a significant risk to the construction
project itself where a repair work of an existing pipeline can potentially burst or collapse
due to the lateral movement. The lateral movement as discussed before can be caused due
to the removal of soil and increased lateral pressure from adjoining building.
The literature review for deep excavations follows to show that the following parameters
are critical to performing a FEM analysis: type of soil and its mechanical behavior; construction method; construction sequencing; initial soil stresses; ground water conditions; support
system; workmanship/ quality of construction; excavation geometry. The Osha Technical
Manual (OTM) categorises soil in two four types. Type A is stable rock or cohesive soils
with unconfined compressive strength of 144kPa or greater. Type B soils are unstable rock
or soils having unconfined compressive strength of greater than 48kPa and less than 144kPa.
Type C soils are granular or sandy soils which have a compressive strength of 48kPa or less.
Type D is usually made up of layered geological strata.

21

2.2.2

Modeling in ABAQUS

The ABAQUS program was one of the first FEM programs to introduce an allowance for
researchers to add elements and materials models. ABAQUS/Explicit is a special-purpose
analysis module that uses an explicit dynamic finite element formulation (Dassault Systemes,
2009). It is suitable for modeling transient dynamic events, such as impact and blast problems, and is also very efficient for highly nonlinear problems involving contact conditions,
such as creating simulations and also helps in modeling continuum problems (Tee, 2005).

2.2.3

Development of FEM model to determine earth pressure in
trench rescue shoring systems

LaBaw et al. (2009), in their research developed a method to predict the earth pressure in
trench rescue operations for the rescue team safety. The team created a 3-D linear elastic
finite element model to examine the effects of different trench rescue shoring components
on earth pressure. ABAQUS/Standard 6.7 was used to in the parametric studies and also
to determine the earth pressure on rigid retaining wall by comparing the model results twit
theoretical vertical and horizontal earth pressure. The model accuracy to predict the load
transfer of the shoring system through multiple layers of materials and varying properties
was also examined.
Half of the symmetric problem was modeled with a 3-D linear elastic model. The model
consisted of three parts: soil, panel and strong back. All three parts were assumed to be
solid and homogenous with elastic isotropic material properties. The soil was also given a
density variable to account for the effects that could arise from gravity. Interaction between
the strongback and the panel was locked and didn’t allow for any movement.
22

Internal piston pressure was set at 1378.951MPa, 5171.067MPa, and 10342.9MPa, producing loads on the strut base of 5.8kN, 10.5kN, and 22kN respectively. Meshing in Abaqus
assigned 820 elements to the panel, 300 elements to the strongback and 130000 to 140000
elements to the soil. Boundary conditions were only assigned to the soil.
The resulting analysis of the FEM model paralleled the calculated vertical stress and
horizontal earth pressures. The FEM analysis brings valuable insight to the methodology to
construct FEM models for the goals of modeling trench excavations. The models are built
and detailed in the research methodology.

23

2.3

Guidance and Training Program for Trench Excavation Related Industry Professionals

The literature review so far concludes that the lack of ability to predict trench soil behaviors
have been a major cause for majority of the cases of trench excavation injuries and fatalities.
The ability to provide such a training program on a large scale industry such as construction
would need a very rapid process dispersion of information. In order to achieve such a
progression over a wide variety of end users the best means to develop a quick training
program would be the method of developing an e-learning and e-training program.

2.3.1

Case study 1

Organizations must continuously learn update themselves to survive in a highly competitive
market, such instances is very much seen in most technology driven industries. But in
construction where aging knowhow and procedures still seem to precede the latest and most
efficient methods of performing construction exist, the need for a faster way of spreading
knowledge is critical. With an approximate number of 884,300 construction establishments
in the United States as projected by BLS National Employment Matrix, 2008-2018, it is a
vast territory to cover for training very quickly. In order to cover the ground of varying
ages and localities within the U.S., the internet provides as the easiest source to provide
the training across the country. Nivar, (2009) described distance learning as a modality
by which instructor and learner are separated by time, space or both simultaneously. In
order to keep up with market stiffness and competition and to favor company reputations
organizations now expect employees to rapidly gain new knowledge, skills, and abilities.
24

As a result companies have turned to training that is available at any time required, and
unimpaired in time with respect to quality and easy to schedule learning opportunities (Sims
et al., 2008).
A key concept extracted from Norman’s research work, was that of universal usability;
which is an approach that provides a feasible solution to attending instructional design needs
for the continuously growing population within the academic and work areas (Norman, 2004).
This concept implements a universal design that may include and help accommodate the
older and less experienced users and at the same time younger and more experienced users.
”While there is certainly a population that has grown in tandem with technology (touch
screen devices, voice recognition devices and others) which are young and computer experienced, there are others that do not share the same common ground and computer experience,
the aging (baby boomers and older adults). This disadvantage when combined with a large
amount of physical and cognitive declining functions of the elderly, may difficult the adoption
and assimilation of technology. Therefore, making questionable the effectiveness and quality
of e-Learning and e-training modules for this special age group” (Nivar, 2009). Marcia’s research aimed at determining a universal design guideline that can be applied when designing
online course for a 25 year old learner and at the same time suitable for a leaner ageing at 65
years. Results of the study aimed at achieving high user satisfaction and information recall,
while maintaining low levels of disorientation and task overload. Marcia’s research concluded
with significantly positive results from the following the approaches of implementing filming
techniques employed in the creation of training video primarily affecting the recall capacities
for all age groups. Narrative content had the most positive impact. The numerical modules
showed inconsistent recall by the different age groups depending on the end user and their
25

backgrounds. Finally the layout arrangements and preferences also had a significant impact
on a successful e-training module.

2.3.2

Case study 2

Rene et al. (2004) offered guidelines for a leaner led instruction, the design of learner
controlled training, and the creation of workplace conditions which help manifest learner led
training. The reason of the researchers to choose e-learning is as a medium of propagating
education is because it allows training to reach diverse and geographically dispersed workforce
in a cost efficient manner; which is highly a reflection of the construction industry. Also
other advantage include where training programs can be structured so that learners have
the opportunity to collaborate remotely on training tasks. Also e-learning may be designed
so that trainees follow a structured program individually. Learner control refers to a mode
of instruction where some decisions are allocated to learner preferences. Learner control as
of today has evolved to include instruction design elements, including the content, pacing,
context, sequence, method of presentation, task difficulties. These options are previously
instructor controlled and the flexibly of these options being available to the diverse e-learning
population makes e-learning tremendously successful.
Preparing the trainees for learner led instruction was a major focus of the study where
the researchers reasoned that trainees who are given control over their training are often
poorly equipped to use that control. The various guidelines leading through the study are
understanding learner control, giving the end user enough time to learn, calibrating the
expected results and feedback, offering help, understanding the varying needs of the diverse
user base and continuously evolving to include all recommendations, designing just the right
26

amount of user control, allowing trainees to skip tasks as needed, Identifying user progress
etc.
Effectiveness of the e-learning programs can be impacted by organizational factors, such
as corporate culture and incentives. This is because if the organization does not provide a
learner led training program the trainees might be reluctant to learn more than that can
be anticipated and training programs can be rendered useless. The team also reasoned that
following such guidelines might run to be more expensive than that of traditional training programs the learning outcomes might be overcome the costs might be recovered from
improved productivity and better skills of the trainees and employees.

27

2.4

System Dyanmics

Zemke (2001) stated that ”Looking at how systems really work can be enlightening -or a
wake-up call” . The corporate culture of most construction companies already incorporate
safety in pristine importance. The fatality rate for excavation work is 112 % higher than
the rate for general construction. BLS data show that 271 workers died in trenching or
excavation cave-ins from 2000 through 2006. Even though the safety protocol implemented
and other management decisions made to keep the trench excavations from ever happening,
they are from being able to deplete these instances. The use of systems thinking and systems
dynamics come into play in such situations, where, even though the corporate decisions made
after careful thought and financial risk assessments to stop cave-ins from occurring, they are
still faced with risks. The maxim as illustrated above have been attributed various times as
an eye-opener to managers who assume that by changing just one part of a complex problem
will cure the entire system of its ailments (Zemke, 2001).
Systems dynamics modeling was born at the hands of engineering and management
Professor Forrester at Massachusetts Institute of Technology (MIT) in 1950’s (Sheriff and
Chinda, 2011). The approach of ”systems thinking” takes into account the synergies of human conduct in business, industry, government, community and other such soft but usually
ignored factors into systems modeling and applies systems modeling to discover emergent
properties or unexpected behaviors from the system’s reaction to policy change or other such
instances. The reason for such unexpected behavior to arise through these simulations is the
length of time the behaviors of the systems is put to test at. The human reaction to systems
and policy changes is very difficult to predict and even more so in the construction industry
which is extremely diverse, dynamic and highly susceptible to economic downturns.
28

An organization must be able to highlight the areas for safety improvement to progress
iteratively through different levels of cultural maturity (Sheriff and Chinda, 2011). Sherif
et al. developed a casual model which aimed not only assist organizations in appropriately
assess their current construction safety standard (CSC) maturity level, but also depict the
potential effects of the enablers to achieve the pre-determined safety goals. The researchers
used Systems dynamics modeling specifically designed to focus on policy analysis and design.
The EFQM (2000)excellence model was adopted as a conceptual framework. To study the
development of systems dynamics models for furthering construction safety a few cases were
studied.

2.4.1

Case study 1

David O’Donald (1997) explained the use of system dynamics a way to look at business, by
focusing on what is happening outside the organization and what is happening inside. It
is a tool that is updated and in tune with modern concepts of business management with
a focus on stakeholder requirements, empowerment of employees and the learning organization while making sure that the decisions are clearly linked to improving the stakeholder
value. The process laid down by the research team has four stages and is based on systems
thinking. First stage is to create a clear statement of objectives. The team proposed that
”sustainable success can only be achieved through maintenance of confidence and support of
all stakeholders”. The statement has deeper meaning which goes to explain that traditional
objectives that meet the requirements of the providers of capital might not be enough in the
future to obtain objectives to greet the organization with success.
The second and subsequent stages require that new measures must focus on the factors
29

which lead to success in addition to the measurement of success itself. Hence the second
stage creates a map of the dynamic links between business activities and the objectives. The
various relationships include cause and effect, delay, feedback etc.
The third stage goes on to map the activities are critical and also identify the key performance drivers. This map is usually a computer generated system dynamics model of
the map. The research team postulated that running computer simulations can yield new
insights and reveal unexpected limits to growth.
The fourth stage indentifies the ways to develop the measurement of key performance
indicators. There are two ways to approach this, one of which is to measure the driver itself,
but is seldom possible. The other is to measure the influencing proxies. Careful examination
of the influence diagram would help identify these proxies and other variables to be measured.

2.4.2

Case study 2

Sherif and Chinda (2011) investigated interactions among five key enablers of construction
safety culture. The team proposed methods for system dynamics modeling of Construction
Safety Culture (CSC). The model was developed on the basis that improving the enablers
will inevitably improve the safety performance in construction. In reality, System dynamics
modeling with the developed index from this research is postulated, to be able to help
organizations to plan the most effective safety implementation process to achieve their safety
goals within a planned time frame.
The conceptual model is based on the adoption of European Foundation for Quality
Mangement (EFQM) excellence model. The model follows the same assumption of the casual
model as assumed in the model. Five enablers identified in the study are leadership, policy
30

and strategy, people, partnerships and resources, processes (EFQM, 2000). The approach
for the model was to develop safety improvement matrix where a weighted score for each
enabler was obtained and then combined to achieve overall score. It is imperative that the
approach requires examining the safety culture holistically; otherwise it would be impossible
to determine the influence of the enablers. A CSC casual loop diagram was proposed which
consisted of seven elements to explain the links between Enablers, Goals and the CSC index.
The sensitivity analysis of the model on close observation of the increasing rate of five
enablers’ values at early stages discovered that the Lds was the weakest enabler. Hence to
expedite the Enablers’ value and to achieve higher CSC figures the companies should employ
better attribute focusing on bettering the leadership attribute. The researchers concluded
that the developed CSC index meets the required goals and objective assist in construction
safety improvements.

2.4.3

Case study 3

Owaba and Adebayi (2006) postulated that system dynamics approach helps in evaluating a
safety program (SP) performance and allows more complex relationships between elements
of the SP (accident causing factors, accident prevention activities and output) and also helps
in identifying the dynamic relationships between the elements in addition to just the statistic
relationships of the elements. As a part of the study the researchers followed the modeling strategy developed by Forrester (1973) and Burns (1975) to measure SP performance
measure. The following steps are religiously followed as part the research: 1. Definition of
system boundary, 2. Specification of set of quantities, 3. Delineation of causal diagram,
4. Delineation of flow diagram, 5. Determination of model equations from the diagram, 6.
31

Implementation of simulation from the model equations using computer program. Using a
set of mathematical equations and the influence diagram a software program was used to
create a manufacturing safety program simulator.
The developed safety program simulator was applied to a bottling manufacturing company to achieve the objectives of the research study and verify the working of the model. The
literature review conducted for the study showed that soft drink industry had the highest
rate of serious injuries and illness and the companies continue to be subject to high incident
rates due to the labor intensive nature and exposure to hazardous operational condition.
Data was collected through sources such as from production clinic, safety and administrative department of the establishment through vetting records, questionnaires and interaction
with heads of various departments. Investigations were also carried out to see budgeting allocations committed by the safety department. Direct and indirect cost elements associated
to the accidents were also identified. The amount dedicated to the safety programs was
discovered to be quite significant.
The model developed consisted of two major parts dynamically linked: 1. Accident causing, 2. Accident prevention. The model was built to understand the dynamic relationships
between the elements of the safety program and depict the future trends. The model was
able to successfully simulate the safety conditions through the years of 1991 to 2004 and
these results coincided with the recorded incidents within the company. The fluctuations in
the effectiveness of the SP was majorly due to varying financial constraints of the company
over the years due to net profit and was also found to directly proportional to the monetary
conditions implemented towards safety programs in the company. The software program
used to develop the simulator was Vensim PLE software and the simulator was found to be
32

effective in facilitating the decision making of the planner of SP.

33

Chapter 3
Research Methodology
Chapter 3 describes the methodology that is used in the present thesis. Three different
simulations are developed to show simple cases of trench excavations using the Abaqus CAE
6.9 software program (Abaqus CAE 6.9). The data for the different cases are obtained from
a geotechnical website that has a summary of geotechnical soil properties for various types
of soil such as very soft clay as shown in Table 3.1, gathered from the US Army Corps of
Engineers EM 1110-1-1904 (Robert, 1990). Plaxis 3D is a FEM suite which has the similar
capabilities of ABAQUS and would have better suited the research objectives. The use of
ABAQUS was fostered by easy availability of the program within the learning laboratories.
Table 3.1: Summary of varios soil properties (Robert, 1990)
Soil Type
Very soft clay
Soft Clay
Medium Clay
Stiff Clay
Sandy Clay
Clay Shale

Young’s Modulus (Es) (kPa)
480-4,800
4,800-20,000
20,000-48,000
48,000-96,000
24,000-192,000
96,000-192,000

34

3.1

Traditional Concepts of Soil Mechanics

In order to understand the parameters required to be input for the FEM simulations and
the assumptions made for the same, there is a need to understand the principles of soil
mechanics. Stress -strain behavior of soils are governed by two principles namely: the
principle of effective stress and the principle of steady state deformation (Poulos, 1989).
The effective stress can be explained as the load F transmitted through the soil at a normal
direction to the cross-sectional area, A, divided by the area. σ = F
A
The effective stress principle implies that the pore fluid pressure should have no effect on
the stress strain properties of the soil and also that, the strength of un-cemented soils will be
zero if the effective stress remains zero. Steady state deformation can be defined as a unique
steady state line given for any soil which is the locus of points at which the soil mass can
strain continuously at constant: void ratio, shear stress, effective normal stress, and strain
rate (Poulos, 1989). The factors that influence stress strain behavior are soil composition,
structure, state, loading method. The Figure 3.1 shows the typical stress strain behavior for
soils.

STRESS

F
F: DENOTES FAILURE
R: DENOTES RELSIDUAL
VALUE

STRAIN

STRAIN

Figure 3.1: Typical Stress Strain curve for soils

35

The traditional geotechnical mathematical models such as Mohr-Coulomb Model are
sufficient to accurately predict the types of shearing effects that soils could be experiencing.
The Figure 3.2 depicts the concept of Mohr-Coulomb Model.

1

'1

'2

Figure 3.2: Mohr-Coulomb Soil Modeling

Computer programs utilize these models to help geotechnical designers to predict slope
stability as detailed in the Literature Review. The concept of Mohr-Coulomb model is
widely used in geotechnical analysis of problems. The Mohr-Coulomb failure theory results
in a linear envelope that is obtained from a plot of the shear strength of a material versus
the normal stress applied. The shear stress at the point where the coulomb line touches
the Mohr’s circle of stress is where the failure shear stress lies. Plaxis is one such computer
program that is extensively used for geotechincal analysis and includes the modules for
Mohr-Coulomb models, Linear Elastic models etc. (Plaxis, 2011).
For Modelling purposes in the research and use of Abaqus software for the FEM simulations, the problem of studying trench failure patterns is studied as a continuum problem.
The choice of element is discussed in the Literature Review Chapter. The assumptions of
Boundary conditions and types of soils etc are mention in the Research Methodology. 2D
36

versus 3D based FEM simulations would yield different results. One of the objectives of the
Thesis in future studies is to perform simulations for dynamic loading; In such scenarios it
is important to perform 3D FEM simulations hence the research is completed using the 3D
extruded trench sections.

37

3.2

Methodology

Three different cases are investigated to show the value of implementing finite element
method in trench excavations. To simplify understanding of the concept the following cases
are developed:
1. Trench with homogenous soil conditions
2. Trench with two layers of soil over lap
3. Trench with failure crack growth.
Research Goals &
Question Objectives Literature
FEM Analysis and
Review
Guidelines

Systems
Dynamics
CONCLUSION

Figure 3.3: Research Methodology

Figure 3.3 breifly describes the various steps taken to approach the problem described
in the thesis. Traditional methods of mathematical calculations such Mohr-Coulomb failure
envelope are applicable to problems as stated in the thesis. The use of the research is
targeted for the construction contractors and trench excavation teams onsite. Geotechinical
designers would still approach these problems with traditional getoechnical approach as
they are designers of such systems. But the FEM tool is used herewith to provide for lack
of visualising hazardous conditions and predicting the same.
The various activities conducted to follow through the research are: 1) Identifying the
need for the research, setting goals and objectives for research. 2) Conducting literature
review to support the need for the research (Case studies, Major reasons for trench cave-ins,
Methods to perform FEM simulation and analysis). 3) Short listing of types of simulations
to be conducted to express the importance and visual effectiveness of the simulations. 4)
38

Completing three FEM simulations. 5) Analyzing simulations and understanding development guideline for performing future FEM simulations. 6) Implementing system dynamics
to understand the impact of change in safety protocol for a construction company. 7) Conclusions. The Figure 3.4 captures the details of the steps through conception of the idea to
completion of thesis.

Steps involving the Research Methodology
Identifying the need for The reasons for existing trench excavation cave-ins are
realized. Lack of preventive and predictive safety
Research
programs is identified.

Literature review

The literature review is conducted to understand the reasons
for continuing existence of trench excavation injuries and
fatalities, the effectiveness of FEM, the impact using Elearning to foster training of new safety programs and use of
systems dynamics to model construction safety cultures.

Simulations Conducted

Three different FEM simulations are carried out to bring
out trends in common instances of trench excavation
cave-ins

The results of FEM simulations are discussed and
Understanding the results of trends the various trends of trench excavation cave-ins
in trench cave-ins
are compared to a safe trench excavation
Building guidelines for conducting
FEM simulations
Use of Systems Dynamics

Procedure to build a 3D static edge crack
development in a trench is laid out.
A proof of concept is developed to help with
understanding of
impact using a new Safety Program.

Conclusions and
suggestions
for future research
Figure 3.4: Research Methodology

39

3.2.1

Trench with homogenous soil conditions

Building ABAQUS simulation for homogenous soil failure began with defining the geometrical shape of the trench. The Figure 3.5 shows the dimensions of the trench, base width=
1.0m, left bank = 3.0m, trench width =3.0m, right bank= 3.0m; extrude depth of 0.3m. The
dimensions input in the system was scaled down by 10 percent to have smaller FEM models
and faster simualtions. This step would not have an impact on the stresses and magnitudes
in the simulation beacuse the units are maintained in proportion.

3.0m

6.0m

7.0m
Figure 3.5: Geometry of Trench Section

The model was constructed to be a single part. The part was assumed to be solid 3D
deformable and homogenous with elastic properties. In order to incorporate the mechanical
properties of the soil, the research focused on homogenous soft clay soil properties with
Young’s modulus of elasticity (Es) 15MPa. To keep the problem simple, the soil was assumed
to be un-drained and various complex parameters of pore pressures etc were ignored. Figure
3.6 is the result of a 3D extruded trench part. A 3D model is more suited to the thesis
since a 3D approach is found to have better impact in the productivty of crews and their
40

understanding of the job and related hazards. The Poisson’s ratio that needed to be fed into
ABQAQUS is 0.25.

Figure 3.6: 3D Extruded Trench Section

Following steps are taken to create the section for the homogenous trench. Once the
section has been created the part needs to be assigned the section properties. The section
properties are completed the next step is to proceed to assembly module. The created
trench part is selected and assigned Dependent mesh. Next, the steps for running the FEM
simulation featuring the surcharge loads is selected to be static, general. The load is applied
in the form of uniform pressure applied to both walls of the trench. Desired output which
needs to be measured is selected as reaction forces and strain displacements.

41

Proceeding to selection of boundary conditions,outer walls of the trench were selected(side1,
side2, side 3) to be fixed by 6 degrees of freedom using symmetry/ antisymmetry/ encastre
for the trench load step. The type of boundary conditions chosen is encastre. The resulting
boundary conditions are shown in Figure 3.7 (a).

Side
3

Side
1
Side
2
((a)) Boundary Conditions

D2

D1

((b)) Partition Domain
Figure 3.7: Boundary Conditions and Cell Partition

The trench part needs to be sectioned in order to obtain the meshed part. The cutting
planes chosen to simplify the object were diagonals at D1 and D2 is also shown in Figure
42

Figure 3.8: Meshed Trench

3.7 (b). The process divides the trench part into three different sections. Once the part is
sectioned, the loads are given into the simulation model. The loads assumed are indicative
of surcharge loads on trench walls due to excavated soil placement within 0.6m of the trench
walls and heavy machinery etc. The load is applied as a uniform pressure at 0.015MPa.
The part is now ready to be assigned mesh properties. First the part is seeded at 0.003 and
then meshed using quadratic reduced integration C3D20R hex element type. The C3D20R
element is a general purpose quadratic brick element. The resulting meshed part is depicted
in Figure 3.8. The homogenous soft clay trench is now ready to be submitted for analysis of
part deformation due to the subjected loads. The mesh deformation of the trench is shown
in Figure 3.9 and the contour deformation of the trench section is seen in Figure 3.10.

43

Figure 3.9: Mesh Deformation

+4.77 e+04
+4.55e+04
+4.03e+04
+3.55e+04
+3.26e+04
+2.93e+04
+2.61e+04
+2.29e+04
+1.96e+04
+1.64e+04
+1.32e+04
+9.96e+03

Figure 3.10: Mesh Deformation Contour

44

3.2.2

3D edge crack in trench

The research adds more complexity to the case done previously by simulating a static edge
crack development in the trench. This is the main reason for the cave-ins to occur in
most situations. Using ABAQUS simply will not yield the most realistic results unless the
attempt to model realitic conditions is performed. To achieve crack growth we manually
feed a cracked domain to the FEM model to predict how much failure can occur due to given
loading conditions, or surcharge and machinery near trench walls.

3.0m
3.0m
6.0m

7.0m

Figure 3.11: Geometry of Trench

The model follows a similar procedure as the previous one, as seen above the researcher
begins with modeling the geometry of the trench as seen in Figure 3.11. The model is built
as a 3D deformable solid. The size of the trench is base width= 1.0 m, left bank= 3.0 m,
trench width=3.0 m, right bank= 3.0 m; extrude depth of 0.3m. The material properties are
assumed to be that of the soft clay hence Young’s Modulus is 15MPa and Poisson’s ratio is
taken to be 0.25.
The resulting model appears now as in Figure 3.12 (a). In the next steps the researcher
assigns damage for traction separation laws and enters a value of 5MPa for Maxps Damage.
The option for damage evolution is selected and displacement at failure is selected restricted
45

((a)) Extruded Trench model

((b)) Meshing Results
Figure 3.12: Trench Extrude and Meshing Results

at 1. The trench part is then assigned sections and meshing is completed as hown in Figure
3.12(b). For meshing sequence the researcher inputs Seeds at 21 number of elements running
along the edges and structured Hex mesh is performed.
To create the crack domain a new separate part is created named as crack which is
modeled as a 3D planar, deformable part; which has a base feature of shell and extruded to
the same size as the trench 0.03. A line is drawn 0.24m away from the trench wall on the
right wall. The depth of the crack is given as 0.24m. Resulting extrusion is now visible as
46

seen in Figure 3.13.

Figure 3.13: Crack Placement in Trench Domain

It is important to carefully configure the interactions between the crack and the trench
parts. From the interactions menu choose the special module which consist a package for
crack. Then, using XFEM type a crack interaction is created by choosing the un-cracked
domain versus the crack domain by specifying the crack location. The crack growth interaction is modeled in the interactions menu selecting the XFEM crack growth module. The
interactions between the crack and the trench domain are completed.
The loads are enforced as a uniform pressure on the cracked trench wall. The magnitude
for top pressure is given as 0.0115MPa. Figure 3.14 represents the top pressure applied over
the trench.
The final step is to set the boundary conditions; which is very similar to the previous
simulation. Outer walls of the trench (side1, side2, side 3) were decided to be fixed by 6
degrees of freedom using symmetry/ antisymmetry/ encastre for the trench load step. The
type of boundary conditions chosen is Encastre. The resulting boundary conditions are
shown in Figure 3.15. The model is now ready to be submitted for analysis. Submitting the
47

Figure 3.14: Loads Applied to Top Surfaces

Figure 3.15: Boundary Conditions

48

model for analysis yielded the following results as depicted in Figure 3.16.

+1.34e+02
+1.23e+02
+1.12e+02
+1.00e+02
+8.95e+02
+7.84e+02
+6.72e+02
+5.65e+02
+4.48e+02
+3.36e+02
+2.24e+02
+1.13e+02

Figure 3.16: Resulting Failure

49

3.2.3

Trench simulation with 2 layer soil

Building the ABAQUS simulation for multi-soil trench began with defining the geometrical
shape of the trench. The objective was to bring in some realistic components of trench
excavation compared to the homogenous simulation in Section 3.2.1. Soft clay and very soft
clay were chosen as the two types of soil mediums to be simulated. Figure 3.17 shows the
dimensions of the trench, base width= 5.0 m, left bank = 2.0 m, trench width =1.0 m, right
bank= 2.0 m; extrude depth of 0.02m

2.0 m

2.0 m
2.0 m
1.0 m
4.0 m

Figure 3.17: Geometry for The Two Layered Soil Trench

The model was constructed using a single part and then partition into symmetrical sections as show in the Figure 3.17. The walls of the trench were considered to be soft clay
and the layer of soil acting as base of the trench was considered to be very soft clay. Both
different soils were considered to be un-drained and homogenous in nature for reasons of
simplicity.
The material properties assigned to the top layer of soil for Young’s Modulus =15MPa and
Poisson’s ratio of 0.20. The material properties assigned to bottom layer of soil for Young’s
modulus of elasticity =20MPa and Poisson’s ratio of 0.25. Section properties to both the
layers of soil were assigned separately. Boundary properties were assigned in a similar manner
as the previos simulations. 6 degrees of freedom using symmetry/ antisymmetry/ encastre
50

for the trench load step. The type of boundary conditions chosen is Encastre. The resulting
boundary conditions are shown in Figure 3.18.

Figure 3.18: Boundary Layer Conditions

This particular trench simulation does not need to be partitioned because the two divided
parts make it easy for meshing and does not cause any conflicts in structured meshing. The
boundary conditions fixes both parts of the trench walls and the and the base of the trench
along the same axis so as to allow for realistic boundary condtions.
The trench part becmoes ready for loading conditions. A uniform ramped pressure was
simulated to be applied at a value of 0.025MPa. As mentioned previously, the loads assumed
are to be indicative of surcharge loads on trench walls due to excavated soil placement within
0.48m of the trench walls and heavy machinery etc. The resulting Figure 3.19 shows the
loading conditions on the top surfaces and the boundary conditions that have been applied
to the trench section.
The trench part is now ready to be meshed. A similar mesh sequence as the homogenous
trench simulation Section 3.2.1 is adopted here.

51

Figure 3.19: Loading Conditions

The mesh is completed using quadratic reduced integration C3D20R hex element type.
The resulting meshed part is depicted in Figure 3.20.

Figure 3.20: Meshed Multi Soil Layer Trench

52

With the completed mesh the analysis starts. The results obtained are shown in Figure
3.21. The results of the simulations are discussed in the Analysis part of the thesis. The

+1.14e+04
+1.08e+04
+1.01e+04
+9.46e+03
+8.70e+03
+8.11e+03
+7.43e+03
+6.76e+03
+4.07e+03
+2.72e+03
+6.98e+02
+2.43e+01

Figure 3.21: Results of Soil failure

comparison of the model to realistic construction aproach is performed in the Chapter 4.

53

Chapter 4
Analysis of FEM Simulations
Chapter 4 includes the analysis of the FEM simulations performed for modelling trench
cave-ins. The results of the FEM simulations are compared to real situations of construction
practices involving trench excavation practices. To approach the problem ABAQUS based
FEM models were built to compare safe practices implemented in construction by modeling
trenches that are considered to be safe, such as, sloped trenches and benched trenches.

4.1

Analysis of Results of ABAQUS FEM Models Discussed in Research Methodology

In order to establish the reliability of the FE models as discussed in the Research Methodology the results of stress failures for the three different simulations are developed. The results
are displayed in the form of Von Mises’ graphs for the stress failure which helps invisualizing
the failures and can give insight to the different types of failure patterns that could occur in a
trench subject different surcharge, loading conditions, soil properties etc. In order to identify
54

the consistency of the results the researcher identified stress failure patterns and comparison
to be the most effective means to identify critical failure points or yielding patterns.
The method to compare such results was developed by checking the stress values at
certain nodes along the Y axis versus the distance along path. The distance implies the
actual distance along the path in the single coordinate direction specified. The graph results
prodcued as a part of the analysis helps identify critical loactions where stress intensity may
be unusually high or cracks developped and the flow of stresses around the cracks. There
are emergent trends which can be analyzed using such graphs as well.

55

The X values used in the XY data can be one of the
following:
True Distance: The actual distance (starting at Distance
zero) along the path in model space.
NormalizedDistance: The normalized distance along the path
(total path distance is normalized to 1).
Sequence ID: The order in which the data point n appears in
the path list.
X, Y, Z Distance : The actual distance (starting at Distance zero)
along the path in the single

d

0
0
0

1

0

n

0
0

0

dx

Figure 4.1: X Values Definition

The first model as designed in the FEM simulation section was trench with homogenous
soil conditions. The results as shown in Figure 4.2, are displayed in graphic contour of S,
Mises stress induced graphs. The S, Mises results are a derived from Von Mises stress; which
is a geometrical combination of all stresses acting at a particular location, in this case the
nodal elements. The various stresses included n such results usually are, normal stresses in
all three directions and all three shear stresses.

+4.77 e+04
+4.55e+04
+4.03e+04
+3.55e+04
+3.26e+04
+2.93e+04
+2.61e+04
+2.29e+04
+1.96e+04
+1.64e+04
+1.32e+04
+9.96e+03

Figure 4.2: Result of Stresses in Trench With Homogenous Soil

56

The results of the simulation are, as observed in the Figure 4.2, compression of the soil
would have resulted in a cave in, if decided to be dug without shoring systems or other
modes of trench protection such as backing or benching the trench. Results such as these
clearly show the significance of finite element simulation and the advantages of predicting
soil behavior prior to trenching. In order to understand the stress flow patterns in such
trench failure cases a stress output graph was generated.
A path was selected along the vertical axis of the trench along the front face of the trench.
The path was critically chosen since it follows along the nearest nodal points to the trench
wall. Apart from the trench edges themselves the chosen path would depict the highest stress
patterns as seen in Figure 4.2. Nodes are selected for analyzing the stress patterns stating
at node 157, 425, 235, 79, and ending at 193 as shown in the Figure 4.3.

152

193

Figure 4.3: Path Selected for Stress Plots

57

The stress versus distance along path yielded the following results as shown in Figure 4.4.
along the various nodes selected to measure the stress patterns, the graph relates perfectly
along to results seen in Figure 4.2 The maximum stress is experienced at the top surface
where the top pressure is exerted and the shearing action occurring owing to the typical
boundary conditions applied. The stress within the nodes of the trench takes a gradual dip
through the increasing depth of the trench. A huge dip in the stress is observed as the path
now moves into the base of the trench where stress pattern is going to be very low since the
top pressure is mostly going to affect the trench walls and not the bottom part of the trench.
Generating such graphs shed more light into detailed analysis of trenches where cracks
appear or can be predicted or in deep trenches to analyze how various types of soils encountered need to be treated, and to ascertain the type of shoring needs.

Stress (kpa/mm2)

Distance along path Versus Stress
50000
45000
50000
35000
30000
25000
20000
15000
10000
5000
0

14
12
8
6
4
10
Distance Along Path (mm)
Figure 4.4: Stress Versus Distance along Path
0

2

58

Results of the simulation run for trench with a crack growth phenomenon the results are
important to analyze in three different paths. This is owing to different stress flows through
the trench model. The Figure 4.5 shows the end result of a crack growth phenomenon in a
trench placed critically 0.12m away from the trench wall.

+1.34e+02
+1.23e+02
+1.12e+02
+1.00e+02
+8.95e+02
+7.84e+02
+6.72e+02
+5.65e+02
+4.48e+02
+3.36e+02
+2.24e+02
+1.13e+02

Figure 4.5: Trench Stress Results in Crack Trench Simulation

In order to understand the stress distributions across the trench, three different paths
were selected to analyze the failure occurring in the trench. This is important since stress
patterns can be different on the left side of the crack plane and the right side. Also there
can be a different stress pattern showing on the left trench wall section. The hypotheses can
be inferred from carefully analyzing the result of trench fatigue in Figure 4.5. However for
scientific understanding and verification of such a hypothesis the stress plot versus the true
distance along path was run.

59

The first stress pattern was plotted on the left side and away from the crack plane along
the nodes starting at 2221, and ending at 239 as shown in Figure 4.6 (a) and the second
stress pattern was plotted along the right side of the crack plane as shown in Figure 4.6 (b)
along the nodes start at 2321, and ending at 244.

2221

239

((a)) Node Selection Path 1

2321

244

((b)) Node Selection Path 2
Figure 4.6: Selected paths for Stress Plotting

60

Stress (kpa/mm2)

26
25.8
25.6
25.4
25.2
25
24.8
24.6
24.4

Distance along path Versus Stress

Distance Along path (mm)
5
10
25 30
15 20

35

((a)) Stress Result 1

Distance along path Versus Stress
Stress (kpa/mm2)

30
25
20
15
10
5
0

Distance Along path (mm)
5

10

15

20

25

30

35

((b)) Stress Result 2
Figure 4.7: Results of Stress Plots

The results of the stress plots along path A and B are shown in Figures 4.7 (a) and 4.7
(b) the graphs for these instances were created from bottom up of the trench section.The
approach was taken in order to capture the increase in stress patterns from bottom up of the
trench sections along crack propagation plane. Since the method of crack development was
modeled to be static a very interesting result was observed. In Figure 4.7(a) at the stress
was tending to 0 at the starting node 2221. As the path progressed to nodes above, stress
values shot up, and at then dipped suddenly. This pattern is noted as a result of the trench
section being split away from the main domain as a result of the crack propagation. The dip
in stress was noted owing to yielding of the soil. As the path progressed beyond the cracked
61

plane stress was noted to increase but at a lower level because the cracked plane still was
being exerted to some forces due to surcharge from top pressure loading.
In Figure 4.7(b) a very curious pattern was observed in the trench where the stresses
experienced were close to 0 at the bottom of the trench. However at the crack plane the
elements shared a part of the forces experienced by the cracking plane and forces from the
main domain of the trench. The wave pattern indicates the nodal elements experiencing
varied forces between the two bodies of the trench. However the wave pattern observed
was increasing in nature, which implied that although the different nodes were experiencing
varied forces; the subsequent nodes were experiencing a gradually incremented value of force.
This is consistent with the finding in the contour deformation results of the trench fatigue.
The third graph plotted was to understand the behavior trench on the left trench wall
section as shown in the Figure 4.8.
The findings of the stress plot in Figure 4.8 (b) is consistent with the previous findings
where the stress gradually increases from the bottom to the top of the trench. It is important
to realize the findings where the stresses experienced on a section of trench where crack
formation is found compared to the section where no cracks are developed. The cracks
are forced to exist in the trench. In realistic conditions, there can be two very different
soil conditions in existence on both walls of the trench. It can be important to model the
crack formation in the weaker and strong soils to compare the hazardous conditions that
may develop due to surcharge loading or change in moisture conditions. The trench section
also clearly portrays the phenomenon of bulging in the bottom section of the trench. Such
phenomenon is commonly found in realistic trenching situations. That is attributed to many
hazards and instances in trenching activities. The approach can help in protecting workers
62

123

440

((a)) Node List 3

Distance along path Versus Stress
30
25
20
15
10
5
0

Distance Along Path (mm)
0

5

10

15

20

25

30

35

((b)) Stress Result 3
Figure 4.8: Nodes Path Selected and Results of Stress Plotting

and keep excavations safer by the performing FEM simulations.

63

The third FEM simulation involving two different soil layers have the results as shown
in the Figure 4.9.

+1.14e+04
+1.08e+04
+1.01e+04
+9.46e+03
+8.70e+03
+8.11e+03
+7.43e+03
+6.76e+03
+4.07e+03
+2.72e+03
+6.98e+02
+2.43e+01

Figure 4.9: Fatigue In Trench with Two Layers of soil

The stress plots for this trench was performed starting at node 123 at the top of the
trench wall section moving towards the bottom ending at node 440. Figure 4.10 shows the
path for stress plot.
In the stress plot as seen in Figure 4.10, the stress reduces from top of the trench section
to the bottom of the trench section. There is a downward slope and a sudden upward
slope observed in the graph at the true distance of 0.55m along the selected path. This
was an unexpected emergent result of the simulation. The explanation of the case is that
the stresses at the dip abruptly between the nodes that act at the interaction layer of the
two soil elements. Since the bottom part of the trench is simulated to be of a stronger
material property than the top layer the transition of stresses from the top layer to bottom
layer diffuses some of the forces. The forces arising through the heaving action which is in
the negative direction of the path of stresses happens to be of a similar value at the layer
interactions and hence there is a cancellation of forces at the point. As the nodal path
64

157

221

((a)) Node List

Stress (kpa/mm2)

Distance along path Versus Stress
50000
45000
40000
35000
30000
25000
20000
15000
10000
5000
0 0

Distance Along Path (mm)
2

4

6

8

10

12

14

((b)) Stress Result
Figure 4.10: Nodes Path Selected and Results of Stress Plotting

continues downward an increase can be seen in stress compared to the immediate previous
elements.

65

4.2

Comparison of The Results to a Safe Practice of
Trench Excavation

To understand how efficient the current safe practices in construction, a model was created
which simulated a trenching for hard clay type of soil. OSHA recommends sloped excavation
as a safe practice for type B layered soils which have mechanical properties enduring the soil
to be excavated without shoring systems required if the trench walls are sloped. A simulation
that features a scenario that assuming a Type B soil which allows for a 1:1 slope for excavation
is constructed. The geometry and the meshed trench are shown in Figure 4.11.

2m

2m

5m

2m
12 m

((a)) Geometry for Sloped Trench

((b)) Meshed Trench Part
Figure 4.11: Geometry of Slope and Meshed Part

Similar soil properties, loading condition and boundary conditions as discussed in Re66

search Methodology Section has been applied in this simulation. The research focused on
homogenous soft clay soil properties with Young’s modulus of elasticity (Es)= 15kPa and
Poisson’s ratio of 0.25. The outer walls of the trench are chosen to be fixed by 6 degrees
of freedom using symmetry/ antisymmetry/ encastre for the trench load step. The type of
boundary conditions chosen is encastre.

Figure 4.12: Boundary Conditions and Loading Conditions

The loads assumed are indicative of surcharge loads on trench walls due to excavated soil
placement within 0.24m of the trench walls and heavy machinery etc. The load is applied
as a uniform stress at 12 kPa. The resulting mesh deformation is shown in Figure 4.13.

+1.58e+02
+1.29e+02
+1.00e+02
+2.71e+02
+2.41e+02
+2.12e+02
+1.82e+02
+1.51e+02
+1.24e+02
+9.61e+01
+6.60e+01
+7.41e+00

Figure 4.13: Fatigue Results in Trench Section

The results as seen in Figure 4.13, show that sloped excavation is safe however when the
same soil sample was treated to similar FEM parameters found considerable soil deformation
67

and fatigue were detected. The simulation correlates the finding of the research to be in
parallel with realistic conditions. However the simulation is considered only to be a proof of
concept since the soils have been considered to be homogenous in nature.
In order to understand the flow of stresses through the trench a path was selected starting
at node 2 and running toward the bottom of the trench ending at node 91. The results of
the stress plot and path selected to measure stress is shown below.

2

91

Stress (kpa/mm2)

((a)) Node Path Selected

Distance along path Versus Stress
300
250
200
150
100
50
0

2
6
4
8 10 12 14
Distance Along Path (mm)
((b)) Stress Plotted

Figure 4.14: Nodes Path Selected and Results of Stress Plotting

The results of the stress plot were nothing short of expected; the maximum stress is
observed at the top of trench wall and decreases gradually toward bottom of the trench.
Such simulations help construction safety managers and excavation crews to analyze the
method of excavation and the degree of safety that can be achieved with confidence. Such
68

confidence in building, construction, and excavation practices will help remove the risk and
much of the contingency preloaded in the bidding policies for most construction companies
helping them compete in the industry at a healthier level.

4.2.1

Sensitivity Analysis

As a part of the thesis, a sensitivity analysis was not conducted but it is mportant to note
that the strategy to perform the trench excavation FEM simulations would have some impact
on the type of results which can be achieved. Use of Plaxis has specialised geotech based
modules in the FEM software program, and this would not rely on use of Young’s Modulus
and Poisson’s Ratio for the soil modelling purposes. Since the simulations were assumed to be
modeled as continuum problems in Abaaqus CAE, the research was completed in the method
described in Chapter 3. The Sue of boundary conditions and type of elements would all be
different when considering different FEM software packages and modelling assumptions.

69

Chapter 5
Guidelines to Build The FEM Model
Due to the lack of availability of procedures to build the models in such programs a guideline
is built in the present section.This chapter discusses a guideline to build a trench model in
ABAQUS. It is aimed to build readily available modules that would contain most forms of
trenches that are constructed, and which is compatible with most commercially available
software programs. This will help trench excavation companies to implement FEM based
safety protocol on a regular basis. E-learning will be an apporpriate program to educate the
construction proffesionals of the proposed safety program with FEM.
The modules reduce the effort needed to build individual FEM trench models for each
project. However since construction work and site locations are unique there might be
instances where such modules would not be able to represent real conditions and the required
FEM model needs to be built individually.
It is focused on a commonly occurring hazardous condition in trench excavations, which
is the appearance of wall cracks and have laid out a guideline to model such instances.
Themodel is meant to be considered as only a framework for building more complex realistic
70

models that can present real world parameters accurately.
The process to create the trench is followed similar to the 3D edge crack for solids in
tutorial 3D Static Edge Crack (Pais, 2010). The procedure to build a 3D static Edge crack
in trench is laid down in the following parts:
1. Creating an un-cracked domain (trench)
2. Creating the crack domain
3. Creating the boundary conditions
4. Solving the system of equations

5.1

Creating the Un-cracked Domain
Creating Trench domain
Create Part

Mechanical Properties

Creating Crack domain
Create crack plane Specify XFEM interactions
Boundary Conditions and Loads
Boundary conditions and Submit model for analysis
specify loads
Figure 5.1: Outline of ABAQUS Model Building

The procedure to create the simulation for 3 D static edge Trench is shown in Figure 5.1

71

Design Steps:

a. Use ABAQUS/CAE 6.9 or later.
b. Create a new model as named TrenchCrack. Do not include spaces in naming configurations in ABAQUS as it runs into errors.
c. Now Select parts and enter the name as Trench, Modeling Space to be used is 3D,
type is deformable, base feature is solid and approximate size is 2. The size is kept small to
make the simulation run faster, as bigger models will take a longer time to run and will also
need a lot of system resources.
d. Using the drawing tool draw a trench section as shown in Figure 5.2, The left side of
the image with menu options of the image is only for visual reference only, small text is not
meant to be readable.

Figure 5.2: Geometry of Trench

e. Complete the drawing and enter depth 5 which is the third dimension of the trench
to be extruded.
72

f. For Materials and enter name as SoftClay. Provide Mechanical, Elasticity. Enter
Young’s modulus as 15MPa and Poisson’s ratio as 0.25. Enter Mechanical, then damage
for traction separation laws, then Maxps Damage. Enter a value of 5 MPa. From the Suboptions menu choose Damage Evolution. Now enter Displacement at Failure as 1 and exit
out of the options. Figure 5.3(a) shows the current stage in the process.The left side of the
image with menu options of the image is only for visual reference only, small text is not
meant to be readable.

((a)) Maxps and Mechancical Properties

((b)) Section Assignments
Figure 5.3: Trench Extrude and Meshing Results

g. In sections, enter the name of section as Main. Accept default settings. Select SoftClay
73

as material and check the box by Plane stress/strain thickness and enter 1 as thickness. In
the parts menu expand the TrenchCrack and double click on section assignments. Select
the domain and exit by accepting default settings. The resulting Figure in 5.3(b) should
represent the 3D trench at currently.
h. To avoid complications in the meshing sequence partition is needed in the trench at
the joints of the base and the walls as shown in the Figure 5.4(a). The left side of the image
with menu options of the image is only for visual reference only, small text is not meant to
be readable.

((a)) Partitioned Trench Domain

((b)) Meshed Domain

Figure 5.4: Trench Extrude and Meshing Results

i. Expand the parts and expanding the TrenchCrack exposes the Mesh menu. Select
mesh and from the menu bar select controls choose Hex, structured. From the menu bar
select mesh and the part. The meshes are developed in the part as shown Figure 5.4(b).
The left side of the image with menu options of the image is only for visual reference only,
small text is not meant to be readable.

74

5.2

Creating The Cracked Domain

a. To create the crack, make a new part. For Parts enter name as Crack, select modeling
space as 3D planar and type is deformable, base feature is shell, type is extrusion and
approximate size is 2.
b. Draw a line from coordinates (8,5) to (8,3) and click done and extrude for the same
depth as the trench which is 0.5m. This generates a 3D plane along the depth of the trench
on the right wall.
c. Expand the assembly menu and double click on instances. Select crack and accept
default settings. The Figure 5.5 depicts the current stage of the assembly process. The left
side of the image with menu options of the image is only for visual reference only, small text
is not meant to be readable.

Figure 5.5: Cranck Instance

d. The next step in the simulation is perhaps the most important step. It is necessary to
specify the behavior of the crack with the domain of the trench. From the menu bar select
75

special, crack and select create. Name the interaction as EdgeCrack. Select type as XFEM.
Select the uncracked domain as the Crack Domain. As shown in the Figure 5.6 on the menu
which appears, specify the crack location by clicking on the line signifying the crack and exit
the options. The left side of the image with menu options of the image is only for visual
reference only, small text is not meant to be readable.

Figure 5.6: Crack Interaction

e. For Interactions and name the module as Growth. Select initial step and types for
selected step as XFEM crack growth. XFEM Crack should have EdgeCrack and exit the
options.

76

5.3

Creating the Boundary Conditions and Loads

The following steps helps in adding the necessary boundary conditions in ABAQUS:

1. Enter name as Loads and accept default settings.

2. From menu bar and enter the name as TopLoads and select the mechanical from the
categories, type is pressure. Select the top surface of the domain as shown in the Figure
5.7 and enter magnitude of 0.025MPa and exit by accepting default settings. The left
side of the image with menu options of the image is only for visual reference only, small
text is not meant to be readable.

Figure 5.7: Loading Condtions

3. To create boundary conditions choose the outer walls of the trench (side1, side2, side
3) to be fixed by 6 degrees of freedom using symmetry/ antisymmetry/ encastre for
the trench load step. The type of boundary conditions chosen is encastre. The Figure
5.8(a) shows how the boundaries domains are selected and the resulting boundary
77

conditions as applied as shown in Figure 5.8(b). The left side of the image with menu
options of the image is only for visual reference only, small text is not meant to be
readable.

78

((a)) Selected Boundaries

((b)) Assigned Bounadries
Figure 5.8: Boundary Conddtions

4. To create the required outputs we first expand field output requests, choose F-Output1. Expand the failure/fracture options and check the box next to PHILSM, level set
value phi. This will allow to view the level set function defining the crack.

79

5.4

Solving the System of Equations

1. From jobs menu on the right hand side menu bar, enter the name as EdgeCrack1. Exit
by accepting default settings.
2. EdgeCrack1 allows to submit for analysis.
3. Check the results of the analysis. Figure 5.9 represents the results of the simulation.
The left side of the image with menu options of the image is only for visual reference
only, small text is not meant to be readable.

Figure 5.9: Results of Crack Growth in Trench

80

5.5

E-Leanring

The literature review established that E-learning is an effective tool which can be adopted
for training purposes of the trench excavation crew memebers and safety managers and
various construction population. It is ideal owing to the diversity of the people from different
backgrounds. To make the guidelines laid out in the thesis available as an E-learning tool,
further work will need to be carried out to place animated videos of step-by-step process of
designing trench excavation simualtions.

81

Chapter 6
Systems Dynamics Model: To Help
Predict Company Behavior Towards
New Safety Protocol
Chapter 6 details the methodology approached to build a systems dynamics (SD) model
using the Vensim PLE research software. The goal of using systems dynamics as a part of
the thesis is to establish the outcome of using FEM as a safety program for construction
companies focused in trench excavation. The simulation aims to help management level
decision for adoption of the FEM safety protocol by highlighting the difference between the
current protocols and predicting a future trend if the FEM safety program is implemented.
The data collection, methodology and analysis of results of the model are discussed in the
chapter 6. The model is presented as a proof of concept to show that systems dynamics would
be ideal to test the impact of adopting the FEM based safety program in the construction
industry.
82

6.1

Model Development

The systems dynamics model adopted is a modification of the simulator program developed
by Owaba and Adebayi, (2006). The simulator program developed by Owaba and Adebayi,
(2006) served to predict the performance of a manufacturing safety program. The Table
6.1 represents the findings by CONN-OSHA for the reported incidents related to trench
excavations between the years 2003-2005 (Rocheleau, 2007).

Table 6.1: U.S Excavation and trenching injuries and fatalities (Rocheleau, 2007)
Year
2003
2004
2005

Fatal
48
41
44

Non-Fatal
310
100
120

Totals
358
141
164

In order to build the model some of the assumptions made were:
Time lag (T) needed for the learning curve and delay in adoption rate of the new safety
program (SP) is neglected because the data used for the model was for short period of time
of three years. The number of accidents prior to the new SP is the average of the sum of
fatal and non fatal injuries from Table 6.1. The total man hours (q) worked assumes a 10
crew team working for 3 years. An average estimated value of $10,000 per year is assumed to
be invested by the company on implementation of safety programs. The difference between
pre-safety accident level and prevented accident at any instant of time step (t) defines the
potential accident varaible (GL). The accident proneness factor (f) was estimated from total
man hours of work force, average number of accidents before SP as 0.01156. From common
practice in the industry an average propotion of 0.8 of acutal budget was estimated to be
implemented for annual SP.
83

6.2

Model Equations

Some of the equations used to develop the safety simulator program are shown below. The
parameter sued in the equations are explained in the Table 6.2. The Figure 6.1 and 6.2
represent the flow of the systems of equations. For Figure 6.2, the text within the image is
for visual reference with respect to the working of equations.

GL = Xp − Y t

λ=

1
Xp

(6.1)

(6.2)

F = qf − Y t

(6.3)

αt = (GL − X) ∗ ρ1 ∗ h

(6.4)

ρ2 = 1 − ρ1

(6.5)

R−Yt
ρ2

(6.6)

γt =

84

Table 6.2: Equation variables and the description
Symbol
Pk
Bk
µ
R
γ
Yt
Xt
αt
F
Xp
q
f
ρ1
λ
h
GL
BT

Description
Proportion of planned budget actually expended on training
Actual budget expended on training
SP activities effectiveness factor
Prevention goal
Prevention rate
Number of accidents prevented
number of accidents caused
Accident causing rate
Hazardous condition
Potential Accidents
Total man hours of work
accident proneness factor
Probability of accidents
Probability of accident caused parameter
Accident causing factor
Potential accidents
Total budget actually expended on the program

85

Budgeting
Budgeting
Factor
Factor
(Planned
Budget)

(Accident
Rate)

The Company Performace to Adoption of F E M
to Adoption of FEM
The Company Performace
based Trench SafetyProgram
based Trench Safety Program
Proportion Of
Proportion Of
Prevension
planned Budget
plannedBudget
Safety Programme
Safety Programme
Time
Prevension Time
Effectiveness factor
Effectiveness factor
LagTT
Lag
Planned
Budget

Actual Budget
Actual Budget

k A
Target
Target

Pre-Safety
Pre-Safety
Accidents
Accidents
Potential
Potential
Accidents
Accident
Accidents
Accident
caused
caused
Prevention rate
Prevention rate
Probability
Probability

Accident Causing Factor
Accident Causing Factor
obtained
obtained fromfrom Company
Company

PievMilive
ate

(Preventive Rate)

ccident Rate

Accident Proneness
Accident
prevention rate
Prevention rate
Proneness Factor
factor
Probability of
Probability parameter
Probability of Probability parameter
accident
, accident
Accident rate 4
Accident rate
Trench HazardousProbability factor
Probability factor
Condition
(Trench Hazardous
Condition)
Total Manhours of
Total Manhours of
Workforce
Workforce

Figure
Program
Figure 6.1: Trench Safety Program Simulator
Figure 6.1: Trench Safety Program Simulator

86
88

Prevented
accidents

The Company Performace to Adoption of FEM based Trench Safety Program
Proportion Of
planned Budget
Safety Programme
Effectiveness factor

Budgeting Factor

ft =

GL = Xp — Yt

R-Yt

^Planned Budget

s
Prevension Time
Lag T

at = (GL — X) * pi * h
p2 = l - p l
Potential

^-Accident
Preventive Rate
ccident caused

revenaon rate
Probability

A
ccident Rate

A =
prevention rate
Probability parameter

o

F= qf-Yt
Accident
Proneness Factor

Accident Causing Factor
obtained from Company

Trench Hazardous
Condition

Probabilit\* of
accident

Accident rate
Probabilitv factor

Total Manhours
Workforce

Figure 6.2: Trench Safety Program Simulator. This figure is not meant to be readable, but is for visual
Figure 6.1: Trench Safetyonly.
reference Program Simulator

89
87

6.3

Analysis of The Results of FEM Simulation Model

The results of the data are compared by plotting the trends of accidents prevented versus
the accidents caused during the implementation of the new safety program. Figure 6.3 shows
the results of the analysis.

Prevented v/s Caused

4 Number
20 Number
3 Number
15 Number

Accident caused: Current

3 Number
10 Number
Prevented Accidents : Current
1 Number
5 Number
0 Number
0 Number

0

2 4

Time (Month)
6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

Figure 6.3: Prevented Accidents versus Accidents Caused

The simulation was run for 36 months of simulation time. This time limitation was
imposed to keep the results in check with reality because of the limited amount of data used
for the simulation. The data acquired as a part of literature review showed the number of
injuries and fatalities occurred between the years 2003 through 2005. The national average
of incidents in the recent year as reported by BLS (2010), states a number of 20 incidents
per year on an average as the incident rates for trench excavation. The results as seen in
Figure 6.2 shows us a trend of accidents caused versus the prevented accidents in numbers.
The accident caused, as initialized to 1 for realistic purposes, increased to maximum of 2.45
at the end of 36 months, while the curve showing the number accidents prevented, initialized
88

at 0, rose to 15 and stayed constant at the end of 36 months. While such numbers do not
reflect realistic accident caused and prevented numbers within the construction industry, they
however represent the expected prevention rates and decline in accidents that could occur
by using a new and realistic safety protocol. The trench hazardous conditions as simulated
by the program show a great improvement by implementing the new safety protocols given
in Figure 6.3.

Trench Hazardous Condition
20
15

10
5
Trench Hazardous Condition : Current

0
0

2 4

6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36
Figure 6.4: Trench hazard Condition

89

6.4

Limitations of Safety Program Simulator

An effective use of systems dynamics is given to explore the impacts of using a new safety
program for a construction company. This model was developed to understand the impact of
safety in a single company. The model does not include the dynamics of cost versus market
competition trends etc. This is one of the major concerns for construction companies as
in the market competition this is one of the major causes for lack of funding imported to
safety programs. Due to lack of data for many parameters such as the total man hours
worked for 36 months within a company and average estimate of the crew size of 5 people
was assumed to work full time for the time period of simulation. The budget for safety
program was estimated from interviews with several trenching companies. These companies
were reluctant to be interviewed formally for the present research purposes.

90

Chapter 7
Conclusions
Chapter 7 gives the conclusion regarding the goals and objectives of thesis. The summary
of results and significance of the various FEM simulations and the importance of simulation
in construction are also identified in the present chapter. Recommendations, limitations of
research are also summarized in the chapter.
As given in Chapter 2 involving the comprehensive literature review, the need for a new
and predictive method to improve safety in trench excavation projects is stated. The element
of unpredictability leads to increased costs of construction and to avoid losses in profits most
small sized excavation contractors tend to avoid the required safety measures and have led
to highest incident rates in construction industry. Although an established OSHA protocol
for trench safety is enough to reduce the incident rates, it is found that the protocol does not
address the adverse affects of the protocol for the trenching, and there continues to occur a
significant number of injuries and fatalities today. Every time such accidents occur, it puts
the construction companies in grave risks of bankruptcy.
The literature review identifies the strength of FEM to predict the behavior of soils of
91

all types. The various aspects needed to successfully model the trench and its associated
environment were identified as the type of soil, the mechanical properties of the soil, the
moisture content and how they affect the mechanical properties, surcharge pressures, loads
imposed on trenches by machinery placed close to trench walls, boundary conditions of the
trench and the geometrical shape of the trench itself.
Three different trenches were modeled as discussed in the Research Methodology section
of the thesis. Trench with homogenous soil study was modeled to show the response of
trench to loads close to the edge of trench walls. The trend of soil cave in as observed in
the results of the simulation coincided well with the expectations of the researcher. The
model was however not suited visualize the cave-in aspect of trenches, and hence the second
simulation aimed at resolving that issue. The 3Dedge crack simulation built to show the
cave-in occurrence in trenches was a successful demonstration of a trench consisting of a
homogenous soft clay material failing due to over loaded trench walls due to surcharge or
soil placement close to trench walls. Finite element analysis performed with ABAQUS of
a scenario involving two layers of soils was lastly presented in Chapter 3. The simulation
introduced a new level of complexity to previous models.
In order to model a realistic trench with multiple types of soil and all of its complexity
a much more comprehensive soil study needs to be performed. The use of ABAQUS CAE
FEM program yields promising results as discussed in the Chapter 4. Chapter 5 enlists the
a guideline to build a 3-D edge crack formation in a homogenous trench.

92

A New safety program is proposed in order to incorporate FEM into existing process of
trench excavation; the steps are shown below:

a. Bore log data collection
b. Investigate the surroundings of the trench to be dug
c. Identify the major factors which could cause instability of the trench walls such as:
Vibration due to existing rail road; Unpredictable weather conditions, etc.
d. Model these parameters in FEM and identify the requirements for a safe trench
excavation
e. If cave-in is predicted use trench box. If not required avoid excessive costs Proceed
with excavation
SD model was adopted in the present thesis to predict the impact of adopting a new
safety program. For making decision, to make a major change in safety protocol in major and
minor companies, it usually takes time and evidence of the working nature of the protocol.
To establish further confidence in the use of the FEM based protocol a systems dynamics
approach was chosen as a suitable option to address such concerns and thereby reducing the
time involved in proving the effectiveness of finite element based safety protocol. The SD
model captured the current safety program and its effectiveness and also took into account
the reliability of FEM predictions to influence a high accident prevention variable in the
model.
The model was suitable to the present thesis because it captured a number of soft variables
and also introduced noise levels into the model to yield as close to realistic results as possible.
The results of the SD model simulation proved to be very helpful in understanding how the
93

new protocol would affect a trench excavation company should they choose to adopt the
safety program. The simulation yielded results for a run of 36 months where only 3 accidents
were caused in the entire span and 16 were prevented. The costs of required budget for the
safety program was also given due importance and the results as observed were from a very
modest planned budget for the safety programs. The increase in budgeting factor does not
yield any significant difference in the results. This observation is important to be noted
for companies looking to invest very little and expecting a change in safety and build more
confidence within the company and all its workers.
The following conclusions are drawn from the work described in the Thesis:
1. The implementation of FEM to simulate trench excavation problems can be done
quickly with relatively higher confidence.
2. Finite element analysis and instrumentation that helps in monitoring the excavations
will be an ideal tool to improve construction safety standards.
3. More detailed soil study prior to excavation and continuous monitoring of soil conditions are highly important in finite element simulations.
4. Systems dynamics models help in predicting behavior for change in company protocol
over a length of time yielding realistic trends.
5. Adopting a FEM based safety protocol would set a higher standard of technology in
construction and help make the industry safer significantly also improving chances for small
sized construction companies to last longer in the industry.

94

Recommendations for further research in trench excavation finite element simulations are
as follows:
1. A more comprehensive real life trench should be dug and simulated using FEM.
2. The bore log details of the soils to be encountered should be accurately investigated.
3. The use of digital sensors to measure the soil stress conditions in real time should be
implemented to validate the FEM model and its accuracy.
4. The future research can focus on modeling impact thermal factors and vibrations
which could cause significant damage to trench stability over time.
5. Dynamic loading can be accurately modeled assuming moving equipment adjacent to
trench walls.

95

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