EARTH ROADWAY FILLS ACROSS
FEM MARSHES

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EARTH ROADWAY PILLS ACROSS FEAT MARSHES

A Thesis Submitted to the Faculty
of
Michigan State College
of

Agriculture and Applied Science

by
C; H. gaeh

Candidate for Degree
of
Civil Engineer

June, 1931

THES‘C

 

 

 

INTRODUCTION

The problem of crossing peat marshes with roads is a
particularly important one in Michigan. The location of
many of our highways has been influenced by swamps. In
many cases the present trunklines, which wind in and out to
evade marshes, will sodn have to be straightened and their
sharper curves eliminated to keep pace with the trend to-
ward high speed traffic. The cost of crossing marshes,
once considered prohibitive, has decreased in the last few
years as modern equipment has lowered the cost of moving
earth. Mr. G. c. Dillman (1)" has pointed out that,"Since
1920 the unit prices for doing work have been decreasing
steadily. On grading today the average cost is only one-
third of what it was ten years ago." The trend toward
higher speed, greater volume of traffic, and lower grading
costs has made avoiding marshes and heavy grading second-
ary in importance to directness of route in the location
of trunklines. The Research and Statistics division of the
State Highway Department have computed the portion of our
paved trunklines traversing peat marshes to be over 3%, or

about 50 miles, before 1930 construction season.(2)

The Michigan State Highway Dapartment has spent much
time and money investigating peat marshes and several papers

have been presented on the subject (2,3). These papers were

of course, of a general nature but resulted in standard

'Numbers in parentheses refer to bibliograph on page 51.

94623

plans which are being used in the field. Progress in this

study is indicated by the frequent revision of these plans.
DEVELOPMENT OF METHODS OF FILLING ACROSS SWAHPS

The following are some of the methods used by the Mich-
igan State Highway Department in the development of standards
for the displacement of peat. This is not intended as a list
of all of the blasting methods tried, but only those that
have been observed by the writer. When the methods first
tried were not successful, field engineers often tried other
methods. In fact so many field changes were made that the
research engineers, who designed the methods, were often at

a loss to know the actual results obtained.

#1. Floating Across Shallow Swamps.

Before hard surfaced roads were generally used, stabil-
ity was not important. Settlement could gradually go on
year after year and when the dip became very bad it could be
refilled to grade and resurfaced at small eXpense. The me-
thod used was simply to fill over the peat and place the
surfacing. The surface mat was not broken and was depended
upon to support the fill. As drainage was particularly im-
portant the first standard called for a ditch extending to
the bottom of the swamp, when this method was used. It is
still being used on secondary roads and is fairly satisfact-

ory although these fills are very hard to maintain during

the spring breakup.

#2. Single Blasting On Centerline After Fill Had

Been Placed. U.S. l6, Ionia County, 1925.

I
NGW} Fill

 

 

 

Old Road +iv
80d - R / Charge of 1 stick of 60%
10' to 20. of Peat Q\iy{f' Dynamite every 2' on
“:7 r::- centerline after new fill
,/47 \ had been built.

 

About 50' was shot as shown above. It was eXpected
that the eXplosion would push the peat out and form a
cavity and that the fill would settle to hard bottom.
When 1 stick every 2' failed to show any effect, the quan-
ity of dynamite was increased to 3 and then 5 sticks in a
hole. In the latter case the fill was cracked up somewhat

but very little settlement occured.

The experiment failed because the mat was strong e-
nough to hold the fill in place. The cracks in the fill
were formed radially from the location of the charge and
indicated that continued increasing of the size of the charge

would tend to blow up the till rather than displace the muck.

#3. Single Blasting To Form Trench At The Toe or

Slope 0n Each Side Of New Fill. U.S. l6, Ionia County, 1925.

l///////, New 5 Fill
1”, Old Road fi\\\\
‘\\¢// I ‘\““‘__¥ ‘4—fl"”’r \kz/r
Charge of 1 stick of 60% dynamite

10' to 20' of Peat Spaced 18" apart, 3' below SWamp
I level, detonated by the propag-

tion method.
After the new fill had been built to an elevation about

#

 

 

 

 

_—--

4' above the swamp level, the trenches on each side were
made by blasting as shown. The prOpagation method of blast-
ing consisted of placing single sticks of dynamite 18" apart
in rows and detonating the first stick by use of a fuse.

The explosion of the first stick detonated the stick next to
it and so on down the line. Only 50 or 60% straight nitro-
glycerine dynamite is unstable enough to be used success-

fully in this manner.

The fill was left this way for several months to give
it a chance to settle. When settlement ceased the trenches
were filled with the excess earth in the fill. This method
was effective in breaking up the surface mat but the fill
was not heavy enough to displace the peat. This method was
effective as a float across the shallower portion of the
swamp but the deeper section, having soundings of about 20',
has continued to settle every year. This demonstrated that

breaking up the surface mat alone was not sufficient to cause

appreciable settlement.

U1

#4. Excavating Peat To Clay Bottom. M lfi, Lenawee

County, 1927.

a‘eo'Pav't I»
._______:"”””"r7 .: _____ : *“~““‘i‘~=g

H———2h'——
Channel Peat h' to 6' Deep

 

 

 

 

 

 

 

Soft . ue ay oiffim

The peat was excavated and the trench was backfilled
with earth. Precautions were taken to follow up with the
backfilling very closely behind the excavating as the ori-
tical height at which the peat would stand vertically was

not known and caveins were anticipated.

The method was successful and no noticeable settlement

or pavement cracks have developed.

The depth of peat excavated has gradually been increased
until swamps up to 12' deep are now being handled hn this
way. Peat less than 12' deep has not been successfully dis-
placed by dynamiting and loading, especially in well drained

swamps.

 

 

 

   

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12
#5. Two Parallel Wedges 26' Apart, 2 Blastings.

U.S. 127, 2% Miles North Of Addison, 1927.

The loading and blasting in this swamp was done as shown
by the details on sheet #7. The preliminary blasting and
loading was done in the fall of 1926 and the final blasting
and grading was done in the spring 0 1927. 20' concrete pave-
ment with mesh reinforcement was placed in July, 1927, after
a series of levels indicated that settlement of the fill had

ceased.

The peat, displaced by the fill, caused heaving on both
sides of the road from Sat. 703450 to 712. After the pavement
was in place, swamp ditches were dug h5' each side of the
centerline throughout, except from 708-711 L where heaved peat
made it necessary to place the ditch 70' out. When these
ditches were dug, further heaving of peat kept fillin them up.
This indicated that cutting the ditches through the surface

mat on each side had disturbed the equilibrium.

Settlement, especially on the centerline, began to appear
soon after the pavement was ooened to traffic. The cross sec-
tions, profiles and pavement condition sketches shown on
sheets 8 to 11, inclusive, show develooments after the road
had been used by traffic for less than three years, in the
spring of 1930. As this swamp is still unstable, plans he"e

been made to excavate the peat, refill and repave in 1931.

Analysis of the cross sections shown on sheets 8 to 10,

inclusive, indicate that the following results were obtained.

13

1. As the wedges settled the peat trapped between them,
tended to spread them apart. The section at Sta. 708%72 shows
wedges that had penetrated about 7' and were still 26' apart.
At Sta. 709%18 there was a penetration of 10' and the wedges
had spread to Al' apart. At Sta. 710423 there was an average
penetration of 13.5' and the wedges had spread to Al' apart.
The quantity of peat trapped between the wedges was smaller

than at Sta. 709¢18.

2. In most cases the wedges had penetrated at least three-

fourths of the way through the peat.

3. The sections were approximately the shapes anticipated

by the designer.

A. At Sta. 711%23 the trapped fiberous peat was suffi-
ciently compacted so that it acted as part of the left wedge

and entirely displaced the sedimentary peat under it.

5. Displacement of some of the soft blue clay under the
points of the wedges indicate that the peat under the fill has

been compacted.

Examination of the profiles and pavement sketches shows

the following.

1. Pavement settlement was greater on centerline than at

either edge for the entire length of the swamp.

2. Although some lake clay was displaced under the wedges,
the depth of this material did not greatly affect the stabil-

ity of the fill.

14

3. The soundings, as they were shown on the plans, in—
dicated the bottom of the lake clay instead of the bottom of
the peat. They were probably taken with an ordinary a" pipe,
which was pushed down until hard sand bottom was struck. Un-
less some provision is made for taking samples at various
depths, it is often difficult to tell when the rod strikes
the clay. If the soundings had been correct, the plan would
have been to excavate the peat through the portion of the
swamp where the depth was 6' or less, and it could have been

to excavate for the entire length.

A. The worst failures occured: (a) where the original
depth of peat was 6' to 10': (b) where the depth of peat was
changing, that is, where the marsh bottom was on a grade;
(c) where the largest quantity of peat was trapped between

the wedges.

The two wedge method of displacing and confining peat
'88 designed to keep the quantity of filling material as low
as possible. It was based on the assumption that settlement
of a fill over peat was mostly due to displacement or flow-
ing of the peat from under the embankment. When the peat
was confined so that flowing could no longer take place, it
was thought that the fill would be stable. Quick sand and
other unstable soils have been successfully confined by

similar methods.

Peat is unlike other soils in that it contains a much
higher percentage of voids. Dr. Dachnowski (A) says that,

"it may be accepted as an axiom that undrained deposits of

15

peat contain from about 70 to 95 percent of water," Compac-
tion of peat is a squeezing out of sons of the water. This
kind of a connection would go on at a gradually decreasing
rate almost indefinitely. Even if the wedges had penetrated
to hard bottom, compaction of the trapped peat would still
continue and the water would soak into the fill or seen
through it. As the center of the pavement is subjected to
greater pressure than either edge, it is probable that settle-
ment would continue to take place along the centerline for
years to come. The proposed plan to excavate the peat before

repaving this swamp is unquestionably the best way out.

#6. Two Parallel Wedges 16' Apart And surcharge. U.S.

223 At Devils Lake, 1928.

The method used is shown on sheets 17 and 18. It was
similar to the method used for the Addison swamp except that
the wedges were closer together, a surcharge was used, and

complete penetration to the marsh bottom was not eXpected.

Part of the preliminary blasting and filling was done in
the late fall and early winter of 1927 and the remainder in
the spring of 1928. The preliminary blasting did not make
ditches 8"each side of the centerline as was anticipated but
made a series of holes separated by pieces of unbroken sur-
face mat. The filling earth was mostly sand and sandy gravel
although there was some loam and clay. The swamp was poorly
drained as a county drain would have been necessary for pro-

per drainage. However, the ground water level could be kept

at least a foot below the surface of the peat.

16

In July, 1928, after settlement had apparently stooped,
the borrow was found to be about 6000 cyds. in excess of the
estimated quantity even though the final blasting had not been
done. It was thought that the settlement must be nearly com-
plete, so borings were taken at Sta. 82l¥50, 823, 824, and
825%50. The borings indicated that the fill had penetrated
nearly to hard bottom at Sta. 823 and 82!L but that very little
peat had been displaced at the ends of the swamp where there
were shallower soundings. Final blasting was therefore carried
out at Sta. 819-822 and 825-827 and a surcharge was maintained
until settlement was less than .05' in 30 days. The pavement
was placed in September, 1928. The cross sections, profiles
and pavement condition sketches shown on shetts 19 to 22,
inclusive, were plotted from levels and borings taken the
spring of 1930 after the pavement had been in use one and one-

half years.

1?
PLAN FOR LOADING AND BLASTING DEVILS LAKE SHAH? (5).

2 nd Loading

 

1 st Loading

’7 \wi'
A ____n___\/__ __ Peat ,

 

 

 

||F111
Final Cross \ I. ’
Section ‘4r-’
\0”
.‘ _. 3 Sticks of 50%
.cq§\ Straight Nitro

Glycerine Dynamite
Bottom of_Mareh

FIRST SHOOTING

Holes 2' to h' deep shall be bored in the peat, spaced h'
apart in rows 8' each side of the centerline. "2 sticks of 501
straight dynamite shall be placed in each hole and unless the
eXplosive is completely covered with water, the hole shall be
tamped with earth. A minimum number of 30 holes shall be shot
at one time by means of a blasting machine. Each line of holes

may be shot separately.
SECOND BLASTING

"After the first loading is complete holes spaced the same
as at first shall be bored to 3' below tyg bottom of the settled
fill provided that in no case shall holes extend to within 1'
of marsh bottom. 2 sticks of 50% straight dynamite for depths
of peat under 10' or 3 sticks for denthgcff 10 to 20' shall be
placed in each hole and unless the exoloslVe is completely cov-
ered with water the hole shall be tamoed with earth. A minimum
of 30 holes shall be shot at one time by means of a blasting.

machine. Both lines of holes must be shot at once.

18

LOADING

"After the 2 trenches have been blasted across the marsh,
the first loading shall be started from one end in these trenches
and both sides advanced equally from the end. The loading shall
be carried full height and width as shown on the above cross
section and any other loading than that shown will not be per-

mitted. The two fills must be carried across by end dumping.

"As soon as the fills of the first loading have been com-
pleted to the stage at which settlement is less than at the rate
of two-tenths of a foot per day, the second shooting may be done.
At the time of this second shooting, both fills must be within
five-tenths of a foot of grade. Should the distances to be
filled by this method exceed 200', after the first 150' is com-
pleted the second shooting of the first 50' may be done, but at
no time shall the distance of shooting be less than 100' from
the end of the uncompleted fill. After the second shooting, the
first loading shall be immediately brought to grade and the sec-

ond loading applied.

"Each loading shall be carried full height and width as
shown on this plan. If at any time during the application of
the second loading the preceding load shall have settled more
than 1' from its original grade it shall be immediately refilled

to the grade shown before proceeding with the additional load-

ing."

Examination of the cross sections profile and pavement

condition sketches of the Devils Lake marsh shown on sheets

19 to 22, inclusive, shows the following results.

 

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23

1. Fill penetration into the peat was 1/» to 1’3 the

depth of the peat for a depth of 16' and less.

2. The fill was the least stable:
a. Depth of peat 12' to 20'.

b. Where marsh bottom was on a 7% grade or greater.

3. The cross sections indicate that the two wedges 8'
each side did not settle together into peat less than 20'
deep and that they combined to form a wedge which penetrated
nearly to the bottom where the depth was greater than 20'.
In the shallower peat the wedge which started first usually
displaced peat or marl in the direction of the other wedge
and tended to retard its settlement. This is indicated by

cross sections for Sta. 820, 821, 822/50, and 825.

h. The shape of the fill was not as anticipated because

the wedges did not continue to settle separately but Joined

to form either one wedge or a mat which acted as a float.

5.The swamp depth was about the same at Sta. 826 as at
Sta. 820 to 822 and about the same penetration was obtained
when the swamp was originally filled but pavement settlement

was greater at Sta. 826 apparently because the marsh bottom

was on a grade.

6. The peat could have been excavated for the fist 300'
and the last 103' of the swamp. Plans have usually called
for using the same method of dynamiting and loading through-

out the entire length. Where there are several hundred feet

2&

of shallow peat, it could be excavated even though the remain-

der of the swamp requires loading and blasting.

7. In this case use of the two wedges apparently retarded

settlement instead of promoting it.

8. Pavement settlement was approximately prooortional to

the depth of the remaining peat under the fill.
SU 1N. ARY

About the only results common to these two swamps were (a)
pavement settlement was proportional to the depth of the re-
maining peat under the fill and (b) the worst failure occured
where the swamp bottom was on a grade but the depths at which
these failures occured was variable. Both are examples of
failure of a method which prOposed to trap peat under the fill

to reduce the quantity of filling earth necessary.

It is apparent that fills over swamps less than 25' deep
will be unstable unless they have penetrated to the bottom of
the peat. If merely an equilibrium is reached it is too easily
upset. Some of the factors which tend to disturb the equili-
brium are (a) drainage of the heaved peat, (b) slow compaction
of the peat under the fill, (c) freezing and thawing , and (d)

flooding.

One of the factors preventing the displacement of peat is
the weight of the heaved peat. After it has stood above the
ground water level for some time, most of the water drains out

or is evaporated. This leaves the heaved peat only a fraction

25

of its original weight. Much of the water usually drains out
before the surcharge is removed but this alone may not suffi-
ciently decrease its weight to start a flowing of peat from
under the fill after it has once stopped. Later standards
which called for placing a heavy surcharge before the final
blasting was done, gained a point here. Most of the heaving
took place before the final blasting and the peat had time to
lose much of its water while the contractor was getting ready

to do the deep shooting.

Compaction of the peat under a fill was discussed on

page 15.

This type of fill is more apt to settle immediately after
the frost leaves the ground in the spring than at any other
time. This is rather conclusive proof that considerable lat-
eral support is derived from the surface mat. Frost action in
soils has been described by Prof. Steven Tabor (5). Certain
soils, including muck, heave when frozen due to ”segregation
of ice in layers or lenses." Swamps seldom freeze to a depth
of more than 2 or 3'. When these layers or lenses of ice melt,
the surface is left in a frictionless and cohesionless condi-
tion and its lateral support is practically released and the

equilibrium is upset.

The finished grade is practically always wider at the top
where the earth from the surcharge has been disturbed over the
peat. This layer of earth is most easily sheared when it is
saturated with water when the internal friction and cohesion

are at a minimum.

26

It can be seen that the factor of safety of the above
type of fill is small. The weight of a 5' surcharge is more
than twice the normal loading of a highway, but it is a static
load. When the vibration, due to traffic, is combined with
the several destructive factors mentioned above, the factor of

safety obtained.by removal of the surcharge is overcome.
LAKE CLAY

Some of the cross sections shown indicate that the plas-
tic blue clay known as "lake clay" has been displaced by the
fill or even by the compacted peat under the fill. This clay
does not have the same characteristics as peat. It is satu—
rated with water which does not readily give up under pres-
sure as does peat. It has been found to satisfactorily hold
up fills where the peat has been excavated. In the swamps
Just studied the clay was displaced at the points of the
wedges only where the bearing was small. It is not subject
to compaction but to flow only. The study made by the Re-
search Division of the State Highway Department indicated
that it is very rarely displaced in swamps less than 25' deep.
For deeper swamps average penetration has been found to be h'
for swamps 40' deep, 10' for swamps 50' deep and all the way

to hard bottom for deeper swamps.

27

#7. Excavating Peat Below Ground Water Level. U.S.27,

South Of Marshall, 1928.

20'Pav't
Excavated Peat

 

 

 

 

 

 

Sandy
Fill
SmLiataraLmL_____________1__
*— 2hL—9- 6' to 10' of Peat
Channel
sort blue—pray Ho.tom

This method was used on several swamps rangingfrom 300
to 600' long. The trench was first excavated all the way
across the swamp by a power shovel with a clam bucket. The
suitable filling earth from the old road embankment was cast
to one side so that it could be recovered, and the peat was
cast to the other side of the road. The trench soon filled
to swamp water level with water which held a considerable
quantity of peat in suspension. This soupy liquid was forced
ahead in the trench as the new fill was placed. The pavement

was placed early in the summer of 1929.

The pavement started warping out of shape shortly after
it was opened to traffic. By the next spring there were sev-
eral dips where settlement of 6" to 1.0' had taken place.
Several theories were advanced to exolain this. First, that
the buoyancy of the ground water had been sufficient to keep
the fill from compacting until it was offset by traffic on the

pavement: second, that there had been lateral displacement

28

causing the fill to widen out at the bottom: third, that the

fill had settled into the plastic lake clay.

Borings indicated that none of these theories were cor-
rect but that soupy liquid in the trench had caused the trouble.
The worst dips were at the ends of the swamps where the trench
was filled last. Most of the water and suspended peat had been
pushed ahead until it reached the end of the trench where it
was trapped under the fill. A slight heaving adjacent to these
places was due to displacement of some of this material by

traffic and not to a lateral movement of the fill.

A new standard, being considered, calls for placing a sur-
charge to help displace this soupy pest and to hasten compaction
of the fill itself. It is necessary to fill the trench by and
dumping to keep the liquid pushed ahead. In doing this an im-
portant specification requirement of placing fills in 12" layer
is dispensed with. If the filling material is clay, compaction
of the fill itself may require a couple of years if it is not

hastened in some manner.

Another suggestion has been to widen the trench 6' to 10'
on each side at the end to be filled last, and at places where
the soupy material is most apt to be trapped. The fill would
then be placed the regular width only, leaving a space on each
side for the liquid. This would make its displacement more
certain and, where the filling material was sand or sandy loam,

would seem to make the surcharge requirement unnecessary.

29

#8. 1929 Standard Plan For Marshes 8-20' DeepJ6)

A ' .1
F1041 .: r1011 Proposed 36' Grade

 

 

 

 

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.ea - — - ——— — ‘
F111 -7 Fill ’
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\~/ Peat: v/

Bottom of Peat

PRELIMIWARY BLASTING

Before any filling is done the surface of the peat shall
be blasted by the propagation method in 2 rows, 10' or 15' each
side of the centerline, using one stick of 50% straight nitro-
glycerine dynamite in each hole, 18" deep and spaced 18" apart

in the rows.
FIRST LOADING

The marsh shall be loaded in 2 runways centered 15' from
the centerline to an elevation 5' above plan grade and to a top
width of 10', with l on 1 slopes to a maximum height above the
surface of the peat equal to a the depth of peat or until rapid
settlement takes place, after which the loading shall be kept
to an elevation 5' above plan grade by additional filling un-
til the rate os settlement is not more than 0.1' per day.

After this stage of settlement has been obtained the "Final

Blasting" shall be done.

30

FINAL BLASTING

2" pipes, plugged at the bottom end, shall be driven lO'
apart in rows 10' apart each side of the road centerline, to
a depth approximately halfway between the bottom of the first
loading and the bottom of the peat. The pipes may be driven
in a vertical position or on an angle at the base of the fill.
After the pipes are placed, the plugs shall be removed and 1
stick of dynamite shall be shot in each hole to form a chamber
after which 20 to 30 sticks of 50% straight nitro-glycerine
or gslatine dynamite shall be placed in each hole. The num-
ber of sticks per charge shall be varied in proportion to the
depth of the peat. A minimum of 20 charges shall be shot at
one time by means of a blasting machine and both rows shall be

shot together. After this blasting the final loading shall be

made.

FINAL LOADING

The loading shall be brought to an elevation 5' above
plan grade and a top width of AO' with l on 1 slopes and shall
be maintained at this elevation by additional filling until
the rate of settlement is not more than .05' in 30 days, after
which the load is spread and thoroughly compacted to form the
cross section and grade shown on the plans: provided however,
that excess earth shall be used to uniformly widen the grade

and flatten the slopes on each side.

LIMITS OF OPERATION

Where feasible, all material for fill shall be sandy or

31

gravelly soil. If the fill material is clay, the fills shall
be built up in compacted layers, the first layer not more
than 3' above the surface of the pest and succeeding layers

not more than 1' in thickness.

Use of this method of loading and blasting was observed
by the writer in 3 marshes, viz.: a swamp about 900' long,
12' to 20' deep on U.S. 27, north of Tekonsha: another swamp
about 200' long with the marsh bottom tilted, located in the
same vicinity: and a marsh about 500' long, 8' to 16' deep on
M 78, southwest of Bellevue. All three were loaded and blast—
ed during the late fall of 1929 and early spring of 1930, and
were paved during the summer of 1930. As borings have not
been taken, the results obtained can only be anticipated by
the apparent stability of the fill and the quantity of borrow
earth required. In this standard as well as others Just stu-
died the designer has not anticipated that the wedge would

penetrate to the marsh bottom.

The longer marsh at Tekonsha, above mentioned, required
about 5000cyds. of borrow earth more than estimated on the
plans. This would be an average settlement of s' more than
expected, so it is probable that the wedges penetrated to the

marsh bottom. The preliminary load was in place during the

spring breakup, at which time considerable settlement occured.

Peat was heaved on both sides for nearly the entire length of

the marsh. The pipes used for the final blasting were jetted

through the fill by a stream of water under pressure. Heaving

of peat between the new fill, which was on relocation, and the

32

old road embankment, which was about 100' away, had blocked
the drainage. The jetting process left the water standing

on both sides of the fill and in the trench between the two
rows forming the surcharge above the wedges. As the filling
material was mostly clay this flooding undoubtedly aided set-
tlement materially. The final blasting was much more effective
than for the other methods described, as the charge, being
over A times as great as those previously used, was sufficient
to raise the entire fill with surcharge several feet into the
air. For about 150' where the marsh depth was the greatest,
peat, which had been trapped between the wedges, was forced up
through the center of the fill to a height of about h'. This
peat was excavated to about swamp level and replaced with bor4
row earth previous to the final loading. The final loading
was placed and left about 2 months until settlement was less
than .05' in 30 days. The heaved peat was leveled off and

the surplus earth in the surcharge was used to widen the grade
and flaten the fill s10pe for the entire length of the marsh.

The fill had been so completely saturated by the Jetting, two

months previous, that, after the surcharge had been removed,

it was necessary to allow the grade to dry for one week during

the dry summer of 1930, before the pavement slab could be pla-

ed. Drainage was considered poor, as water had several times

risen above the old road, but dry seasons have prevailed since

the new road has been built and water has always been kept at

least 2' below swamp level or about 5' below the pavement grade.

To date no noticeable settlement has taken place, In as

33

much as the fill, while loaded with a surcharge, (a) was sub-
Jected to freezing, and thawing, (b) was flooded, (c) the heaved
peat given ample time to drain before the final blasting , and
(d) the factors of safety increased by widening the grade with
the surcharge material, therefore, this fill should be as sta-
ble as possible for the two wedge method. It would not be sur-
prising if some settlement took place along the centerline due
to compaction of the peat between the wedges. In this partic-
ular case, most of the marsh is on a super-elevated curve where
the crown was left in the pavement. A small settlement would

merely remove the crown and the super-elevation would still pro-

vide drainage for the slab.

The other marsh at Tekonsha was handled in the same manner
and at about the same time as the one Just described. In this .
case the marsh was only about 200' long and the old road was at
the same location. The marsh bottom sloped toward the middle
of the fill longitudinally and was also tilted laterally. The
old road fill was nearly to hard bottom on the shallow side.
As the old gravel road was a trunk line which carried fairly
heavy traffic, the peat under the old fill was probably com-
pacted to the extent that it was not readily displaced. Drain-
age was much better at this location. The filling material was
partly sand and partly clay. The fill settled quite rapidly on
the low side and heaved peat on that side only. Since it has
been paved some settlement has taken place at the point where

the marsh was the deepest. The steep tilt to one side was pro-

bably the cause.

34

The marsh, on M 78 southwest of Bellevue, was about 500'
long and from 8' to 16' deep. Following preliminary blasting
and loading the quantity of peat trapped between the wedges
was unusually large. As the wedges settled the peat heaved up
between them along the centerline of the road. At this point
the loading and dynamiting plan was abandoned and the peat was
excavated and replaced with borrow earth. This was expensive
but the heaving of an abundance of peat along the centerline
indicated beyong doubt that the two wedge method of displace-
ment would be a failure so excavating the peat and backfilling

with earth was the best and surest way out.
Comparison Of The 1926 And 1929 Standards.

The 1929 standard plan used on the Tekonsha marshes was
unquestionably an improvement over the 1926 standard plan used
for the Devils Lake swamp., The principal changes were substi-
tution of the propagation method of preliminary blasting, in-
creasing the size of the charge of dynamite, and increasing the
size of the surcharge. The propagation method of preliminary
blasting required 1/3 more dynamite but fewer caps than the old
method. It was easier to execute and more successful in cutting
the mat. The first standard required a very small surcharge
before the final blasting and therefore did not cause much hea-
ving until after the final blasting. By displacing as much peat
as possible by the first load better results are obtained by the
final blasting for the heaved peat has had a chance to dry out
and become much lighter. The charge for the final blasting was

increased to three times that required by the 1926 standard and

35

made large enough to raise the fill and surcharge into the air
so that the wedges struck the peat with an impace. The final
load for the 1929 standard was almost h times as heavy as that

required by the earlier plan.
The Two Wedge Method Being Replaced.

Although the 1929 standard plan is still in use the two
wedge method is gradually being discarded and is being replaced
by the method recommended for deeper swamps on the 1929 standard
plan. This method calls for the same preliminary blasting but
for starting the load in the center and building it to a height
of 5' above grade with a top width of 20' with l on 1 slopes.
This is in the form of one wedge instead of two and is designed
to penetrate all the way through the peat. The final blasting
is the same as for the two wedge method. The final surcharge

is only 20' wide on top for the width of grade and pavement being

considered.
A PrOposed Standard.

A new standard plan which is being considered but which
has not yet been adopted is practically the same as the last
method mentioned above except for the final blasting which is
as follows: (6) "2" pipes plugged at the bottom end shall be
driven 10' apart in 3 rows spaced 7' apart for future 20' pave-
ment, or spaced lh' apart for future h0' pavement. The center
row to be driven along the road centerline, and all rows to be
driven to a depth approximately half way between the bottom of

the first loading and the bottom of the peat. The pipes may

36

be driven in a vertical position or an angle at the base of

the fill.

"After the pipes are placed, the plugs shall be removed
and one stick of ..............dynamite shall be shot in each
hole to form a chamber, after which 20 to 30 sticks of 504
straight nitro-glycerine or 60% gelatine dynamite shall be
placed in each hole. The number of sticks per charge shall
be varied in proportion to the depth of peat except that in
all cases the number of sticks in the center blasting row are
to exceed those of the other two rows by five sticks; and the
center blasting row charges are to be fitted with Delay Deton—
ating Caps. A minimum of 20 charges shall be shot at one time
by means of a blasting machine and all three rows shall be
shot together. . ............ In lieu of the above method of

placing charge, contractor may use the Jet method."

This method is proposed for all depths but is aimed par-
ticularly at depths of 12' to 25' where complete penetration
has been most difficult to obtain. The charges for the out—
side rows have been demonstrated to be sufficiently large to
raise the fill. While the load is in the air the charges
along the centerline, which were equipped with delay electric
caps, explode and start displacement of the peat and the im-

pact of the fill, as it falls, continues the action.

The manufacturer recommends the old type waterproof de-
lay electric blasting cap for this kind of work as the new

type caps are designed for use with ammonia dynamites only (7).

These delay caps are made in five different sizes for different
periods of time. First delay would probably be sufficient in

this case.
THE MINNESOTA METHTD

A method of displacing peat is being used in Minnesota
Which has been described by Mr. P.W.Riedesel, former resident
engineer of the Minnesota State Highway Department (12). It
is being briefly outlined here and compared with the Michigan

standard.
PRELIMINAVY BLASTING AND LOADING

Step #1. Place charges of one stick of 601 dynamite l'
apart and 5' or 6' deep in 3 rows, one on the centerline of
the road and one 8' each side of the centerline: and shoot by

the propagation method.

Step #2. Build a fill the width of the trench at the toe

of the lepe and to a height 5' above the swamp level.

Step #3. Place charges, consisting of 1 stick of 60% dy-
namite, 1' apart and 5' to 6' deep, along the toe of the fill

and shoot by the propagation method.

Step #h. Fill the resulting trench to a height 10' above

swamp level.

FINAL BLASTING

Place charges of 150 lbs. of A0“ dynamite 20' to 30' apart

on the centerline at points three-fourths the distance from the

38

bottom of the fill to the bottom of the peat, and shoot 6 holes
at a time. Maintain a surcharge 10' above swamp level and re-
peat blasting until the fill penetrates to hard bottom. For
wider fills, use charge of 100 lbs. of LOT dynamite 20' apart

in rows 10' each side of the centerline.

The above method was said to be successful but rather ex—
pensive. It will be noted that the preliminary blasting is
very complete.‘ It requires 250# of 60% dynamite per station as
compared with 6”# of 50% dynamite per station called fOr by the
latest Michigan standard. Mr. Viedesel gives A reasons for re-
quiring a very complete preliminary blasting; First, it Opens
the way to immediate natural settlement; second, it makes set-

tlement from the final blasting more certain: third, it saves

dynamite; and fourth, it lessens the danger of side slipping.

Shearing of the surface mat is very important especially
in shallower marshes and it is possible that the Michigan stan-
dard does not place enough emphasis on the first blasting. The
Minnesota method is designes to remove a large protion of the
mat while the Michigan plan is to merely shear it. The quanti-
ty of dynamite required to break up the mat depends upon the
densenees of the mat. In some cases where the mat is tightly
woven together by roots it might be advisable to excavate to a
depth of 3' or A'. In other instances where the surface is
very soft no preliminary blasting at all is necessary. The
stiffness of the mat depends on the water content of the peat
as well as the quantity of vegetation. The Minnesota method

calls for 60% gelatine dynamite while the Michigan requires

39

50% straight nitro-glycerine dynamite. The latter is less sta-

ble and therefore more easily detonated by the prepagation me-

thod.

There is less difference in the final blasting. Minnesota
uses 750# of AOT gelatine dynamite per station while Michigan
requires fi15# of 60% gelatine dynamite or 501 straight nitro-
glycerine dynamite. 60% dynamite has only 1.23 times as much
energy as hot dynamite but it eXplodes with a more shattering
blow. It appears that the Minnesota method was designed to
actually displace the peat with the dynamite while the Michigan
standard depends on a sudden impact to start the flow of peat

and the weight of the fill to continue action.

40

UARSHES 0V3? 25' D???

An entirely different problem has arisen in the settlement
of fill over marshes more than 25' deep than that encountered
in the shallower marshes. The first Michigan State Highway De-
partment investigation of marshes that had been filled without
the use of dynamite, indicated that in nearly every case where
the depth of.peat was over 25', the fill had settled to the
marsh bottom. The first standard plan developed, did not call
for a second blasting of these swamps. The surface mat was
broken and the sinkhole was filled until settlement stOpped.
Excessive overruns in the quantity of borrow earth required
for some of these marshes led to an investigation which showed

that lateral displacement was causing the loss.

None of the deeper marshes are being studied from borings
in this paper as was done with the shallower marshes. Some re-
sults that were obtained in the Highway Department investiga-
tions are being used together with some facts and theories of
earth pressures to determine the probable causes of lateral dis-

placement and precautions that might be taken to prevent or de-

crease it.

INTERNAL FRICTION AND COHESION

As George Paaswell, C.E., has said (11), "The correct in-
perpretation of the character, distribution and amount of pres-
sure throughout an earth typical of ordinary engineering con-
struction, cannot be eXpressed by exact mathematical analysis.

The usual earth mass retained by a wall contains so many uncer-

i1

tain elements that can neither be anticipated nor refermined
by typical tests, that it becomes very hard to assemble suf-
ficient data for a premise upon which to found any satisfac-

tory conclusion."

In the definition and discussion of the resistance to
movement of a prism of earth, Charles Terzaghi, Research Con-
sultant, U.S. Bureau of Public Roads, is quoted (8,9). "The
resistance to movement is a combination of forces due to the
attraction existing between the individual soil grains and is
independent of the pressure acting upon the surface under con-
sideration. It is a constant value for any unit area within

the embankment if the material is uniform.

"The internal friction is caused by the resistance to

soil grains sliding over each other and varies with the pres-

sure upon the sliding plane.

”It should be understood that cohesion and internal fric-
tion in every respect correspond to the shearing strength of
solid bodies. They represent the combined result of the pro—
perties of the soil constituents and of the conditions under
which the soil exists. Thus they include friction and true
adhesion between the individual particles, skin friction be-
tween soil particles due to capillary pressures, etc. When
these conditions change, both the cohesion and thelinternal
friction of the same material are apt to change, similar to
the change in shearing strength of a solid body due to change
in the temperature. Therefore, strictly speaking, the terms

cohesion and internal friction refer not only to definite

52

materials but also to definite states. In general, the finer
the soil constituents the more variable both quantities become.
...........for materials with even a trace of cohesion, the
"angle of repose" has no relation to the angle, a, and may, for
the same material, be very different, depending on the height

of fill."
NONUNIFORMITY OF MICHIGAN SOILS

As Michigan soils are glacial deposits they vary greatly.
A recent bulletin resulting from several years of soils re—
search along Michigan State Highways, lists 51 different kinds
of soils which have been classified into 6 groups. While car—
rying on this survey to determine the effect on pavements of
various types, observations were taken mostly in cuts as the
fills consisted of mixtures of different soils. It can be seen
that a fill, partly made of earth taken from cuts 1000' or more
each side of the swamp and the remainder taken from one or more
borrow pits, is by no means a homogeneous mass. Taking this
into consideration, it is altogether unlikely that the soils
making up the average fill would have constant physical proper-

ties.

PHYSICAL PROPWRTIES OF FEAT UNKOWN

An attempt by the Research Division of the State Highway
Department to determine the physical qualities of various kinds
of peat by laboratory methods failed. The reasons are given by
Mr. V,R.Burton (3) as follows: "First, because it is impossible

to maintain the peat in its original condition on eXposure to

43'
air and after drying. Peat as it comes from the ground is only
partially oxidized and at the greater depths there is actually
some reducing action. Consequently, immediately it is handled
in air, oxidation begins and if it is attempted to dry it, a
totally different material results. Next, it is certain that
the amount of compression under loading in a vessel such as is
used in the standard bearing value test which does not permit
of a free displacement in all directions, will not give results
at all indicative of the behavior of the same material in its
natural place." It is doubtful whether the stability of peat
should be eXpressed as internal friction and cohesion which
acts according to Coulomb's hypothesis. Fiberous peat, which
usually composes the tOp five to twelve feet of Michigan mar-
shes, will stand vertically, when a trench is cut through it, to
a critical height of 12', more or less depending on the height
of the ground water level and the consistency of the peat.
Cross sections have proven its horizontal thrust to be suffi-
cient to offset the opposing thrust of a fill up to 25' deep.
Yet research engineers for the Bureau of Public Roads (8) state
that ”Bearing properties depend on the combined effect of co-
hesion and internal friction" and in the same article describe

peat as affording ”exceptionally low support".

COHESION AND INTEQNAL FRICTION VARY WITH FIELD CONDITIONS

The method used in building a fill can greatly effect the
cohesion and internal friction of the mass. Experiments by
Jacquinot and Frontard in 1910 on earth taken from a dam which

had failed, showed the following results:

AA

"1. f (‘ tan d) varied only 0.111 for the soft, pasty
earth to 0.18 for earth nearly fry. Neither the amount of
water used for the cuddling or ramming caused much variation
in f.

"2. On the contrary, the amount of water used effects
the cohesion very much, and sufficient ramming can more than

double the coefficient of cohesion."

In 10 out of 12 cases studied by the Research Division
of the State Highway Department, where lateral displacement
of fills had taken place, the fills were built with industrial
railway equipment. This material was not compacted and was
often dumped from a trestle directly into water. In such a
fill the soil would have a minimum coefficient of cohesion due
to the high water content and lack of compaction. The method
ofbuilding a fill of cohesive earth may greatly effect the
water content even though the fill later becomes submerged.
The interior of these fills usually are found to have a lower
water content than the outer portion. If a fill can be built
before it is submerged its internal friction will be greater.
Charles Terzaghi, Sesearch consultant, U.S. Bureau of Public
Roads,(9). says, "The coefficient of internal friction, tan 6,
of a cohesive material is very different depending on whether
the water content of the earth has or has not an Oppurtunity
to adapt itself to changes in pressure,” Clayey soils are some

times almost impermeable and when they once become saturated

are very loath to give up their water. Lateral displacement

apparently takes place in a series of slides. The fill is built

55

up until the weight is sufficient to shear the weakest place,
at which time a portion of the fill slides. The resistance of
a fill to shear is entirely dependent on the combined effect
of its cohesion and internal friction and these factors are

greatly affected by mthods of construction.
EFFECT OF BUOYANCY

The buoyancy or hydrostatic uplift of a submerged fill of
porous soil has been described by William Gain (10) as follows:
"If this filling earth weighs 100# per cu. ft. in air and has
h0% voids, then 1 cu. ft. contains 0.6 cu. ft. of solids, and
since the buoyant force of the water equals the weight of an.
equal volume of water or O.6x62.5 equals 37.5 pounds, the true

weight of the filling in fresh water is 100-37.S=62.5 pounds

per cu. ft.

"This method can be applied to get the approximate weights
in water of such materials as rip-rap, cobble-stones or clean
gravel. It cannot apply to finer materials, as clean sand,
bank sand or loam, because in such earths free circulation of
the water is not realized: the water cannot get under the ma-

terial everywhere and its full buoyancy effect cannot be ex-

erted."

The above quotation was given in connection with the de-
sign of tunnels and was intended as a warning against depend-
ing on hydrostatic uplift of a submerged fill over a tunnel
unless the soil was very porous. Buoyancy has been found to

have caused a decrease in bearing capacity of cohesionless

46

sand by research engineers for the Bureau of Public Roads (8)
who are quoted as follows: "If the water rises near to the sur-
face of the ground, the specific gravity of the sand apparently
decreases on account of the hydrostatic uplift; and, as a con-
sequence, the bearing capacity also decreases." The bearing
capacity of cohesionless sand was found to be decreased 351 due
to buoyancy, which was prooortional to the loss in weight. As
long as water is free to flow under a mass the buoyancy will be
equal to the weight of the water displaced. In case of a fill
of sand or gravel the buoyancy would probably be about 35% dur-
ing the time that settlement was taking place and probably
slightly less than that after settlement was complete when com—
paction would slightly decrease the ease with which water could
circulate. in case of a mere impermeable fill, the water dis-
placed would be nearly the volume of the fill and the buoyancy
would appear 50% during the time that settlement was taking
place and water could flow under the mass. As soon as complete
settlement had taken place, the lake clay at the bottom of the
marsh, into which the fill sinks would almost entirely stop
circulation and buoyancy would no longer affect the mass. At
this time the internal stresses in the fill would be more than

doubled.

From the above it appears that lateral flow does not start
until the fill has penetrated into the lake clay. If the co-
hesion and internal friction are sufficient to hold the fill
together, the increase in weight will compact the fill and in-
crease its stability; if not, lateral flow will start and a

portion of the fill will break away and form an angle of repose

A7
with the marsh bottom. The plane of the shear on the remaining

portion of the fill, becomes wet so that when the fill is again
built up another slide takes place more readily than the first.
Cases have been found where one side of the fill stood nearly

vertical while the other formed an angle of repose of as low as

20° with the marsh bottom.

The Coulomb hypothesis is (10), "that the friction on a
plane in the interior of a mass of earth, is equal to the nor-
mal pressure multiplied by the coefficient of friction, f=tant¥,
and the cohesion is equal to the area of the plane considered
multiplied by c, the cohesion per unit of area. In other words,
the friction is proportional to the normal pressure on the area
considered, whereas cohesion is simply proportional to this

area and is independent of the normal pressure."

The following formulas are developed in Cains' text (10).
E = horizontal thrust of earth asainst a vertical plane.
w 3 weight of earth per cubic foot.
d = angle of internal-friction.
f I coefficient of friction I tan 5.

c - coefficient of cohesion in pounds per square foot.

h = depth of the fill in feet.
For non-cohesive earth:

E = 3h tan2(h5°- é) Equation #1
2 2

For earth endowed with both cohesion and friction:

E a wh tan?(h50- 5) - 2ch tanfhso- g) Equation #2
’5 ‘2' 2

48

Most soils weigh about 100% per ft. and it is seldom that

there is more than a 10¢ variation from this figure.

Taking

buoyancy into consideration values for w will be used as fol-

lows: sand 60#, loam 70#, clay 80$.

Coefficient of cohesion,

will

be considered as follows:

sand 0, loam 200# 1 sq. ft., clay £00# 1 sq. ft.

The angle of internal friction, 5. will be taken as fol-

lows: sand 260, loam 16°, clay A9.

Substituting the above values into Equation #1 or 2 and

solving;

marshes,
h
25'
30'
A0'

33'

the total horizontal thrust,

is found to be as follows:

Sand

E
7330#
10550#
18750#
29300#

Loam
E
1080#
3330#
8A30#

15800#

Clay

E
3100#
8960#
25830#

A97h0#

E, for various depths of

If buoyancy acts on the clay fill while it is submerging

so that w becomes 50# per cu. ft. the values of E are ~5070,

-13h0, -h950, and 17100 for the A values of h considered . It

is evident that the effect of buoyancy is probably the most

variable factor.

The above partially explains why clay fills

do not penetrate the peat-as readily as granular soils and why

lateral displacement is so apt to take place in fine grained

soils.

A soil which is endowed with both internal friction and

cohesion and yet_is porous enough to be affected by buoyancy is

the ideal 8011.

119

CONCLUSIONS mo RaccwwwoATIo'Is
NARSHES o - 12' DEED

Displacement by dynamiting has not been successful but
the peat can be readily excavated. If the ground water level
is below the bottom of the trench the fill should be built up
in layers and compacted. The same specifications used for
normal fills of the same height may be used. If the trench
contains ground water some provision should be made for the

displacement of the resulting soupy liquid in the trench.
MARSHES l2 - 25' DEEP

More emphasis on preliminary blasting is recommended.
A solid mass fill which has penetrated to the bottom of the
peat is necessary. Even though a fill which has a slatic load
shows no signs of farther settlement it may be suSpended in
peat and continue to settle when subjected to the impact and
vibrations of motor traffic. The quantity of borrow earth used
is an indication of the depth of penetration but complete bor-
ings are desirable as a check. Where water is available Jetting
the pipes through the fill is preferred to driving them. A pro—
perly built fill is too stiff to permit driving pipes without
first making holes with an auger. Having surcharge in place
during the spring breakup, or during a time when the fill is
flooded, will help to force the fill through the peat or aid
compaction of a fill already on hard bottom. Drainage is im—
portant but swamp ditches should not be cut through heaved peat

as there is danger of disturbing the equilibrium. Particular

50
attention should be paid to portions of marshes where the bot-
tom is on a steep grade either laterally or to one side. If
there are sections several hundred feet long of peat less than
12' deep and the remainder deep enough to require blasting, the
shallow portions should be excavated and a method of loading
and dynamiting used on the reainder. In case there is and old
road fill, the preliminary blasting should include shearing the
old embankment near the ends of the swamp and possibly at other

points so that the fill will be free to settle.
MAQSHES MORE THAN 25' DEEP

In the selection of borrow pits sandy, gravelly or loam
soils are most desirable and heavy clay should be avoided as
much as possible. Where sand or gravel can be alternated in
layers with clay the latter is much less objectionable. Clean
sand is less desirable than sand containing some cohesive ma-
terial. The danger of lateral displacement of the fill in-
creases rapidly with the depth of the marsh. Rigid enforcement
of specifications for properly building fills will increase the
internal friction and cohesion of the soil and decrease the dan-
ger of lateral displacement. The quantity of borrow earth used
does not indicate the depth of penetration as much of the earth
may be comsumed by lateral flow. Heavy overruns in borrow
earth over the original estimate indicates that ther has been
lateral flow. Although the unit weights of different kinds of
soils do not vary greatly in air it is probably that the effect

of buoyancy makes this variation much greater for soils compos—

ing a submerged fill.

51
BIBLI9GwAPHV

(l) G.C.Dillmen, address, Michigan Roads and Airports,
March 12, 1931. P 16.

(2) V.R.Burton and A.C.Benkleman, paper, "Further Study
of Settlement in Peat Marshes."

(3) V.R.Burton, "Fill Settlement in Peat Marshes", 1926
Proceedings, Highway Research Board. Also in Public Roads,
‘Vol. 7, No. 12, February, 1927.

(A) Dr. Alfred P.Dachnowski, "Stratigraphic Study of Peat
Deposits", Soil Science, Vol. 17, No. 2, February, 192A.

(5) Prof. Stephen Taber, "Freezing and Thawing of Soils
as Factors in the Destruction of Road Pavements", Public Roads,
Vol. 11, No. 6, August, 1930.

(6) Michigan State Highway Department Standards, No. E-A
-A-88, E-h-A-BBA, and Proposed E-h-A-SBB.

(7) DuPont "Blasters' Handbook", 1928, Ph2.

(8) C.A.Hogentog1er and Charles Terzaghi, "Interrelation-
ship of Load, Road, and Subgrade", Public Roads, Vol. 10, No. 3,
May, 1929.

(9) Charles Terzaghi, "The Mechanics of Shear Failure on
Clay Slopes and the Creep-of Retaining Walls", Public Roads,
Vol. 10, No. 16, December, 1929.

(10) Willaim Cahn, "Earth Pressure, Retaining Walls and
Bins", First Edition, 1915.

(11) George Paaswell, "Retaining Walls, Their Design and
Construction", 1920.

(12) P.W.Riedesel, "Blasting Settles Road Fills in Minn-

esota Muskeg", Engineering News-Record, May 16, 1929, P 733.

 

 
    

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