1|

H

1

101
122

 

.THS_

SHEfiAR STRENGTH Q? A COMPACT'ED glL‘E’Y CLAY

Thesis for tin Degrea oi: M. S.
MECEEMN STAE‘E UNEVERSE'E’Y
Anthony Frank Aveiiano
1959

 

This is to certifg that the

thesis entitled

SHEAR STRENGTH OF A COMPACTED SILTY CLAY

' presented by

ANTED NY FRANK AVELLAICD

has been accepted towards fulfillment
of the requirements for

I'lASTEI—t 0F scmm: degree in CIVIL ENGINEERING

W

Major professor

 

Date May 13, 1959

0-169

_-‘ m

LIB RA R Y I" .
Michigan Stab:
University

  
  

 

SHEAR STRENGTH OF A COMPACTED SILTY CLAY

by

Anthony Frank Avellano

AN ABSTRACT

Submitted to the College of Engineering
Michigan State University of Agriculture and
Applied'Science in partial fulfillment of
the requirements for the degree of

MASTER OF SCIENCE

Department of Civil Engineering

. ,/ .
Approved: w'wfl.,££4>41,"

 

ANTHONY FRANK AVELLANO ABSTRACT

This thesis is an investigation of the nature of shear
failure of a compacted silty clay.

The investigation consists of the determination of
the true angle of internal friction, an analysis of the
interaction of the friction and cohesive components of
shear strength, and a study of the deformation character-
istics of the material.

Consolidated undrained (CU) triaxial tests with
porewater pressure measurements were used to measure the
shear strength. Triaxial creep tests were made to determine
deformation characteristics.

It was found that there is a point of incipient
failure at which the shearing stress equals the frictional
resistance of the soil 5% tan ge. This point occurs at
the maximum positive porewater pressure. At lower shear
stresses the measured pore water pressure agrees with that
computed on the basis of elasticity. At higher stresses,
the pore pressures are influenced by the displacement
of particles along the failure plane. It was also concluded
that cohesion becomes stressed at incipient failure.

Excessive deformation occurs when cohesion is stressed.

SHEAR STRENGTH OF A COMPACTED SILTY CLAY

by

Anthony Frank Avellano

A THESIS

Submitted to the College of Engineering
Michigan State University of Agriculture and
Applied Science in partial fulfillment of
the requirements for the degree of

MASTER OF SCIENCE

Department of Civil Engineering

1959

ACKNOWLEDGMENT

The author wishes to express his indebtedness and
deep gratitude to Dr. T. H. Wu, Department of Civil
Engineering, Michigan State University, without whose
generous help and encouragement this thesis could not
have been written.

The assistance of the National Science Foundation
is also acknowledged for granting funds for this investi-

gation.

ii

 

 

 

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TABLE OF CONTENTS

ACKNOWLEDGMENTS.

LIST OF FIGURES.

LIST OF TABLES

Chapter
I.
II.
III.
IV.

V.

DEVELOPMENT OF CURRENT KNOWLEDGE
THEORY .

METHOD OF INVESTIGATION

RESULTS.

CONCLUSIONS

APPENDICES--Figures

Tables.

BIBLIOGRAPHY.

iii

Page
ii

iv

vi

Figure

\OCDNONUl-twm

11.

12.

13.

14.

15.
l6.
17.
18.
19.

LIST OF FIGURES
7: vs ER on the Failure Plane.
Element Under Strain
Grain Size Curve.
Graphical Solution for fie.
Triaiial Unit for CU Tests
Schematic Diagram of Triaxial Cell.
Mohr's Envelope for 2333 psi Loess.
Mohr‘s Envelope for 333 psi Loess
Mohr‘s Envelope for 1333 psi Loess.
Void ratio cf vs 55;

True Phi Determination.

Stress-Strain and Pore Pressure-Strain
Mississippi Loess 333 psi

Stress-Strain and Pore Pressure-Strain
Mississippi Loess 1333 psi.

Stress-Strain and Pore Pressure-Strain
Mississippi Loess 2933 psi.

F: vs 35 Mississippi Loess 333 psi
7: vs 55 Mississippi Loess 1033 psi.

7“ vs 5% Mississippi Loess 2030 psi.

Unconfined Compression Test 333 psi
Unconfined Compression Test 1333 psi

Creep Curve

iv

Page
23
23
21
22
23
23
24
24
25
26
27

28

Figure Page

21. Creep Curve 1333 psi. . . . . . . . . 36
22. Consolidation Curve 333 psi . . . . . . 37
23. Consolidation Curve 1333 psi . . . . . . 38

Table

JI'UJD.)

\fi

\10\

LIST OF TABLES

Soil Index Properties

Mississippi
Mississippi
Mississippi
Mississippi

Mississippi

Loess
Loess
Loess
Loess

Loess

333 psi CU Tests.
I333 psi CU Tests
2333 psi CU Tests
333 psi Creep Tests.

1333 psi Creep Tests

Calculated and Measured Values of Pore
Pressure Up at Maximum + U for Mississippi
Loess 303 psi .

Calculated and Measured Values of Pore
Pressure UD at Maximum + U for Mississippi
Loess 1333 psi.

vi

Page
39
A3
41
A2
43
43

AA

AA

ERRATA SHEET

Chapter I

l. P.l, line 2, Coulcomb should read Coulomb.
Line A relatifs should read relatif.

2. P.2, par. A, line 2 and par. 5 last line,
Ruthledge should read Rutledge.

Chapter II

I. P. 7, line 1 modulus should read modui.
Equation (111 N (6, - 63,) should read N (6. - 55

Equation (12) ;£%3L__ should read .14211
2.
Equation (13) -— N61 1— MM,’ should read - Ne. t N6,
‘ *- T r: '2"

respectively.
Chapter III
1. P. 9, line 1, studies should be studied.

2. P. 10, line a, killograms should read kilograms.
Last line #00 should be 300.

P. 11, par. 2, line 1, diped should be dipped.

A. P. 12, par. 2, line 1, thest should be these.
Line 9 respresnt should be represent.

5. P. 13, par. 3, line 1, Tergaphi should read
Terzaghi.

6. P. 15, par. 2, line 2, diped should read dipped.
Bibliography

1. No. 5, Ruthledge should be Rutledge, P. C.

I. DEVELOPMENT OF CURRENT KNOWLEDGE

The history of the shear strength theory of soils
dates back over a century and a half to 1773 when Coulcomb
wrote his essay "Essai sur une application des regles de
Maximis et Minimis a'quelques problems de Statique, relatifs
a' 1' Architecture." In this paper, he expressed the
classic equation

S = C + (T'tan fl, [1]
in which C is the cohesion, and cr"tan Z a frictional
resistance proportional to the normal pressure on the plane
considered. Although the shear strength equation is simple
in appearance, the determination of the parameters C and 3
in a cohesive soil is a delicate and trying problem.

In 1937, after years or experimentaion, Hvorslev (l)*
introduced the modified Coulcomb equation

S = Ce +-( a: - u) tan 2e [2]

where S shear strength

Ce = true cohesion

2e = angle of true friction

0? = total normal stress on the failure plane
u = porewater pressure
(on - u) = 5% , the effective normal stress on the failure
plane

 

*Numbers in parentheses indicate reference listed in
Bibliography.
1

Hvorslev also concluded that true cohesion was a function
of the water content. These two criteria of true cohesion
and true friction were profound advances in the under-
standing of the fundamental strength properties of soils.

Rendulic 1937 (2) made the first attempt to measure
porewater pressures occurring in a triaxial test. He also
showed that the void ratio depends upon the deviator and
hydrostatic stresses in a test specimen.

Simultaneously, Hvorslev investigated rapid shearing
in soils, and found that a negative pore pressure may be
developed, apparently increasing the shearing resistance.(3)

As rapid advancements in the understanding of shear
strength were made in Europe, work on triaxial apparatus and
shear problems were brilliantly carried out in the United
States by Jurgenson and A. Casagrande. Casagrande was
influential in determining the effective pressures on
specimens in the undrained triaxial test. (4)

Climaxing the Corps of Engineers' Soil Mechanics Fact
Finding Survey on Shear Strength (1939-l9u7), Ruthledge
prepared a review of the results obtained in the survey.

Of paramount importance was his finding that the shear
strength of a saturated soil depends only upon the water
content at failure, being independent of the confining
pressure47—3, porewater pressure or the method of testing.(5)

In 1955, G. A. Leonards (6) approached the shear

Strength problem in a manner very similar to Ruthledge's.

He found that for a given set of initial conditions, the
relationship between compressive strength and void ratio
at failure is unique, regardless of the confining pressure,
drainage, water content or method of testing.
In the Coulomb-Hvorslev equation S = Ce +- 5% tXHIZE,

5%" is calculated by (0? - u). It was Just a short
period of time before a mathematical expression was derived
to solve for 5% and the pore pressure u,in terms of the
applied stresses in an undrained triaxial test. In 1948,
Skempton (7) (8) developed his 7K theory for saturated
normally loaded clays, based on the assumption that the
soil is an elastic material. He also developed an expres-
sion relating the pore pressure to the deviator stress and

7k . The term 7k was introduced as a ratio of the

expansibility of the soil to its compressibility. ;1_varies

 

 

Cs
from .5——+ 3, and is expressed by A. = C . The pore
c
pressure U3 is equal to'
_» D
1+27L [31*

where D is the deviator stress.

As determination of pore pressure is so vital in the
analysis of effective stresses, Bishop and Henkel (1953)(9)
made further studies of the problem. They found, as did
Hvorslev, that a negative pore pressure will result in a

preconsolidated clay specimen sheared in the consolidated

 

*-
See derivation on pages 6 and 7.

 

undrained test. This negative pore pressure developed
during shear, remained even after the load was removed,
causing the soil to absorb water and subsequently fail.
Bishop and Henkel explain the negative pore pressure as
being caused by dilatancy (expansion of the soil when
sheared due to particle movement along the failure plane).

The tendency to undergo volume change during shear
develops an additional pore pressure U; . u% can
be expressed using the ‘7e theory and shear deformations
by the equation

3 PD

U" = - ______ 4 *
D 1+27k [ 1

I
where P = 35, N , N being a constant and E, the principle

2 Cc '
vertical strain. Knowing U5 , the shear equation for

I

saturated dilatant soils becomes

S = Ce + (Gi- u) tan as = Ce + [‘3 -'( U5 + U3 )] ta? pe.
5

The triaxial test only measures UD , but using the 7.
theory U3 and U2 may readily be found.

Finally the pore pressure in a partially saturated
soil must be considered. J. W. Hilf has analyzed the
pressure in air and water contained in the voids of a soil
in the undrained test. (13) The simplified equation for
porewater pressure in a partially saturated soil can be

expressed as

Va
U = C D+ —-————-—
D C {Pa-ucluc [5]

CC + 2CS + Va
Pa-uC

 

 

 

*See derivation on page 7.

atmospheric pressure

where Pa
Va = volume of air/unit volume of soil after
application of D
u = Capillary pressure between grains, varies
from 1/2 Pa——+3 " [11]

Shear strength has thus far been regarded as the sum
of Ce and 5% tan as. Retrogressing to 1948, A. W. Skempton
(12) performed extensive field investigations on saturated
impermeable clays using the as = 3 analysis. This interpre-
tation of S = C, may well suffice for clays having little
or no drainage.

Skempton found that the ze = O method obtained good
agreement between the computed and measured factors of
safety, but the failure plane in the field did not agree
with that calculated.

Opinions vary as to the interaction of the shear
strength components. One hypothesis is that of P. W. Rowe
(13). Rowe postulated that when a shear stress is applied
to a soil, it is first resisted by the frictional component.
The cohesive component is brought into action only after
the stress exceeds the frictional part. Through circum-
stantial evidence, Rowe concluded that any shear stress
applied to true cohesion results in creep or progressive
deformation. In other words, equilibrium of a soil mass
is attained only if the applied stress is resisted by the

true friction.

II. THEORY

Incipient Failure

The purpose of this investigation is to study the
shear strength characteristics and the behavior of the
material under stress. One may expect the behavior of
the porewater pressure and effective stress to undergo
considerable change as a soil specimen is stressed to
failure. From Rowe's hypothesis, it seems likely that at
some point the shear stress equals the frictional component,
and further increase in the stress mobilizes cohesion.
This may be called the point of incipient failure.

The shear and normal stresses on the failure plane
may be examined from a plot of 71 and 01;, . See Figure 1.
It is seen that the curve crosses the true O line at point A.

After point A is reached, cohesion is mobilized. Previously

the shear is resisted entirely by the frictional component.

Porewater Pressure

If an elastic soil in a CU test is subjected to a
deviator stress D and hydrostatic stress 65' , the principle

strains may be expressed as follows: See Figure 2.

5' = W _ 2%.. ,0; = DEM. axe—Lla— [7]
/ E: E3

5’: M + (was) A = —/re 2E2; —|-/¢45 UE—B
’ Ec E

S

6

where ES and EC are the modulus of expansion and compression,
and,/43 and ,A% are Poissons' ratios for expansion and

compression,respectively.

Also (Lg/fie.) = CS and @‘E/j‘) = CC [8]

where CS and CC are the expansibility and compressibility,

respectively.

In the consolidated undrained test A\/ = 3 for a

saturated soil, so
I I

6, =-—2<.i. [9]
We may combine 7, 8, and 9 and solve for the pore pressure

Us

US __. D = D CC [10]
1+2 R. CC + 2 CS

 

Equation 10 is based on the assumption that the
Inaterial is elastic, and that no volume change occurs
when shear deformation is produced. If the soil tends to

undergo volume change during shear, this volume change can

be expressed by

34€=NJ‘ = N(€,-€3) [II]
where A E is the normal strain, /' the shear deformation,
and N a constant. If equation 9 is substituted into
equation 11, then

3 A6 = NI‘= ’32" N [12]

The total principle strains become

6,: E,’ ‘Aé = D-Uo + Z/fisuo -- flé,
E E 2. [13]

c J

I

6:: 63‘ ‘36 = '?4&(Z%%E@) “(71743)1é%.+yéi
. _, Z-

Combining 13 with 8 and 9,

 

 

C
U = c (pl-3P) D D -— 3 PD 1A
0 cc + 2 CS or 'IT2 71" ‘I+‘2‘ 71 [ 1
where P = 3 6:;N
2 cC

Equation 1A is seen to consist of two parts. The first
part represents the porewater pressure in an elastic
material, and the second part the additional porewater
pressure due to the tendency of the soil to undergo a volume
change. Since the volume change is brought about by relative
displacement of particles along the failure plane, it seems
that the second part becomes important only at large strains
near failure. Therefore, at low strains the pore pressure
may be computed by the expression UA = .2... As

l+221

stress increases, soil particles are displaced along the

failure plane and cause a decrease in pore pressure by the

- 3 P D
1 + 2%.
the point of incipient failure. This point may possibly

amount Particle movement is initiated at

coincide with the maximum porewater pressure.

gases

A further objective of the investigation is to study
the deformation characteristics of the soil under slow
loading. Rowe's hypothesis states that after true cohesion
is mobilized, the soil undergoes excessive progressive
deformation at constant stress. This phenomenon is called
creep, and occurs after shear stress exceeds the frictional

resistance.

III. METHOD OF INVESTIGATION

Soil Studied
The soil studies was Mississippi loess . Mississippi

loess is a Pleistocene Aeolian deposit found along the east
bank of the Mississippi River, extending the entire length
of the state. This particular soil came from Vicksburg,
Mississippi.

In its natural state, loess is a calcareous clayey
silt containing a variety of fresh water and land shells.
Loess is light buff in color and rather fine in texture to
the touch. The index properties of the loess are given in

Table 1 and Figure 3.

Preparation of Soil Specimens

 

The soil was received in a disturbed condition with
most of the natural water content retained. It was put
into an air tight metal container and stored until ready
for use. The natural water content remained substantially
the same during storage.

It is very important to produce quality specimens
for triaxial testing. This criterion demands a uniform
distribution of soil particles, moisture, and void ratio.
These properties were obtained by a tedious process of

hand grinding the soil in a commercial meat grinder and

 

10

then mixing the material in a 12 quart mechanical mixer for
lO--15 minutes at a moisture content of about 25%. The soil
was thoroughly mixed until a homogeneous substance was
obtained. Approximately five killograms of soil were

ground and mixed to make one batch of specimens. The
preparation procedure resulted in a very satisfactory soil
mix.

To obtain a constant void ratio throughout the length
of a specimen, it is essential that the compactive effort
be uniformly distributed. A CBR mould (6" diameter and
8" high) was used to contain the soil.

A fine copper screen was placed on the bottom of the
mould to facilitate drainage during compaction and give
the soil a smooth surface. Approximately 4-1/2 inches of
soil were placed in the mould in four increments, each
layer being kneaded with a rubber tamper. A second copper
screen was placed on top of the soil cake.

A 2” high aluminum compacting piston was used having
a diameter l/'" smaller than the CBR mould, thus eliminating
the friction between the wall and piston.

The mould was then statically compacted in a 60,000
lb. capacity Tinus-Olson testing machine. The rate of
loading was applied at approximately 2% of the total load
per minute, and held at the desired value for ten minutes
by the automatic load holder. Removal of the stress was
instantaneous. Compaction pressure of MOO, 1003, and 2000

psi were used.

11

After a 2A hr. period in a 133% humidity moisture

romn,tfim soil was extracted from the mould yielding a cake

roughly 6" x 3-1/8". The soil cake was cut into six speci-

mens with a coping saw. Steel plates and a "C" clamp were

used to restrict movement while cutting. This procedure

was laborious, but proved satisfactory in obtaining uniform

undisturbed specimens.

Each specimen was diped in wax, placed in a sealed

then stored in the moisture room for a period of

bottle,
10 days. This was done to reduce the effects of thixotropy.
Seed and Chan (1A) showed that Mississippi loess

increases in strength with prolonged storage time. The

greater part of the thixotropic strength increase was found

to occur during the first 13 days of storage. The time

interval between the testing of the first and the last

specimen of a batch was about five to six days. Thus, a

period of 10 to 15 days of storage was incorporated to

minimize the effect of thixotropy.
All test specimens were 2.8" high and 1.43" in dia-

meter. They were trimmed on a hand operated lathe to

reduce disturbance. The uniformity obtained was very good.

Experimental Program
The experimental program consists of the determination

<of the true friction, study of the porewater pressure

behavirn7,1neasurement of the compressibility and expan—

sibiliiny, and the measurement of creep.

12

Tb study the shear strength, the value of the true

angle ofirmernal friction 3e must be known. The true

angle of friction may be found by producing two soil speci-
mens with equal void ratios, but different effective normal

stresses at failure. Cohesion depends upon the void ratio

of a partially saturated soil. If two specimens have equal

void ratios at failure, their true cohesion values must

be the same. If these two specimens have equal cohesion,

the difference in strength must be attributed to the dif—

ference in the frictional component, 5% tan 3e.

If the effective principle stresses of thest two
as the ordinate

specimens at failure are plotted, with 7‘

and <5— as the abscissa, the Mohr's envelope will intersect

the ordinate at Ce with a slope of Ce. Figure 4 shows the

graphical construction used to compute the value of the

true angle of internal friction. Figure 4a shows two void

ratio vs Jig, curves obtained for one soil. The corre-
sponding effective stress envelopes are plotted in Figure 4b.

Points 1 and 2 respresnt equal void ratios at failure, and

the Mohr's circles for these conditions are labeled 1 and

2 in Figure 4b. The common tangent then makes an angle

,Oe with the horizontal and intercepts the y—axis at a dis-

tance Ce above the origin.
Consolidated undrained (CU) tests were performed to

obtain the necessary envelopes. Each sample was placed

in.tflua‘triaxial unit and consolidated under a hydrostatic

13

until the volume change had ceased. After

a deviator stress was applied at an approxi-

Pore pressure

pressure 4?

consolidation,
mate rate of 4% of the maximum deviator.

meamnxmmnts being taken simultaneously.

The bulk of the triaxial tests and all of the pore

presmnxaneasurements were performed on the three bay
triaxialinut shown in Figure 5, designed by Dr. T. H. Wu.
This unit is similar to that developed at Harvard with
certain modifications that greatly facilitate pore pressure
The unit was

measurements and over-all ease of operation.
of

designed to provide an axial load of 240 Kg. and a 03'

4.22 Kg. The right cell in Figure 5 is equipped with a
harness that allows the application of a constant axial

The machine has one instrument panel from which

load.
and pore pressure measurements can

saturation, drainage,
be controlled. Each cell functions independently of the

other two.
Porewater measurements are made by balancing the

capillary tube A in the schematic diagram Figure 6.
Zhi 1936, Tergaphi (15) showed that the inclination

of‘tflua.failure plane with respect to the minor principle

d. = 45° +-J%9— .

.Specimmnns tested in the unconfined compression test provide
This method

axis was included at an angle of

a convenient means of measuring this angle.

of determining De is not very accurate, but if the angle

is measured as it first appears, a fairly constant value

14

of the true angle of friction is obtained. In this study,

a large number of tests were made (approximately 23) and

the mean value of file constitutes a reliable estimate.

Unconfined compression tests were performed at 2% strain
per minute with a standard controlled strain apparatus.
Creep tests were made to study the deformation of
the soil. They are very analogous to the CU tests. The
specimens were consolidated under a 0} pressure until
drainage had stopped. Then allowing no drainage, load
increments equal to a fraction of the maximum deviator
stress were applied. This increment was maintained until
all deformation had terminated, then another increment
was added. The procedure was continued until the specimen
failed. Creep tests were done only on samples compacted
to 1000 and 300 psi.

It was imperative to obtain values of CC and CS to
evaluate the porewater pressures for a partially saturated
clay. They may be determined from the consolidation curve,
to an arithmetic scale of e vs 7" .

c = _A_\/___ = __e_e-_ [15]
VAU‘ (/+c.)Aa'
Consolidation tests were carried out on 303 and 1000
981 specimens . The first load increment was 125 grams,

each increment thereafter being double the previous load.

Void Ratio Determination

A critical factor in the determination of the true
angle of internal friction is the void ratio at failure.
Extreme care was taken in measuring this void ratio as it
is highly sensative to the least error in weight or volume
measurements.

After failure each specimen was immediately weighed
in air; it was then diped in wax at 1240c and again weighed
in air. Thereafter the sample was suspended from a thin
thread and weighed in water. Knowing the specific gravity
of the wax and soil, the moisture content, and using
Archimedes principle, the void ratio can be found. Because
of the accuracy needed to evaluate the various components
of the void ratio, extensive practice in perfecting the

technique was necessary.

IV. RESULTS

True Phi

From the consolidated undrained tests, void ratio at
failure vs CE' at failure curves and Mohr's effective
stress envelopes were determined. Figures 7, 8, 9, and 10
contain the results for soils compacted to 303, 1330, and
2000 psi. The graphical procedure explained in Chapter III
was used to compute the true friction. A value of 340
was obtained from the lOOO--3OO psi specimens and a value
Of 310 from the 2000--lOOO psi specimens. See Figure 11.
As the number of tests for the 1000 and 300 psi groups is
Inuch greater than that for the 2000 psi series, it is felt
that the value of 340 is more reliable. The unconfined
compression tests resulted in a as of 390. The value of
340, being derived from a more accurate method, was used

in all ensuing calculations..

Incipient Failure

The results of the CU tests performed are given in
'Tables 2, 3, and 4. Each table is a summary of data
ggathered from specimens of one given batch. The data
presented does not include all the tests performed, but

is representative of the results obtained.

16

17

The deviator stress-strain and porewater pressure-
strain curves are shown in Figures l2, l3, and 14. Plots
of 7: vs 0-7, are shown in Figures l5, l6, and 17. Also
shown in Tables 2, 3, and 4 are the measured normal and
shear stresses on the failure plane at the point where
the shear stress is equal to the frictional component of
the shear strength. These stresses are found to be in

excellent agreement with the calculated stresses at the

\ '%¢”~ fi-fi ..‘-n

point of maximum porewater pressure. In other words, the

maximum pore pressure occurs at the point of incipient ,

'2."

failure.

Unconfined Compression

The 23 unconfined compression tests gave an approxi-
Inate value of 390 for 3e. The stress strains curves for

21 300 psi and 1000 psi specimen are illustrated in Figures

18 and 19.

Creep Tests

The results of creep tests are shown in Figures 20
arui 21. In these figures, the deviator stress is plotted

angainst the strains that occurred under the particular

iruerement of loading.

In essence Rowe's therom is assoctated with the creep
(Reformation in clay. Upon examining the creep-deviator
curves 20 and 21, it is evident that at a particular deviator

stI%BSS a break in the curve takes place. After this break

18

the strain increases at a much faster rate, terminating
in failure of the specimen.

When the 300 and 1000 psi creep tests are compared
to CU tests of similar samples, the sudden increase in the
rate of deformation occurs at the same stresses as the
Inaximum positive pore pressure (Tables 5 and 6). This
finding verifies Rowe's hypothesis and further substantiates
the fact that the behavior of the soil at stresses above

the point of incipient failure is different from that at

lower stresses.

Consolidation

The void ratio vs. pressure curves from the
consolidation tests are drawn to an arithmetic scale in
IFigures 22 and 23. For the 300 psi soil CC = .0319 and

C -.OO23. CC and CS were also determined for the 1003

S
psi compacted loess and found to' be .0039}; and .00181,

respectively.

By the use of equation (6) the values of U5 for uC of
ZL/2 of Pa and 0 were computed and are given in Tables 7 and
8, together with the measured porewater pressure at the
pnoint of incipient failure. The agreement between computed

arui measured values is within an allowable range.

 

V. CONCLUSIONS

The value of he for this soil was found to be 340.
Incipient failure takes place when the shear

resistance is equal to 5; tan 3e. r
Maximum positive porewater pressure occurs at

the point of incipient failure. The pore pressure

".firnm. A -'-e.. -41 r

is seen to be positive at low strains, then ;

decreases as failure progresses. Up to incipient

w!"-

failure the pore pressure agrees well with

that predicated by the elastic theory. Subse-
quently the displacement of particles along the
failure plane greatly affects the porewater
pressure.

Equilibrium of a soil can not be maintained
without large deformations after cohesion has
been mobilized. Cohesion is initiated at

incipient failure.

l9

2O

 

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SOIL. CHARAQTE RISTIC. VALUE.
PLASTIC, LIMIT 2,301,
LIQuI‘D LIMIT 29%
PLASTICITY INDEX (.1.
Ac-rIVITY .lob'le
SPECIFIC GRAVITY 2.72.

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CONSOLIDATED UNDRAINEDTEST 5PEQIHEN1¢4 SPECIMEN#5 SPECMENasb 5PECMENI¢7
MISS. Loess 300 PSI
6‘3, Kg/cm" .5 1 Z 4
INITIAL e. ~02
FINAL e. .021 .018 .013 .573
INITIAL w "7.: 23.2 22.4 22.2 22.1
FINAL w °/o 22 21.7 21.2 2.1
FINAL SAT. ‘71: 90.7 911» 70 98.5
m, s/cc. 1.1.8 1.4.8 1.1.3 1.72
(no, 3/ee 2.05 2.05 2.05 2.07
MAX (‘1 ”’13), Kg/cm‘ 3.14 4.53 (9.71 11.38
3, m "hunt, K's/Cam‘- 4.56: 5.98 9.12. 15.31
53 HT FaniuaeLKq/cm‘ .948 1.45 2-21 3.73
Dav: STDRAOI. 19 1+ 13 13
€ A1 MAX 0‘1- ”3) .100 .128 .11? .125
€ A1 MAX + A... .0071 .0142 .0187 .030+
6 AT 12: O .023 .037 .071 .130
MAX +M, Kg/cm‘ .105 .21 .343 .84
Mm: —M, Ka/cw- .440 .148 .2011 .052.
D AT max +22, Kg 10 174- 37 73
Eff“ m mx +41, Pkg/cum" .414 .705 1.4? 2.33
3E 5". m max +11, K3/Cm1 .01 1.10 2.45 4.77
7» an», 114/5, Kg/cmz .41 .8": 1.71. 3.35-
5.. ,Ir 2 A/NE, Nix/em" , .01 1.2:. 2.1.2 5.0
TAHLFRR. MIss.Lcrss 300 PSI CUTFST

 

141

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CONSOLIDATED UNDRAINED TEST SPECIM£N#4 specmervfis specmmfl. SPECIMEIW:
Ml§$.-LOESS 1000 'PSI

"613‘, Kg/cmz .35" .75 I .2 .2!
“INITIAL e .600 .5'90 .587

L We. ‘_ _ * .011 ’ ' .57: , i ".57: .580
617151.5me ’ * - ' 20 20.3 - 20- _- “1:202.
TIMI. wo/o - 22.3 ‘- 31.5 _ _ - -201. 20.:
’pru. 521:7. -97 673.5 93 f: 77

751/... 1.1.7 1.71 4 1.72 “-7 1.71
Riga/cc. 2.0:. 2.07 "I" 2.08 ‘ 1' 2.07
-MAX (firms), Keg/cm" 2.07 4.18 1, 7-41 11.1
,aAT FAILuRr , Kg/cm‘ _._, .2371 - .. 4.3b 9.74 13.17
{ 5:2. magma/ma ' .71 - 1.0+ “ * , 2.13:; :'--__2.87
’Dnvs 570mm: " ” ”‘ 12 f: +12 .. -- ”L .18 1.. .15 '
‘e m MAX (cm—r.) .05 .044. '5'“ .072 : .0413
g m ,w + a. *‘ f .0003 , .0075" i‘: .0137? _ .0132.
IS AT .0. =0 5 .007 #:017 .0193 ' -—
’MAx + 221 Kg/cmt .07 .14 . 245 .25’
MAX «2.4... Kg/‘cmg _ “ .42. .412 .124 -—-‘
'D“AT“.MA‘X3M, & 4 1112’ ‘ 33‘ .20
fgfi" m MAX-IMLKg/cm" .3 £55“ 1.5+ ‘ J 2.37
f3” 5?; ax-i_mx&‘.2,-K3/m?l_ 3.55;-.- Li_;‘:‘.20.. “Ti“:JIEiL . 4.2+ _
mimnémvéwfi‘ffnffiftimjf: fiizi'fi’i‘ "2:37 *
..17aT¢:L:~a.R§L/¢w*' (.5? -1711”: 2:22 ‘ ' 4.2+
"YARN “it Was... mm (:0, 1000 PM C U TEST

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CONSOLIDATED UNDRAINEDTESTISPEUMENufl SPECIMENfil SPECINEN‘tZ
1 ”155. LOESS 2000 PSI
6:31. Kgcm‘ .5 1 Z.
TNI'fIhL ‘6.” ‘2 7 7 ' "'"" 7— ""
£11451..- g. .582. .54‘1 . 545'
+INITIAL. 14070 19.2. 18. 3 13.4
’FEJAL L070 20.1 1‘7. 2.. 17. 2.
FINAL 5:47-070 ‘28 J 74.4 94.5
73L3/cc. 1.73 * ‘ 1.75 -_ 1.77
Agra/2. 2.07 2.04 2.10
MAX (em-r3). Kg/dmz -415 _ 0.77 7-7 *
-01... mmem/cm‘ 5.08 f 8.28 12.00
if; AT FAMMRE; Keg/mm“ .931 1.2.? 2.10
PM: STQHAGE. ‘ 19 11 15
€ -141 MAX (0“ ‘03) _ .010 .0445 .0401.»
EAT MAX 1' 2%.. .00b 7’ 7 .0ch ’ .014
’E, AT ,u. = O , , .017 .02.! __ . .02.?
353412, Kg/gm‘ , .119 .101 .245‘
13133130... 53/ch .g , .50121-._t.p: 5.37.? “":::“'“'.‘15+
D AT MAX 7+.“ , K; 8.5 ‘ 21.5 ' 44
32b" 111 max 7 MJKg/cvn: ..’._'.-35. .87 1198'
j 5i. aiimAmeg/m _- 14.50 * ....... 1.2.1-11: -281.
j“ a“: :02; we) Kazan} '* 7.55.1" ".87 1.30-.
in“ m (be st Kg/cm * " , .50 1.27 ’ 2.75“
TARLF #4 Mus. Lows 20 :3;

CU TEST

02

 

 

 

 

 

 

 

 

 

 

“‘“CREEP TEST ”SPEC-AMEN SPECIMEN. SPECIMEN ngmeu
-44--_;300--E§I4 -4-. ..... #2. 4- -- #:34- - #4 4-40.
4°‘3,K3Am" ' $4.54 “ ".5‘ "J 1 1
INITIAL w°fo 20.5 F 20.5 20.8 20.7
:ELNAL4LJ 7. - 444.44.20.14 .- - 2,0,0 --zo-.7 20.8
LImrmL c. -- 4.051,. .505 .008 “ .uo
-‘sat‘;aahxuo~,% * '* thin *99 ‘J 93. - 92.
anu w CU_TE$T,Kg/cw\z .105- .105 " .24 .21
JD 41¢. um. amt/5mg g-‘f‘4104_--.--.----10 4120.4- * ‘20
DSMBKEAMNCREEPQLRVE Kg "T” "”V' ' 9‘ " 14: . -J-. -12.

 

 

 

 

 

 

TABLafi‘5 M155. Loess 300 PSI

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4485545?” -TEST -..3PEL\MEN. Spectrum Spa-0mm SPECIMEN
444 AQQOPSJ .4. 44331444444 #3, 4.4."?51. “- 5
“‘3, Kg/cm" ’ 11.35 44.- .354 -.75 ‘ . .75
-Isnm; LJU/o' _ -. 4.1.8.14;- ‘ ‘17-;O_f---”;_‘.-18.3-. . 17.0
fwAEWfid‘f.‘ ” ‘ - .i-f-“I‘iii'il-ijm-3Q.-Lo-l.-ii:ZIY-eb.4 ‘ 17.7
:INJTJAL--§-_ -_ __ ,4'111-4511.‘ ,T :.TI"mZ:;‘-‘.:iim.j; 566'
BEQMW:;44_ ‘.[;4“12.:Z:LT'_ 15 :‘-443}4‘-44"_- 7.5-- ‘
THAMI'UWN‘C‘UTEST',K9%&H*"“.O'T '”““““.D7' ' * ‘ .14 ‘ _ .1}
-Dm ¢¢LINE Fwy/61W; 1.0 10 Ila lb
’D m BREAK IN CREEECuRVE Kg '75 7-5 _ 15 . - . .17

 

 

TABLE-2*" é M155.Loass 1000 PSI

1414

 

 

 

 

 

 

 

 

 

 

 

 

41/55. 1.0555 spam/m SPECIMEN SPEC/mm spams/v
30019.5/ #- f- u 5 #‘ 6 4 7
0:. Ky/cm‘ .5 /.o 2.0 {0
van: RA W0 '62/ '6/8 w; -5 73
5A 7: ‘7; 76-7 95 7. 78.:
2 «ex: 2 u = A’y/a.‘ .47 x575 ‘m' /.07
i «=0 . Uffifla.‘ '07? -/_5' 027 H/
MEASURED 0mm“: was gL 3+3 -e+

 

 

TABLE #7 CALCULATED AND MEASURED VALUES 0F
FORE PRESSURE U0 AT MAX + U

 

 

 

 

 

 

 

 

 

 

 

 

 

M/SS LOESS SPICININ SPEC/MEN 5P£CIIV£N $P£CINEN
/ 000 PS/ # + # 3 at 6 4’ 5
_g§,"A’.V/.covt “55' '7! 2- 0 3'0
voxo RA 7/ o -e// '57; -5 7: -5 30
5" T 7° 77. 0 98-5 930 97. 0
_§ Aa/a/z, gain/6.: +5 . are .976 /.30
i «=0, u.= kr/«v‘ -/oz '35 .953 43
MEASUR so (Jam: 07 v4 2*: '2 a

 

TABLE #8 CALCULATED AND M£A$UR£D VALUES 0/:
P0145 PRESSURE 0. A7- MAX + u

6.

13.

ll.

BIBLIOGRAPHY

Hvorslev, M. J. "Uberdie Festigkeitseigenschaften
gestorten bindiger Boden, Ingeneiovidenskabelige

Skriften. A. No. 45. Copenhagen, 1937.

Rendulic, L. "Relation Between Void Ratio and Effective
Principle Stresses for a Remoulded Silty Clay,"
Proc. Ist. Int. Conf. on Soil Mech. and Found.

Eng. V3, p. 48, 1936.

Hvorslev, M. J. "Conditions of Failure of Remoulded
Cohesive Soils," Ist. Int. Conf. on Soil Mech.
and Found Eng. V3, p. 51, 1936.

BJerrum, L. "Theoretical and Experimental Investi-
gations on the Shear Strength of Soils," Norwegian
Geotechnical Inst. Publication #5, 1954.

Ruthledge, "Soil Mechanics Fact Finding Survey
Progress Report: Triaxial Shear Research," U.S.
Waterways Experimental Station, Vicksburg,
Mississippi, April, 1947.

Leonards, G. A. "Strength Characteristics of Compacted
Clays," Trans. A.S.C.E., V. 120, p. 1423, 1955.

Skempton, A. w. "A Study of the Immediate Triaxial
Test on Cohesive Soils," Proc. 2nd. Int. Conf.
on Soil Mech. VI, p. 192, 1948.

Skempton, A. w. "Geotechnical Properties of Post
Glacial Clays," Geotechnique VI, 1948.

Bishop, A. w. and Henkel, P. J. "Pore Pressure Changes
During Shear in Two Undisturbed Clays," Proc.
3rd. Int. Conf. on Soil Mech. VI, p. 94, 1953.

Hilf, J. w. "An Investigation of Porewater Pressure
in Compacted Cohesive Soils," Technical Memorandum
#654, United States Department of Interior, Bureau
of Reclamation, 1956.

Wu, T. H. "Pore Water Changes in Clays Under Shear
Stress," Report #1 Project G-4l58, Eng. Exp.
Station, Michigan State University, 1958.

45

46

Skempton, A. w. "Practical Examples oflfi = 3 Analysis
of Stability in Clay," Proc. 2nd. Int. Conf. on
Soil Mech. V2, p. 63, 1948.

Rowe, P. W. "Ce = O Hypothesis for Normally Loaded
Clays at Equilibrium," Proc. 4th Inst. Conf. on
Soil Mech. V1, p. 189, 1957.

Seed, H. B. and Chan, C. K. "Thixotropic Character-
istics of Compacted Clays," Proc. Am. Soc. of
c. E. V83, 8M4, 1957.

Terzaghi, K. "The Shearing Resistance of Saturated
Soil and the Angle Between the Planes of Shear,"
Proc. lst Int. Conf. on Soil Mech. and Found.
Eng. V1, 1936.

 

Jun-u «.41- !I ,
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