A CASE STUDY ON THE PERFORMANCE OF EMBANKMENTS ON
TREATED SOFT GROUND
by
Isabella Santini Batista
Diploma in Civil Engineering
Universidade Catolica de Pernambuco - Brazil
Submitted to the Department of Civil and Environmental Engineering
In Partial Fulfillment of the Requirements for the Degree of Master of Engineering
In Civil and Environmental Engineering
at the
Massachusetts Institute of Technology
June 2003
MASSACHUSETTS I NSTITUTE
OF TECHNOLO GY
003
C 2003 Isabella Santini Batista
All rights reserved
LIBRARI ES
The authorhereby grants to MIT permission to reproduce and to distributepublicly paperand
electronic copies of the thesis document in whole or in part.
Signature of Author........................................
.............
......
..
Department of Civil and Environmental Engineering
. t May 09, 2003
A
Certified by...................................................
.....
Andrew J. Whittle
Professor of Civil and Environmental Engineering
Thesis Supervisor
Accepted by.......................
.---..-
O
Oral Buyukozturk
Chairman, Department of Committee on Graduate Studies
A CASE STUDY ON THE PERFORMANCE OF EMBANKMENTS ON
TREATED SOFT GROUND
by
Isabella Santini Batista
Submitted to the Department of Civil and Environmental Engineering
on May 9, 2003
In Partial Fulfillment of the Requirements for the Degree of Master of Engineering
In Civil and Environmental Engineering
ABSTRACT
The proposed construction of a new high-speed rail link between Amsterdam and Brussels
involves construction of embankments overlying soft clay and peat deposits with very stringent
requirements on the time frame of construction and allowable settlements. A field program of
instrumented test embankments, referred to as No-Recess, has compared the performance of five
schemes for stabilizing the soft ground behavior. This thesis summarizes the No-Recess project
and compares the measured performance for two instrumented using finite element analysis. The
results confirm the benefits of geotextile encased sand columns as a practical technique for
stiffening underlying soft soils, accelerating consolidation and reducing settlements.
Thesis Supervisor: Andrew J. Whittle
Title: Professor of Civil and Environmental Engineering
A Case Study on the Performance of Embankments on Treated Soft Ground
M.Eng 2002 - 2003
Acknowledgments
I would like to dedicate this thesis to the memory of my beloved grandfather Gracio
Barbalho and my cousins Mairson Bezerra and Eduardo Carvalho.
First and foremost, I would like to thank Professor Andrew Whittle, my thesis supervisor
and academic advisor, for introducing me to this topic and for guiding me through my work.
My warmest appreciation also extends to Dr. Lucy Jen and Professor Herbert Einstein,
who introduced me to the 'art' of soil and rock mechanics; to Cynthia Stewart, for helping me on
several occasions regarding academic and bureaucratic problems; to Robert F. Woldringh for
providing me the data for this research; and to all my friends at MIT, especially to Tina, Kartal
Toker, and Patrick Lee.
I am grateful to my parents, brother and sisters for their continuing love throughout my
life and to my parents-in-law for their encouragement in this challenging phase of my life.
Jozeane Bezerra helped me take care of 'our' Guiga - many thanks!
Finally, but most importantly, I offer my deepest gratitude to my beloved husband
Valdner Batista and my son Antonio Guilherme for 'everything': without their endless love and
support, finishing up this work would not have been possible.
3
A Case Study on the Performance of Embankments on Treated Soft Ground
M.Eng 2002 - 2003
Table of Contents
I
2
3
4
5
6
8
..............................................
Introduction ..........
No-Recess Project..................................................................................................................10
10
Objectives of No-Recess Project ..............................................................................
2.1
11
Site Characteristics .....................................................................................................
2.2
16
Ground Improvements Options .................................................................................
2.3
16
1)............................................................
(HW
Conventional Test Embankment
2.3.1
19
Stabilized Soil Columns (HW 2).......................................................................
2.3.2
21
3)............................................................................
(HW
W
alls
Stabilized Soil
2.3.3
23
Geotextile Coated Sand Columns (HW 4) ..........................................................
2.3.4
25
Stabilized Soil Test Embankment on Piles (HW 5) ............................................
2.3.5
28
M onitoring .....................................................................................................................
2.4
30
Performance of an Unimproved Embankment ...................................................................
30
Site Characteristics .....................................................................................................
3.1
31
Soil Parameters and in situ Stresses .........................................................................
3.2
34
One-dimensional Settlement Predictions...................................................................
3.3
34
Calculations for undrained, initial settlement....................................................
3.3.1
34
Calculations for Consolidation Settlement........................................................
3.3.2
35
Calculation for Secondary Compression..........................................................
3.3.3
37
Total Settlement.................................................................................................
3.3.4
38
Simulation using Plaxis ..............................................................................................
3.4
model..........................................................................................39
Finite Element
3.4.1
41
M aterial Properties ............................................................................................
3.4.2
42
Calculations .......................................................................................................
3.4.3
49
Ground Improvement Options..........................................................................................
49
Vertical W ick Drains .................................................................................................
4.1
52
The effects of the smear zone in wick drains .....................................................
4.1.1
Geotextile Coated Sand Columns...............................................................................55
4.2
FE Simulations of No-Recess Instrumented Embankments...............................................59
59
Conventional Design, HW I.......................................................................................
5.1
62
(HW
4)........................................
Predictions for Geotextile Coated Sand Columns
5.2
66
Conclusion .............................................................................................................................
4
- 2003
02-20
MEng
. n 2002
List of Figures
Figure 2-1: The Hoeksche Waard (HW) test site (Geotechnical Engineering for Transportation
11
Infrastructure, 1999)..............................................................................................................
13
Figure 2-2: Soil profile and in-situ properties at HW site .............................................................
Figure 2-3: Surcharge sequence for constructing embankment HW 1, high section .................. 17
Figure 2-4: Plan view of the embankment HW1.......................................................................18
Figure 2-5: Cross-section LL of embankment HW1................................................................18
19
Figure 2-6: Cross-section AA of the high embankment section of HW1 ................................
20
HW2
..............................................................
Figure 2-7: Plan view of the trial embankment
Figure 2-8: Cross-section LL of the embankment HW2..........................................................20
21
Figure 2-9: Cross-section AA of the embankment HW2 ..........................................................
Figure 2-10: Plan view of the embankment HW3.....................................................................22
22
Figure 2-11: Cross-section LL of the embankment HW3 .......................................................
23
Figure 2-12: Cross-section AA of the embankment HW3 .......................................................
Figure 2-13: Plan view of the embankment HW4.....................................................................24
Figure 2-14: Cross-section LL of the embankment HW4........................................................24
25
Figure 2-15: Cross-section AA of the embankment HW4 ........................................................
Figure 2-16: Plan view of the embankment HW5.....................................................................26
Figure 2-17: Cross-section CC of the embankment HW5........................................................27
27
Figure 2-18: Cross-section AA of the embankment HW5 .......................................................
Figure 2-19: Instrumentation of test embankments (Barennds, 1999)......................................28
Figure 3-1: Initial effective stresses and piezometric head in the FE model.............................40
Figure 3-2 Cross-section of a road embankment on soft soil...................................................41
43
Figure 3-3 Tim e vs. height of fill ..............................................................................................
Figure 3-4: FE meshes for unimproved embankment and location of 3 reference points on the
43
ground surface .......................................................................................................................
44
Figure 3-5: Total displacement after construction = 1.04m. ....................................................
Figure 3-6 Total displacement after consolidation = 1.62m......................................................44
Figure 3-7 Excess pore pressures at the end of construction = 87.OOkN/m 2 .............. ............. . . 45
Figure 3-8: Settlement and Surcharge vs. time for the unimproved embankment....................46
Figure 3-9: Horizontal Displacement in the toe of the embankment for the unimproved
em bankm ent...........................................................................................................................46
Figure 3-10: Relationship between maximum horizontal displacement and maximum settlement
...........................................................
47
elevations...............................................................48
three
Figure 3-11: Excess pore pressure at
49
Figure 4-1: Em bankm ent with wick drains ..............................................................................
1996).............50
et
al.,
(Terzaghi
drains
of
vertical
of
installation
pattern
Figure 4-2 Triangular
Figure 4-3 Vertical drain of radius r., smear zone of radius r,, and consolidating soil cylinder of
53
radius r, (Terzaghi et al., 1996).........................................................................................
Figure 4-4 Relations between degree of consolidation and time factor for radial flow into a
vertical drain. Loading is assumed to be instantaneous and excess pore water pressure
constant with depth. (a) different drain spacing; (b) different mobilized discharge capacity
(T erzaghi et al., 1996)............................................................................................................55
56
Figure 4-5 Geotextile coated sand columns ..............................................................................
58
Figure 4-6 Process of installation of geotextile encased sand columns ...................................
5
2002 - 2003
MEng
. n,202-20
Figure 5-1: FE model of surcharge vs. time and comparison between measured and predicted
59
ground settlem ents for H W l..............................................................................................
Figure 5-2: Comparison between horizontal displacement predicted and that measured in the
1
field for H W I.........................................................................................................................6
Figure 5-3: Relationship between maximum horizontal displacement and maximum settlement
61
for HW .................................................................................................................................
62
Figure 5-4: Excess pore pressures dissipation at three elevations in HW1 ...............................
Figure 5-5: Imposed surcharge sequence and comparison of predicted settlement by plaxis and
63
that m easured in the field...................................................................................................
64
Figure 5-6: Horizontal displacement predicted by Plaxis ..........................................................
and
maximum
Figure 5-7: Relationship between the maximum horizontal displacement
65
settlem ent in H W4 .................................................................................................................
embankment
the
high
beneath
locations
Figure 5-8: Predicted excess pore pressure in three
5
(H W4 )....................................................................................................................................6
6
A Case Stud on the Performance of Embankments on Treated Soft Ground
M .lng 2nn2 - 2003
List of Tables
Table
Table
Table
Table
Table
Table
Table
Table
2-1:
3-1:
3-2:
3-3:
3-4:
3-5:
3-6:
3-7:
29
M onitoring program .................................................................................................
31
Total head for each layer..........................................................................................
Engineering properties from lab and field testing at No-Recess site.......................31
Engineering design parameters used to model soil behavior...................................31
In-situ stress at No-Recess site.................................................................................33
Vertical stresses due to embankment construction based on elastic theory.............33
One-dimensional calculations of settlement for the high embankment...................38
Material properties of the road embankment and subsoil........................................42
7
M.Engz 2002 - 2003
1
Introduction
The new high-speed rail link between Amsterdam and Brussels will require new techniques
for construction of embankments overlying very soft soils. The principal factors affecting the
design of the embankments are related to the high train velocities, short construction time, and
very strict requirements on residual settlements. After analyzing these requirements, a research
team initiated a program in 1997 to evaluate different techniques for stabilizing railroad
embankments constructed on very soft clay and peat foundation soils.
The program, formally entitled "New Options for Rapid and Easy Construction of
Embankments on Soft Soils" (No-Recess), was a joint venture of the railroad project manager
High-Speed Rail South, the Ministry of Transport, Public Works and Water Management, Road
and Hydraulic Engineering Division (RWS/DWW), and the European Community (EuroSoilStab
lime-cement columns project).
The No-Recess project comprises a series of test embankments constructed in the Hoeksche
Waard polder near Gravendeel in the Netherlands. Five ground improvement techniques were
selected during a workshop in 1997 in Delft based on advice from a panel of international
experts. Direct comparisons of field performance were intended to demonstrate the viability of
the new methods for constructing the railway embankments for the new high-speed line. Since
there was no prior experience of these construction methods in the Netherlands, the instrumented
embankments were monitored for two years after the end of construction.
This thesis presents a summary of the No-Recess field project. A two-dimensional finite
element model is developed to represent the in-situ conditions and soil properties at the site for
one case of an unimproved embankment and two of the five ground improvement schemes used
8
M.Engz 2002 - 2003
in the No-Recess project: (1) using pre-fabricated conventional drains, and (2) reinforcement of
the soft soil with geotextile encased sand columns.
9
A Case Study
2
2.1
on
the Performac
ofEbnmnso
rae
oft
Ground
M.Ena 2002-- 2003
No-Recess Project
Objectives of No-Recess Project
The goal of the No-Recess research project was to investigate alternatives for constructing
embankments on soft clay foundations, which would provide a cost effective alternative to slab
track (i.e. concrete structure). The techniques in the No-Recess project are compared with a
reference technique of embankment construction using with a traditional free-draining sand fill
and pre-fabricated, vertical wick drains, together with surcharge loading to accelerate
consolidation. Since the purpose of the No-Recess project was to address design requirements
imposed by the high speed train, the following specification were formulated:
" Short construction time (less than 18 months)
*
Low residual settlement (less than 30mm in the 30 years after 24 months after starting
construction)
*
Minimum surplus of soil
" Sufficiently stiff behavior of the construction under dynamic loading by high "speed trains"
(mitigate potential problems associated with surface waves in the fill and sub-grade soils)
*
Minimum impacts caused by widening of existing rail or road constructions
The purpose of selecting the new techniques was to fulfill the requirements to accomplish the
specifications above. Five instrumented embankments were constructed in the Hoeksche Waard
as follows:
" HW 1: Conventional test embankment with PV wick drains
" HW2: Lime stabilized soil columns
" HW3: Stabilized soil walls
10
A
Case Study on-- th~ P~rform~nr~
--
Of Fmh~nL~npntQ
.N,
Tr~t,.A ~rd'~ ~
a------~--
%,
%a
"
HW4: Geotextile encased sand columns
*
HW5: Stabilized soil test embankment on piles
J
2
.n1 1r
ME
ijVUii
2
-
LUJ
The site characteristics and embankment construction methods are described in the next
sections.
2.2
Site Characteristics
The test site has a plan area of 400x125m, with level ground surface at EL -0.75m +
NAP*. Each embankment was constructed with a high part, 5m above the ground level, and a
low part, im above the ground level. Figurel illustrates the lay out of the five embankments
area.
-
Figure 2-1:
The Hoeksche
Waard (HW)
.
7C
test site (Geotechnical
Infrastructure, 1999)
* NAP: Niew Amwsterdam Piel,
datum
11
:- JE
-
Engineering for Transportation
A Case Study on the Performance of~
nTetdSf
rud
S vothPefracofEbnmnsoTraeSotGud
Cas
~ ~~
A~~~~~~~~~
~ ~ Embankmet
1
)A,
NA~~~~2002
MEn
2003
n l
The underlying soft foundation soils are typically 9.6m thick and comprise the following
main layers (Figure 2-2):
" Clay 1: average thickness of 3.1m of clay, very silt, moderately organic, grey, with sand
lamination;
*
Peat 2: average thickness of 2.Om of peat, slightly clayey, wood fragments, brown, grey
(Hollandveen);
" Clay 2: average thickness of 3.Om of clay, extremely silty, moderately organic, peat traces,
wood fragments, grey;
"
Peat 2: average thickness of 1.6m of peat, highly organic, brown, with sand lamination
(Basisveen).
12
A
tfnt
Profile
W1I
I" %,
m
y(kN/m 3)
Wn, WP and W,(%)
. n11 2002
V
- 200
I 3J
ME
IU
Stresses and Pore Pressure(kN/m 2 )
0
-1
-2
0
-3
I
-4
e0
A
Peat 1
-
0
z
c6 0
A
-7
-8
0
-9
A
I1
Peat 2
F
50
100
WP
150 200 250 300
Wn
1011 12131415161718
W1
0
50
100
-0
(7' (kN/m2
-U--
u(kN/m 2)
'P(kN/m 2 )
A
-y-
Figure 2-2: Soil profile and in-situ properties at HW site
13
150
',(kN/m
2
200
M.Eng 2002 - 2003
All of these layers are Holocene deposits. Below the Basisveen peat layer is a thick sand
layer comprising medium-fine, slightly silty sand, which is considered to be relatively
incompressible and free draining.
Many in-situ and laboratory tests were carried out before the construction of the
embankments. Fugro Ingenieursbureau B.V. (Masterbroek, 1998) performed the geotechnical
soil investigations.
In-situ tests carried out in the site consisted of the following considerations.
0
35 piezo-cone penetration tests (PCPT) with measurements of sleeve friction and pore
pressure to a depth of 15m below ground level.
0
35 dissipation tests at the end of each PCPT
The tests were performed with the electrical Fugro-sleeve friction cone unit (with a
capacity of 200kN).
The dissipation tests were performed at the maximum depth, which are
usually done when the penetration process is stopped to allow the excess pore pressure to
dissipate.
0
6 mechanical soil borings with undisturbed continuous sampling, 12m below ground level.
The borings have been carried out using a truck/crawler type mounted drilling rig (according
to the Dutch standard NEN 5119). Undisturbed samples have been taken during the borings to a
maximum depth of 12m below ground level. After the samples were taken in the thin-walled
Shelby tubes with a diameter of 70mm and a length of 400mm, the samples have been
transported to Fugro testing laboratory in Amhem and stored in a temperature conditioned room
before laboratory testing.
14
0
36 field vane tests (FVT) at four locations, 9 meters below ground level, at Im vertical
intervals, performed close to the locations of the piezocone tests beneath embankments 1
through 4.
*
16 cone pressuremeter tests (CPM) at four locations, 9 meters below ground level, depth
interval of 2m, performed close to the locations of piezocone tests (DKMP 1-1, 2-1, 3-1 and
4-1) CPM 1-1, 2-1, 3-1, and 4-1.
0
Installation of two fixed cone points to a depth of 15m below ground level in the southwestern and south-eastern corner of the test site, coded (FP 6-1 and FP 6-2)
*
Installation of 6 standpipe piezometers at two locations with 3 depths of filter screen (each
Im long) indicated as B 6-1 to B 6-3 and B 6-4 to B 6-6, the standpipes were installed at 2
locations (3 in each location) closed to the Fixed Cone Points. The filters were located at
depths of 3.5m, 5m and at 10m with respect to the ground level. The filters were isolated
with clay pallet packers and the ground water was measured after a period of time.
The laboratory test program was carried out on samples at lm vertical intervals and
consisted of:
"
Description of the soil
"
Photographs
"
Water content
*
Bulk density
" Density of solids
" PH values of the soil
"
Atterberg limits
*
Particle size distribution (one per boring)
15
M.Engi 2002 - 2003
Some more specialized laboratory tests also were carried out in the site, including:
" Organic content (Im intervals)
*
Carbonate content (1 m intervals)
" Fall cone strength (tm intervals)
*
Sulfate content (Im intervals)
" Isotropic consolidated undrained multistage triaxial (3 per boring)
* Oedometer compression tests (3 per boring)
The in situ vertical stresses that are shown in Figure 2-2 take into account the artesian pore
pressures in the underlying sand layer.
2.3
Ground Improvements Options
The five embankments were designed to have a high part (5m above initial ground surface)
and a low part (1 m above initial ground surface) with side slopes of 1:2 (v:h) with a transition
zone of 10 m located at the end of the high part. Each embankment was to be constructed within
eighteen months (after the working platform was made), followed by a period of six months prior
to assume surface construction (rail bed installation). The residual settlements expected after 24
months from the start of the construction must be less than 30mm over a 30-year period.
2.3.1
Conventional Test Embankment (HW1)
The conventional test embankment (with pre-fabricated, vertical wick drains) was
constructed to provide a base reference on performance compared with the other test
embankments.
The conventional embankment was constructed in stages within a 6-month
period. The surcharge of the low embankment was 1.8m of sand whereas for the high
16
M.Engz 2002 - 2003
embankment was 2.5m of sand, both for a year. In both, low and high embankments, the vertical
drains were installed to a depth of 1 m above the sand layer and were designed with 1 m triangular
grids. Figure 2-3 shows surcharge sequence used to construct the embankment with vertical
drains. Figures 2-4, 2-5, and 2-6 illustrate the details of the embankments and also show the
locations of the important instruments stated earlier in this chapter.
5
4
E
3
0)
(0
2
1
0
1
10
Time(days)
100
Figure 2-3: Surcharge sequence for constructing embankment HW1, high section
17
1000
A
Cni
4Ztimd
nn
thi-
AfF
Prfnrmi~r,
~
m..sp~Q
~
~
N
AV
kAl Ir
3
-
A6
Q
4,l
fI
KAI
A~l~
!-5
1-6
.6
/
o0-, 1
19-
A'-
-21
~
'-24
I 11-317 41-741 0l-20
33
5
21-1-12
-,
-
/
V
TO.:
*,-ZS
71K
1-
22
12513
-23
-3
~12
a
-3
-6
'-40
Figure 2-4: Plan view of the embankment HW1
2a~C1
j
SEIT
ittIQ
SE~~h
V.R1KALE DRAJNt HO.-
1'rv
Figure 2-5: Cross-section LL of embankment HWI
18
JE
E SRAII N
200002000C
4
CJUUI
ilL
I
IA
A Case Study on the Performance of Embankments on Treated Soft Grud
o Trate Sof GrundM
n te Prforanc
ofEmbakmets
A Cae Sudy
Ency 2A(Y
S0020=2
-
IQA(Y
200
Figure 2-6: Cross-section AA of the high embankment section of HW1
2.3.2
Stabilized Soil Columns (HW2)
The stabilized soil columns method used in HW2 is the Scandinavian lime-cement
mixing method of stabilizing clay soil mixing 200kg/m 3 of new binder mix comprising 80% of
blast furnace cement and 20% anhydrite (instead of the combination of Portland cement and
unslaked lime that would be used in Scandinavia).
This mixing method of stabilization
technique (dry mix technique) uses dry air to transport the binder. The reason for the change in
the binder mix is to have better performance with this new mixture in the very soft Dutch soils.
The embankment was constructed in 1 month. For both, high and low embankment, a
preload of respect to 1.5m and 1.Om for a period of 260 days was needed to accelerate the
consolidation process. The lime columns were 600mm in diameter and were placed in 1.6m
square grids for the low embankment whereas for the high embankment they were placed in the
range of 1 m to 1.2m square grids. For both cases, the columns extend 0.5m into the underlying
sand layer. The low embankment uses stabilized blocks (overlapping short columns) of 1.5m,
which were produced with a speed of 500m per day. More details can be seen from Figures 2-7,
2-8, and 2-9.
19
A Case Study on the Performance of Embankments on Treated Soft Ground
T
r
M Eno, 2002
-------
s
unr
9
Figure 2-7: Plan view of the trial embankment HW2
~r~rF1rrrFnr7~tF
L~
nir ~nr~rri
F
L L
Figure 2-8: Cross-section LL of the embankment HW2
20
it
-
2003
A
C~i'~e
Afl
-
thc~- PPrfArm~riPP
-
r~fFmnvi..nt~~
"*
~**n
t~.i
Trj~x~ti~.A Qr.4~n flrr.,,..A
'J-sue
Utnu
PW
ME
1V.ndlg~J~
2003
2002 - ~.U
7C~-NA
Figure 2-9: Cross-section AA of the embankment HW2
2.3.3
Stabilized Soil Walls (HW3)
The technique used in the test embankment HW3 comprises the stabilized soil walls
installed by the FMI process (Barends et al., 1999), a process of constructing a soil mix wall
using a specialized cutting tree device.
The machine has a speed around im/min with a
maximum depth of 9m and width of 500mm. The cutting tree device is inclined up to 80 degrees
and is dragged behind the FMI machine. Due to the cutting blades rotated by two chain systems,
the soil is not excavated but mixed in place with cement slurry. The embankment was built in
two weeks.
In the high embankment, a load transfer platform was used, which is placed at the height
of 0.5m with 3 geotextiles (tensar geogrid type SS20, SS30 and 80RE with crushed rock). The
stabilized walls, which have dimensions of 2 x 500 mm, are placed at an interval distance of 1m
to 2.5m in both embankments, but in the case of the stabilized block, it is placed at a depth of
1.5m only in the low embankment.
The binder dosage used for both stabilized walls and
21
A Case Study on the Performance of Embankments on Treated Sof Grud
M
~
Pnv
-
stabilized block is 150 kg/m3 , where 80% consists of blast furnace cement and 20 % of
anhydrite. Figures 2-10, 2-1 1,and 2-12 show the details of the embankments.
-
3-4
1443-
3-,
-
34
2
-I4
i
~44-
a3-
11
44 4-3
2.2
-~
3
*
~
~loon
1-4
31
_______
SE11
3-
3-'
-3
--
414
4-
37
3-V
~ ~ ~'4-
3414x
34
-3
54-
0
342
3
-54
-4 44'
3.4
3-4
1-4
3-1
33
~4
344
3-V
3-33RN
--- O1
3-3
3~~~
~~~~1
4
5
-
~
1
~ ~
3-34Dal
3
33353-
Figure 2-10: Plan view of the embankment HW3
Figure 2-11: Cross-section LL of the embankment HW3
22
3-
41
" "
A Case Study on the Performance of Embankments on Treated Soft Ground
I
nlP -A2003
MEne 2002
I OCCO
De:Wceo~
e
if
(q.O
Cno(
60CC+
z
Mv
M)
It
Sr-S
S"2X
voe' (C
340
MV'
NAP
2
G
WS
700
-
MV
I
NAP
PR 4AP
Dr3
2000
CCC0
2000
CC,
2000
000 C0CICCC~
inc
030_1000
1
_ _
2200
_
1000
2000
-000,
__T
2000
0CC
200
Figure 2-12: Cross-section AA of the embankment HW3
2.3.4
Geotextile Coated Sand Columns (HW4)
Geotextile coated sand columns (GCC) system is the technique used in HW4.
This
process consists of vibrating a casing with two valves at the bottom until it reaches the sand
layer. A 40-cm thick layer of sand is made, a geotextile stocking is installing and filled with I m
of betonite/sand mixture to create a geo-hydrological barrier (12%).
Then, the rest of each
800mm diameter column is filled with sand to the top of the casing, which is vibrated and pulled
upwards, compacting the sand. This system can have a rate of construction of 40 columns per
day.
The embankment with a load transfer platform of geotextiles in the HW4 test was
constructed in 6 weeks.
For both embankments, the depth of the columns is 9m below ground level and the
distance between columns is 2.4m to 3.4m triangular grids for the low embankment, whereas for
the high embankment, it is in the range of 2.Om to 2.4m. For the low embankment, a surcharge
23
A Case Study on the Performance of Embankments on Treated Soft Ground
M.Engz 2002 - 2003
of I m of sand for one month and one geogrid Fortrac 80/80-10 were used, whereas for the high
embankment, they used three geogrids Fortrac 200/30-30 and no surcharge was necessary. More
details can be seen in Figures 2-13, 2-14, 2-15.
XT.
3
VD__
~
C~0 C
~
C
a0.0
.'0C~c
0
~~
~
00
No -
~
~2~,t
W-3
0
0)
0C
C
~
0
4)
~
0-
-~
C
0k
.0 t
0'-2
0~~
Pi
.-
PI
C2
W
V
Figure
2-14: CrsssctinL
~~
0
11
-
Figure
2-3 Plnve0fteebnmn
t'~
'a
a~
w"
00
*1
hm..~
W
G--
f h
0
manmn
A-
W
24
A Case Study
MPng
. 2MW - 2003
on the Performance of Embankments on Treated Soft Ground
10000
100CC
000
-
7m
970
- \P
'00-
170
31520
Figure 2-15: Cross-section AA of the embankment HW4
2.3.5
Stabilized Soil Test Embankment on Piles (HW5)
The piled foundation in the technique used in the HW5 trial is made with wooden piles
and AuGeo pile system (Barends et al., 1999), and the sand used in the embankment is replaced
by stabilized soil from another site, which is made by using mix-in place plant installation
(ARAN installation). The embankment was built in six weeks, with a production rate of 200
piles a day.
The AuGeo pile system basically consists of a Cofra stitcher that pushes a steel plate into
the ground with a casing (1 80x 1 80x6.3mm) using maximum pressure of 25tons. After that, a
PVC pipe, which is sealed at the bottom, is inserted in the casing and filled with the foamed
concrete with unit weight of 1200kg/m 3 . The last step of this process, after having sufficient
hardening of the concrete, is to lift the casing, cut off the PVC pipe, and cover the top with a
concrete tile of 300x300mm.
25
~
-
~
%1 m
i
-
5I
nt' l
4U Cl
M.Eng 2 002 - 200
.3
iuUEU
The depth of the piles for the low and the high embankment is 12m below the ground
level but the pile type used for the low embankment was AuGeo pile system only, whereas for
the high embankment wooden piles also were used. The distance of piles in the case of the low
embankment is Im square grids, but for the high embankment a 0.8m square grid is used.
For the low embankment, Im of stabilized soil is used with a binder dosage of 120kg/m3
where 80% is blast furnace and 20% is anydrite, whereas for the high embankment 5m of
stabilized soil is used. The load transfer platform consists of two geogrid Fortrac with a thickness
of 30cm and with a fill of crushed concrete for both cases. The embankments can be better seen
in Figures 2-16, 2-17, and 2-18.
howter paler
5-7
SchuimoeOnpolen
5-11
5-9
eta
A55-5
-95-1721
5-23
5-26-
3~4 5-24
5-
2
3
5-8
55-42
0000
-210000
50005-
1000
5
'Ice
8
5-0
3
5-14
$Ic10-000
10
g551
Figre2-6:Pln ieCo the emakmn H
51-
26
0
0
5-22
109
ME iV.L~2002 - 2
ACase Study on the Performiance nf Fmhnnk~mpnt,_ nn Trp,,tpi 4Ztff I-gr% nAl---------...~-.~......
"Y'J
pen
:aer
4 crvor
£'A
C
v~
R243,'rW,
R32LH/5!v
7
/1jL4
OC0 A
-m
sets
' -. --
KQ
1%>
Figure 2-17: Cross-section CC of the embankment HW5
25000
7500
4
7
6c00
50C
:C0---
:30
m3 gG
cp
-lso.
500--,
4 50
zoo3
-530-
:a:
"t
-
9"C
r
r
''
40C,-na
et
XM=
----------
P"
300--N3
4
Figure-
Figure 2-18: Cross-section AA
of the embankment HWS
27
V wx
Z21
B Ct
4,- P
A Case Study
2.4
on the Performance of Embankments on Treated Soft Grud
onTretedSof GrundiAt.o Emankent
n t fomane
A ae Sud
n
I')AIQ ~ -A 2003
MEn 2002
Monitoring
Each of the five embankments was monitored to provide detailed information on vertical
and horizontal soil displacements, pore pressure, and vertical stresses. Each design is expected
to satisfy stringent rail requirements for high-speed embankments (with reduced construction
time and long-term deformations). The monitoring was carried out from the start of construction
through the end of 1999. The instrumentation for monitoring the embankments was installed
after the construction of the foundation and before the construction of the embankments. The
monitoring program for each embankment is shown schematically in Figure 2-19, while the
rotation for the instruments is given in Table 2-1.
High
54M
Z(NAP)
t~c
~=iL1L=r
Figure 2-19: Instrumentation of test embankments (Barennds, 1999)
28
xL
A Case Study on the Performance of Embankments on Treated Soft Ground
M.Enix 2002 - 2003
MEt 02-20
Since each technique has its own features, each embankment trial has a different
monitoring starting time in 1998 as follows: February 20 for HWI, June 29 for HW2, March 27
for HW3, July 17 for HW4, and September 29 for HW5. The description and location of the
monitoring program for each embankment can be seen from previous figures shown in this
chapter.
The monitoring time is based on the average construction time of 3 months.
A
monitoring time of 100, 300 or 600 days corresponds with a construction time of 6, 12, or 24
months. The settlement S =(Z - Z,) will be monitored and evaluated. The logarithmic rate of
can be used as an evaluation parameter for the residual settlement
settlement LRS =
log(
(Barends et al., 1999).
Description
Location
Instrument
SETT
Settlement top layer
5x5m grid at the top of embankment
SETH
Settlement hose
longitudinal section and low and high cross-section
SETP
Settlement plate
longitudinal section every 10m and 2 cross-section
EXTM
Extensometer
5 levels in soft layer under high embankment
INCP
Inclinometer
3 in cross-section high embankment
PWSP
Pore water stand pipe
1 at ground level under high embankment
PWPT
Pore water pressure transducer
SPCL
Soil pressure cell
3 levels in soft soil layer
1 beside the column and 1 at the top
Table 2-1: Monitoring program
29
3
Performance of an Unimproved Embankment
This chapter considers the behavior (or a hypothetical) unimproved embankment with the
same site characteristics, soil parameters, and in-situ stresses as the No-Recess project, in order
to provide a basis for evaluating the performance with the ground improvement solutions.
3.1
Site Characteristics
Since this analysis is being done for the same site in the Netherlands, an analysis of this
site was made and a general profile was designated for the study of the unimproved
embankment. This site basically consists of 9.7m of soft soil underlying a relatively
incompressible, artesian sand layer. The ground surface is at EL.-O.9m and the ground water
table is located at the elevation -2.2m.
Three piezometers were installed in the high section of the embankment, at elevations
-2.7m, -5.Om and -7.5m.
of
The pore pressure was calculated from the results given by the
piezometer installed in the top of the sand layer (EL.-7.5). The piezometer gives the total head in
the top of the sand layer as +1.25m and the hydraulic conductivity in the clay and peat layers, is
k=8.64x1 0 5 m/day and 8.64x 104 rm/day, respectively.
After some calculations, it has been
inferred that there is minimal head loss in the peat layer and the head loss in the clay layer is
0.72m/m, the total piezometric heads in the layers are summarized in Table 3.1.
30
M.Engz 2002 - 2003
Elevation
Stratum
-2.2m (wt)
Clay 1
-2.2m
-4.Om
Clay 1
-0.9m
-6.Om
Peat 1
-0.9m
-9.Om
Clay 2
1.25m
-10.6m
Peat 2
1.25m
Total Head
Table 3-1: Total head for each layer
3.2
Soil Parameters and in situ Stresses
The soil parameters were based in the parameters given in the report of the No-Recess
Project (Masterbroek, 1998). Calculations and correlations were done to find other parameters to
be able to analyze the unimproved embankment.
Stratum
Clay
Peat
Clay
Peat
Thickness(m)
3.1
2
2
1.6
wn(%)
66.4
98.5
120
131.6
wI(%)
160
267
160
267
wD(%)
81.9
186
81.9
186
suFV(kN/m 2) e
1.9
38
4.8
45
1.9
37
1.9
36
Stratum
Clay
Peat
Clay
Peat
cc
0.27
1.33
10.33
0.45
CR
0.093
0.23
0.113
0.155
RR
0.0093
0.023
0.0113
0.0155
c
0.017
0.034
0.011
0.01
c(m 2/year) k(m/day)
2
8.64E-05
8.64E-04
3
8.64E-05
2
8.64E-04
3
Table 3-2: Engineering properties from lab and field testing at No-Recess site
Stratum
0'(0)
Ko( 0 )
E(kPa)
V'
4
Clay
Peat
18
15
0.69
0.74
40
38
650
295
0.35
0.35
0
0
Clay
20
0.66
38
610
0.35
0
Peat
15
0.74
37
530
0.35
0
c'(kPa)
Table 3-3: Engineering design parameters used to model soil behavior
31
A Case Study on the Performance of Embankments on Treated Soft Ground
M.Eng 2002 - 2003
The drained young's modulus (E') and apparent cohesion (c') parameters were calculated
from the available soil data as follows:
Drained young's modulus.
.
By definition the one-dimensional modulus, D = -, where m, is the coefficient of volume
Mv
CR
compressibility M, = 0.435 ,' , and G'vave is the average vertical effective stress during the
loading event.
Equating D with the constrained modulus from elastic theory.
E'(l-v')
)
(1 +
(I + v')(1- 2V)
-
-045CR
, where CR is the measured compression ratio, u' is the possion's
0.43v5
(71'A
ratio, and E' is the drained young's modulus.
*
Apparent cohesion
The Mohr-Coulomb, the undrained shear strength su (in plane strain), can be related to the
drained shear strength parameters (c',$') as follows:
s, = c'cos#'+p'sin #'
where p'= -(1 + K,
2
)o',,, and
KO = coefficient of earth at rest
Assuming su is reliably measured in the field (by the field vane), a'v, is also well known,
and $' has been reported from laboratory tests (Masterbroek, 1998), then:
S
s14
cos '
p'sin#'
Cos b'
32
M.Eniz 2002 - 2003
Elastic stress analyses were used to estimate the vertical effective stresses in the subsoil
due to embankment loading at ground level.
Elevation(m) Stratum
G'p(kN/M2)_
2
32
Y(kN/m 3) u(kN/m2 ) ao(kN/m 2) a'ydkN/m2) Lab
-0.9
Clay1
14
-12.7
0
12.7
-1.2
-2.2
Clay 1
Clay 1
14
17.5
-9.81
0
4.2
21.7
14.01
21.7
-3
Clay
17.5
13.3
35.7
22.4
-4
Clay 1
17.5
30
53.2
23.2
44
-5
-6
-7
Peat 1
Peat 1
Clay 2
10.5
10.5
14.8
40
50
66.6
63.7
74.2
89
23.7
24.2
22.4
42
-8
Clay 2
14.8
83.3
103.8
20.5
50
-9
Clay
14.3
100
118.1
18.1
-10
Peat 2
11.8
110
129.9
19.9
136.98
20.98
116
11.8
Peat 2
-10.6
Table 3-4: In-situ stress at No-Recess site
&Vf(kN/m
Low
20
20
20
20
20
20
20
20
20
20
20
20
2)
-1D
High
O'f(kN/M2) -1D
High
Low
134
134
134
134
134
134
134
134
134
134
134
134
20
20
20
20
20
20
20
20
20
20
20
20
134
134
134
134
134
134
134
134
134
134
134
134
& 'f(kN/m 2) -2D
High
Low
33.1
34.3
35.5
36.5
38.9
42.6
46.3
48.9
51.4
53.8
54.8
55.9
133.8
133.4
133.0
131.9
131.2
130.6
130.1
129.9
129.8
129.0
128.0
126.9
18
96
G'A(kN/m 2) -2D
High
Low
45.8
48.3
57.2
58.9
62.1
66.3
70.5
71.3
71.9
71.9
74.7
76.9
Table 3-5: Vertical stresses due to embankment construction based on elastic theory
33
146.5
147.4
154.7
154.3
154.4
154.3
154.3
152.3
150.3
147.1
147.9
147.9
. 2002 - 2003
MEng
3.3
3.3.1
One-dimensional Settlement Predictions
Calculations for undrained, initial settlement
The calculations for the elastic settlement were based on the elastic theory of Janbu et al.
(1950) and corrected by Christian & Carrier (1978).
In the case of multi-dimensional
deformations and settlements for more complex layer structures and/or complex load
configurations, finite element programs employ realistic stress deformation relationships. The
elastic settlement are computed from
qB
EU
where q = applied load (considering the crest of the high embankment)
p and i are factors accounting for the foundation embankment depth and compressible layer
thickness, respectively.
q = applied load
B = width of the foundation
The undrained young's modulus, Eu, can be estimated approximately from the drained
modulus assuming vu = 0.5
3.3.2
Calculations for Consolidation Settlement
Calculations for consolidation settlement were based on the following equation:
o'vf~
a'p
Pcf=X Hi RR log , +CR log ,
a vo th
P
where Hi = thickness of the layer
34
M.Eng 2002 - 2003
RR = recompression ratio
CR = compression ratio
C'R = initial effective vertical stress
Y'vf= final effective vertical stress
T', = pre-consolidation pressure
3.3.3
Calculation for Secondary Compression
The calculation for the secondary compression were based on the following equations:
A = Ca
t
t,
log-
where ca = secondary compression index
t = duration of load
tp= end of primary
Tables and graphs based on simple differential equations usually predict consolidation
behavior under a fill area as a function of time. One-dimensional consolidation problems can be
solved using general calculations, but computer programs are required in the case of twodimensional consolidation problems.
In the case of one-dimensional consolidation, Terzaghi's consolidation theory explains how
saturated soils compresses with time under some restrictions such as
" the soil is homogeneous and saturated,
" the granular material and water are incompressible,
35
2002 - 2003
MEng
. n,202-20
" a linear relationship exists between compression and increasing of effective inter-granular
stress,
" the consolidation coefficient is constant throughout the consolidation process,
" the compression is small related to the thickness of the layer.
Su32u
,
&2
where, cv = KV
u = excess pore pressure (kPa)
t = time (sec)
c, = vertical coefficient of consolidation (m2/day)
z = coordinate in z direction, depth (in)
kv
vertical permeability coefficient (m/s)
mv= vertical coefficient of volume compressibility (m 2/day)
7w
T
unit weight of water (kN/m 3)
cAt
(H,1)2
where T = time factor
t = time duration (sec)
a = drainage constant, a
h
= layer thickness
=1
for one way drainage and a = 0.5 for two way drainage
(in)
The relationship between the average degree of consolidation U and time factor T can be
expressed graphically or by numerical expression.
36
-*-,*..-,~,
.. *..-,
C~i~i~
.
~tiuI~, nn
~
~"th~ Pprfr~rn-i~.np~.
.II
C.~L11s
mmemS U~
on II IV4L~U
eae 3UIL
S:>on ~IIUUIIU
Gwroud
~L~UIJNIEi~jIL~
%1%.~
~m
m,~j
M.Eng 2002 - 200.3
o~.
The end of consolidation happens when all excess pore pressure has totally dissipated, in
theory t = oo. For a practical end of consolidation, an average degree of consolidation U=90% is
widely used. Therefore, the end of primary can be calculated as:
0.848(Hd) 2
P
Cv
where, tp = end of primary time
cv = vertical coefficient of consolidation
Even though the calculations for one-dimensional consolidation can be analyzed
numerically, calculation for two-dimensional consolidation process requires computer programs.
A finite element program has the ability of giving the solution for plane strain consolidation
problem, simulation of a non-homogeneous soil structure, and force, displacement calculation
together with a flow rate or water pressure.
3.3.4
Total Settlement
The total settlement is calculated by adding the elastic undrained, primary and secondary
settlements. It is done by sub-dividing the soil profile into four layers, with soil parameters cited
in Table 3.2. Table 3.6 summarizes the hand calculations of one-dimensional settlements for the
high part of the embankment. The height of the fill assumed for this calculations is 6.7m and the
unit weight used is ytiI= 20kN/m 3 .
37
M.Eng 2002 - 2003
A Case Study on the Performance of Embankments on Treated Soft Ground
1-D High Embankment
_
p
1
Ea(kPa)
P,(M)
1
0.1
134
35
520
0.902
Layer
CR
RR
thickness(m)
a'yo(kN/m 2) a',(kN/m 2 ) a'p(kN/m 2 )
0.093
0.23
0.113
0.115
0.0093
0.023
0.0113
0.0115
3.1
2
3
1.6
21.7
23.7
22.4
19.9
Clay
Peat
Clay
Peat
q(kN/m 2 ) Bm)
__
_
pD(M)
121.7
123.7
122.4
119.9
31
42
50
96
0.176
0.227
0.144
0.030
t,(year)
8.66
8.66
8.66
8.66
log t/tQ0
0.540
0.540
0.540
0.540
p (m)
_0.577
Layer
Clay
Peat
thickness(m)
6.1
3.6
Hd
cv(m 2 /year)
tp(years)
5.55
3
8.66
Layer
Clay
Peat
Clay
Peat
co
0.017
0.034
0.011
0.01
eo
1.9
4.8
1.9
1.9
thickness(m)
3.1
2
3
1.6
t(years)
30
30
30
30
0.010
0.006
0.006
0.003
0.025
1.500
pt(m)=pe+pp+ps=
Table 3-6: One-dimensional calculations of settlement for the high embankment.
3.4
Simulation using Plaxis
The one-dimensional hand calculations suggest a total settlement of 1.50m.
Two-
dimensional analyses of the embankment can be studied using finite element program. As stated
earlier, in order to perform two-dimensional calculation, it is necessary to have the tools of a
finite element program. The FE program can also account for real time consolidation occurring
during phases of embankment construction. During the process of the dissipation of excess pore
pressure, the soil obtains the shear strength needed to continue the construction stage process.
38
3.4.1
Finite Element model
This part of the chapter covers the consolidation analysis of the embankment on soft soil.
Figure 3-2 illustrates the cross-section of the embankment. The embankment is 46m wide, and
6.7m high (this accounts for the actual height of the fill placed) with a side slope of 3:1 based on
the settlement vs. time graph in the No-Recess project report.
The embankment itself is composed of compacted sandy fill.
The subsoil comprises
9.7m of soft soil sub-divided into four layers. The underlying artesian sand layer is not included
in the model but is treated as a rigid base with prescribed piezometric head, H=+1.25m. The
model calculations assumed that phreatic surface coincides with the original ground surface, and
the total head calculations were based on the results of the piezometer located on the top of the
sandy layer. This model assumes a steady upward flow through the soft clay and peat layers.
Figure 3-1 summarizes the initial effective stresses and piezometric head in the FE model.
.39
A Cae Study
- Ca-s S
nfFmhnn1m~nrc
- th~ PPrfArm2ncP
- onn
An
Profile
~
Tr~t~A ~,-.44 fl..~-....A
r
'Ui VJuII
M.EJng
Effective Stresses(kPa)
2.0U02 - 201J3
Total Head(m)
-1
-2
-3
-4
Peat 1
CL -5
z
C
0
-6
E
(V
-7
-8
-9
Peat 2
-10
-
K
-
--
-11
o',(kPa)
a',.(kPa)
i
,
0
5
10
15
20
25 -1.5
-1.0
-0.5
Figure 3-1: Initial effective stresses and piezometric head in the FE model
40
0.0
0.5
1.0
1.5
A Case Studv on the Performance
of
Embankmet
onTetd9
rud
n Teatd Sot Goun
te Prfomanc
ofEmbnkmnts
A Cae Sudyon
- 2003
2002 -20.
M-Env
M~n 202
46m
27m
27m
Cs B
Say
11LR
I I
I
41
9 .7m
ClayII
1H-
-qn-H-
Figure 3-2 Cross-section of a road embankment on soft soil
The embankment was analyzed as a plane strain FE model using 15-node (cubic strain)
triangular elements. Lateral boundaries are set 27m from the embankment where deformations is
expected to be negligible, the lateral boundaries are also specified as closed flow boundaries,
imposing one-dimensional vertical flow conditions far from the embankment.
In the initial
conditions, the embankment must not be present in order to generate the initial conditions. The
suggested Ko-values of the layers were calculated based on Jaky's formula.
Ko =1 - sin #
3.4.2
Material Properties
The properties of the different soil types are given in the Table 3-7 where five material
sets have been created. The type of 'undrained behavior' as set up for the clay and the peat
layers enables simulation of excess pore pressures due to embankment construction.
41
A Ca~ ~tiidv
nn
rh' P~rf~rm~nr~. nf Fm
~
,~.,
A Case Stud .L nn th,- P,-rfnrm5,n,, ,,f Pmhnl, 111%,11 +a VIj
Parameter
Material Model
Type of behavior
Unit Weight (kN/m 3)
Permeability (m/day)
~
Q,..44
~
Ir IVIM11-A Q V 4 t-- JOU11 A
%A .U
1jr, Inn')
Fill
Clayl
Mohr-Coulomb
Drained
20
10
Mohr-Coulomb Mohr-Coulomb Mohr-Coulomb
Undrained
Undrained
Undrained
17.5
10.5
14.8
8.64E-05
8.64E-04
8.64E-05
Peati
Clay2
Infill
Peat2
Mohr-Coulomb
Undrained
11.8
8.64E-04
Young's modulus (kN/m2)2.50E+04
6.50E+02
2.95E+02
6.1OE+02
5.30E+02
Poisson's ratio
Cohesion (kN/m2)
0.35
1
0.35
40
0.35
38
0.35
38
0.35
37
Friction angle (degree)
35
18
15
20
15
0
0
0
0
Dilatancy angle (degree) 0
Table 3-7: Material properties of the road embankment and subsoil
3.4.3
Calculations
The embankment construction was simulated in 5 stages based on the reported sequence
used for the reference embankment (HW 1). Figure 3-2 shows that partial drainage was modeled
during each fill phase (using specified time increments for loading) with intervening
consolidation phases (no change in fill height).
'Undrained' - refers to the capability of modeling excess pore pressures within a soil layer in Plaxis; 'undrained'
materials can represent the full range of drainage conditions within the soil depending on the type of analysis and/or
time frame specified in the calculation steps.
42
M'Fncr'?00')
-')(InI
AY-9Af
n
A Case Studv on the Performance of Embankments on Treated Soft GroundM
Construction and Consolidation time(days)
807.5 7.06.5
6.0
5
-
5.5
5.04.5.
3
4.0-.
-3.52
i3.0 -
2.52.01.5.1.0 -
0.5 0.0 -0.5 0
50
100
150
200
250
300
350
400
450
500
550
650
600
700
750
800
850
900
950
1000
Time(days)
Figure 3-3 Time vs. height of fill
Long-term embankment behavior was examined by allowing consolidation to proceed
until the minimum excess pore pressure, Au=lkN/m*
A
4
5
B
1
+
t
+
+
+
t..44+
+
t
+ +
t
+
+
+
t/'
4.4
4..44
4.4
++494 4.4
4.4.4.4.4.4.4.4.4.4.4.~~~~~~~"1
+.4 +..44
.. 44
4.+
+
+. +4.4++
..+ +............
4
4
.
4
+..44
+..4
4
.
tI
4
.
.
t~~
+.4
+.
4.
+4
t 4.444
t4444444444444444444444444
4 ...
.. .
44
4.
...
.
1~
+..4
+..4
4
t..44
+.
'++
t..44
4
4.4
4.4
4.4
+..
44
A
B
Figure 3-4: FE meshes for unimproved embankment and location of 3 reference points on the ground surface
* The current analysis does not consider secondary compression and hence,
does not simulate measured creep
properties of the clay or peat layers.
43
A Case Study on the Performance of Embankments on Treated Soft Ground
A Cae Sudyon
Q~mbnkmntson
he erfrmane reaed Sft roud
I l
P -A2003
1M
MEne
)(A).
N 1~ 2002
Figure 3-5 clearly shows that the settlements of the original surface and the embankment
increase significantly during the last phase due to the dissipation of all excess pore pressures,
which cause consolidation of the soil.
It can be concluded that even though consolidation
already occurs due to the time interval needed for the construction of the embankment, there is
still a great amount of settlement occurring in the last phase (consolidation phase).
6
6
I
Figure 3-5: Total displacement after construction = 1.04m.
i
6
MV -Io
Figure 3-6 Total displacement after consolidation
I
= 1.62m.
44
A Case Study on the Performance of Embankments on Treated Soft Ground
Figure 3-7 illustrates the remaining excess pore pressures at the end of construction.
Au
indicates how much consolidation has occurred during construction.
So, it can be
concluded that 65% of consolidation occurred during the construction time.
Au = 87.OOkN/m2
Aav= Yt HfI = 134.OOkN/m 2 , therefore;
Au
Aoa
-
87
=65%
134
Figure 3-7 Excess pore pressures at the end of construction = 87.OOkN/m
2
Figure 3-8 illustrates the surcharge sequence used for FE analysis and settlement
behavior vs. time predicted in the centerline. Figure 3-9 shows the horizontal displacement
predicted in the toe of the embankment predicted during the construction and consolidation
phases.
45
A Cnctp 4Zn
n.n th- DPrfnrmn,.p
~
Jt~ r
4. P all
m kT1 1ILa mIs
t.f
A
iI
"6 2002
- 2003
ME
KA . 1C,AU1
Al'
IAl
Q,.A -'..JJUUEI
8
I--
6
E
4
CL
2
a)
0
0
-2
s
I er
-4
10
100
1000
Time(days)
10000
Figure 3-8: Settlement and Surcharge vs. time for the unimproved embankment
.
0
-2 4--
-4
E)
-6
-8
-- - - -
-
-e - Day 25
-0 Day 50
-0 Day 85
-e-Day 90
,A- Day 120
M- Day 130
-
-
-10 +-
--
-
y
-- -18--0
- 0-a ;
-0
-12
0.0
0.1
I
0.2
0.3
0.4
0.5
Day 5000
I
0.6
0.7
Horizontal Displacement(m)
Figure 3-9: Horizontal Displacement in the toe of the embankment for the unimproved embankment
46
A Case Study on the Performance of Embankments on Treated Soft Ground
M.Eng 2002 - 2003
E
E
E
OD0
0.2
0.4
0.6
08
1.0
1.2
1.4
15
1.8
Madimun Settlement (m)
Figure 3-10: Relationship between maximum horizontal displacement and maximum settlement
Figure 3-10 illustrates the relationship between maximum horizontal displacement and
maximum settlement for the unimproved embankment.
In this correlation for unimproved
embankment, significant drainage during initial loading causes less horizontal displacement than
predicted from undrained analysis (Ladd, 1991).
Figure 3-11 shows the excess pore pressures at three elevations where the piezometers
are installed such as EL.-7.5m, -5.0m, and -2.7m.
47
A Case Study on the Performance of Embankments on Treated Soft Ground
MEng
. nQ2002 - 2003
X0
100
---EL. -7.5m
--- A- EL. -5.OmA,
+ EL. -2.7m
80
E
z
e
60
(D
L..
L-
40
CL
0
20
Ch,
x
0
0.1
1
10
100
Time (Days)
Figure 3-11: Excess pore pressure at three elevations
48
1000
10000
A Cnupi
4
4.1
.L P~.rfnrm-ani-r rnf~ml n1,
" 111%111
Qtiidj nn thrnt
%il
a 'Tt %I 1A
Q %4-
uu
Jw
A
200
. Hr 2002
- InlJ
ME
InnJU
I.Ai
AJI~
Ground Improvement Options
Vertical Wick Drains
The primary consolidation of a compressive soft clay foundation may take place over a
long time depending on the drainage path length. At the HW site, Hd = 5.55m and hence, 90%
consolidation takes eight years and eight months to reach the end of primary consolidation (refer
to previous calculations). The purpose of installing vertical drains is to accelerate the primary
consolidation. When the soil has characteristics of stratified soil, in which the permeability is
larger in the horizontal than in the vertical direction, the vertical drains are more effective. At
the HW site the PV drains cannot be installed into the underlying sand due to artesian pressures
that exist in this layer.
Figure 4-1: Embankment with wick drains
49
A Case Study on the Performance of Embankments on Treated Soft Ground
M.Eng 2002 - 2003
The reason for not fully penetrating the consolidating layer to reach the permeable layer
is that the sand layer is an artesian layer (the total head in the sand layer is higher than that in the
compressible layer). As a result, the PV drains were installed to the depth 1m above the sand
layer (approximately to El. -8.7m).
The vertical drains are often installed in a triangular pattern at a spacing S in the range of
1 to 5m. The circular drains have normally the radius r, in the range of 80 to 300mm and the
radius re of the soil cylinder discharging water into a vertical drain is in an order of magnitude of
0.525S. To use wick drains with rectangular cross-section, the equivalent rw has to be calculated
such as
r (a+b)
IT,
where, a = thickness of the drain, typically values between 3.2 to 4.0mm
b = width of the drain, typically ranges from 93 to 100mm
D D
/Vertical
Drain
105 DS
Figure 4-2 Triangular pattern of installation of vertical drains (Terzaghi et al., 1996)
The discharge capacity of drains is limited in many cases due to some soil and drains
characteristics. The discharge capacity of the drains can be calculated as
q
71
k4
50
M.Eng 2002 - 2003
where qw = discharge capacity
kw= permeability of the drain
r= the equivalent radius
The discharge factor D was created to determine whether the water can flow freely or if
there is any well resistance.
This factor is dependent on the horizontal permeability of the
consolidating soil and the maximum drainage length of the drain (Terzaghi et al., 1996).
The factor D can be computed as
D =
"
k,, ,,
where q, = discharge capacity of the drain
1= maximum drainage length
kh= horizontal permeability of the consolidating soil
In the case of soft clay deposits, analyses of field performance point out that when D is
greater than five, the well resistance is no longer an important factor and the minimum discharge
capacity of the drains can be calculated as
q,(min) = 5k,.,72
where kho =the initial horizontal permeability
Since the minimum discharge capacity is calculated from the initial horizontal
permeability, the magnitude of discharge capacity changes as the permeability decreases during
51
ri. '...-a~.
31UUY
U II
LII I..~
I L~I ILJI *II~II'.A
Uk LIIIUaIINIII~IIL~
UII
I I~4L~U
~3UIL '~Ji'Juu*..z
A 1Pn
2)002 - 2)O3
consolidation due to the factor that less water penetrates the drains during a given time,
therefore: qw(min) gets smaller with decreases in permeability.
The effects of the smear zone in wick drains
4.1.1
A smear zone or cylinder of disturbed soil is created when the drains are installed (due to
displacement of the surrounding soil) which forms external radius rs around the drains. In the
smear zone, the compressibility increases, whereas the permeability and the pre-consolidation
pressure decrease. The equation below illustrates one-dimensional vertical compression together
with vertical and radial flow.
, CDZ u2
ar 2
where
au= Du
j(2u +I
+
ch
=
r
ar
at
, u is the excess pore pressure
Since the excess pore pressure u is a function of time as well as of vertical and radial
flow, the same result can be extracted by superimposing the solutions for vertical compression
by vertical flow and one by radial flow.
The degree of consolidation, at any time, can be
calculated as
U =1-(1- UzXl-Ur)
where, U, and Ur are the vertical and radial degree of consolidation, respectively
The excess pore water pressure can be defined, at any time, as a function of vertical and
radial flow.
U
uur
U.i
52
M.Eng 2002 - 2003
where u, and ur are the excess pore water pressure for vertical flow only and for radial flow only,
respectively.
The vertical flow is often ignored in situations where drainage occurs principally in the
horizontal direction. It is necessary to take into account the contribution of the vertical flow
when the height of the drains is small and the spacing is large.
rwr
at,DepthB
thouhot h
I
I
(
4zad
Flow -ine
Fni
compressible layer with fully penetrating vertical drains as
U= I
exp
53
e
e
f
s
a
M.Eng 2002 - 2003
where U = degree of consolidation for radial flow only
3
Fn = ln(n)--n , where n is greater than 10
4
n=
Cht
r
2
In expressions for vertical compression with radial flow into vertical drains for the smear
zone, the factor of radius as r, = sr,, has to be taken into account.
When the effect of smear zone is taken into account, the permeability is different from
the permeability of the undisturbed soil, whereas the compressibility is the same as that of the
undisturbed soil for the relationship between U and Tr. Permeability can be calculated as
ks = k
S2
Figure 4-4 illustrates the small effect of n in the relationship between U and T, whereas
for the rate of consolidation, n plays a great role due to its influence on r, = nr,. in the time factor
Tr.
54
M.Eniz 2002 - 2003
A Case Study on the Performance of Embankments on Treated Soft Ground
.
00
, . .
(b)
20
20 -
n & r,/rv 25
q, =q,(min)
s =4
K.
s , r, /rw 4
I0
80
ch t
f
Figure 4-4 Relations between degree of consolidation and time factor for radial flow into a vertical drain.
Loading is assumed to be instantaneous and excess pore water pressure constant with depth. (a) different
drain spacing; (b) different mobilized discharge capacity (Terzaghi et al., 1996)
4.2
Geotextile Coated Sand Columns
Ground improvement has been developed very rapidly to suit a wide range of ground
conditions and foundation problems. It is now a major area of geotechnical engineering and has
become increasingly popular as a cost effective alternative to piling in areas with highly
compressive soils. Treatment of compressible soils is designed to improve the load carrying
characteristics of the ground and to mitigate the total and differential settlement of the structures
subsequently constructed on the ground.
55
A Case Study on the Performance of Embankments on Treated Soft Ground
A Cae Sudyon
ofEmbnkmets
te Prfomanc
n Teate Sot GoundM
2002
M Fna
Fo
9~~. -'2003
Figure 4-5 Geotextile coated sand columns
The Geotextile coated sand columns technique involves the in situ joining of large
diameter, closely spaced, vertical sand columns within the compressible layers. This technique
was used in the No-Recess project (HW4) in order to stabilize the compressive soft soil and
minimize the long-term settlements. The geotextile confines the sand and acts as a filter to
prevent intermixing with adjacent clay and clogging. It also provides the stiffening effect to
ensure integrity of the sand piles. These features allow the columns to act as piles, considerably
mitigating settlement and deformations due to dynamic loads. Another advantage of the sand
column technique is its relatively rapid construction time.
Many foundation benefits have been found with the use of sand columns in soft soils
such as increasing the bearing capacity for overlying structures or embankments, accelerating the
consolidation process of the consolidating layer surrounding the granular columns, and
improving the load-settlement characteristics of the foundation (Davies, 1997).
56
2002 - 2003
MEng
. n,202-20
Inclusion of the columns reduces soil drainage path length, and increases the rate of
excess pore pressure dissipation. Installation of the sand column by displacement techniques
may minimize the soil permeability due to formation of the smear zones along the column
boundary, but since the column are closely spaced, the dissipation of the pore pressure can still
be achieved quickly (Bredenberg, 1999).
One of the major factors to be considered when
improvement of the treated foundation is to be predicted is the stiffness of the sand columns
relative to that of the soil in which they are installed.
The installation method consists of vibrating a steel casing into the ground to the load
bearing layers. The casing has two flaps at the bottom, which are forced closed during driving,
displacing the ground. The geotextile is connected to a funnel, which is put in the top of the
casing.
The tube is filled with sand through the funnel and the casing vibrated out, causing
initial compaction of the sand and stressing the geotextile (Nods, 2002).
Sand columns generally have a diameter in the range of 0.8m, and they are normally
placed in a triangular pattern with a spacing of 1.7m to 3.4m. In the case of the No-Recess
project (HW4), the columns had a diameter of 0.8m and were installed in a triangular grid in the
range of 2.4m to 3.4m for the high embankment. A geogrid is laid over the columns in a load
transfer platform in order to give extra horizontal stability and help to transfers horizontal
railway loads. To avoid large long-term settlement, surcharge is used to accelerate movements
(Nods, 2002).
57
A~q~-
(~
~itu~
nn th- Perfnrm-nnr
f
P
,
1 ,
t
rtA Q
-4
us
'ill
Figure 4-6 Process of installation of geotextile encased sand columns
58
w
sit
CAUI 1
aIUIU
vI.EAng 2002
-003
A Case Study
5
on
the Performance of Embankments on Treated Soft Ground
M.Ene 2002 - 2003
FE Simulations of No-Recess Instrumented Embankments
This chapter compares results of finite element simulations using the Plaxis program, with
measured performance for two of the embankments constructed at the No-Recess (HW) site: (1)
the conventional design with PV wick drains (HW 1), and (2) the geotextile coated sand columns
(HW4).
5.1
Conventional Design, HW1
The engineering properties and the design parameters used to model the PLAXIS finite
element code analysis in these two cases are identical to the case of the unimproved embankment
(chapter 3). Figure 5-1 illustrates the imposed surcharge sequence and the centerline ground
surface predicted settlement for the embankment with wick drains. This figure also shows the
good accordance between the measured settlements and those predicted by Plaxis model.
8
sc l
_I I I I I II
6
z
4
I
111H
2
CU
0
i
I
I
i
I
III
.
.
.
,
i
I
i
!
1
I
.
.
.
I I III
-0-U-U-
I
III
-2
- I
~FTTTTii
I II
-4
10
I L.. I
100
settleme t
1000
Surcharge(m)
Settlement(m)-Predicted
Settlement(m)-Measured
liii
If
10000
Time(days)
Figure 5-1: FE model of surcharge vs. time and comparison between measured and predicted ground
settlements for HW1.
59
A Case Study
on
the Performance of Embankments on Treated Soft Ground
M.Eng 2002 - 2003
The total vertical displacement calculated for the high embankment with vertical wick
drains by the FE model was 2.15m. The settlement predicted by Plaxis in figure 5-1 shows
agreement with that measured in the field. This large amount of settlement in the first 4 months
is due to the wick drains, which help to accelerate the dissipation of pore pressure, thus
accelerate the settlement rate.
The development of the horizontal displacement is shown in figure 5-2. The predicted
horizontal displacements by FE model are not exactly plotted on the same days that the measured
results are plotted, but one can see that the predicted results have a small deviation from those
measured in the field.
The largest horizontal displacement occurs around the EL.-4m, which also happens to be
the same behavior with that measured in the field. Figure 5-3 shows the correlations between
horizontal displacement and vertical settlements that can provide useful insight regarding the
relative importance of undrained versus drained deformations within embankment foundations
(Ladd, 1991). In the HWl, the pore pressure was measured at three elevations (-2.7m, -5.0m,
and -7.5m).
Figure 5-4 illustrates the predicted excess pore pressure dissipation in those
elevations. In the case of vertical drains, the highest pore pressure dissipation is at EL.-7.5.
60
. 2002 - 2003
MEng
0-
-2
L
S-6
K-
8A-8-10
-
--
-------
-
----
--
7
-
Day 90 (Predicted)
- Day 180 (Predicted)
- Day 840 (Predicted)
Day 98 (Measured)
-
-U-Day
182 (Measured)
-4-- Day 850 (Measured)
-12
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
Horizontal Displacement(m)
Figure 5-2: Comparison between horizontal displacement predicted and that measured in the field for HW1.
2.-5
-
E
C
a)
E2.0
U
CL)
(U
E
1 0.0
2
0.0
0.1
0.2
0.3
0.4
0.5
Maximum Settlement (m)
Figure 5-3: Relationship between maximum horizontal displacement and maximum settlement for HWJ.
61
A Case Study
on
the Performance of Embankments on Treated Soft Ground
Eng 2002
efrac
rae h otGon
fEbnmnso CaeSuyo
A~~~~~~~~~~~~~~~~~~
-
2003
80
60
N)
E.
I-,-
a)
V~
a-) 40
a)
0)
-
EL -7.5m (Predicted)
-s-- EL -5.0m (Predicted)
- EL -2.7m (Predicted)
20
-f
0
10
100
1000
10000
Time (Days)
Figure 5-4: Excess pore pressures dissipation at three elevations in HW1
5.2
Predictions for Geotextile Coated Sand Columns (HW4)
In the Plaxis simulation of the geotextile coated sand columns (GCS columns), the
properties used for the sand columns were those used for the fill. Since the last meter of the sand
columns is filled with soft material, the sand columns in the Plaxis model were set up Im above
the sand layer, leaving this last meter of original soil (peat). By using geotextile coated sand
columns a reduction of the settlement of 1.05m is achieved under the high section of the
embankment, relative to the embankment with wick drains predicted by FE model.
Figure 5-5 illustrates imposed surcharge sequence and the comparison between predicted
settlement and measured settlement in the field. The results accomplished by FE model are close
62
2002l
MA 1n
1n gA ?A
A Case Study on the Performance
off Embankments
on
Sft Grud
Goun
on Treated
reatd Soft
Ebankent
n th Peformnce
A Cae Stdy
- 20M3
to those measured in the field. In the case of sand columns, no surcharge was used for the high
embankment, only the weight of the embankment.
6
surch rge
4
E
0
2
I
I II
U,
-0
Settlement(m)-Measured
0
-
-2
Surcharge(m)
I II I--Settlement(m)-Predicted
I I III
I
-4
1
10
100
1000
10000
Time(days)
Figure 5-5: Imposed surcharge sequence and comparison of predicted settlement by plaxis and that measured
in the field.
Figure 5-6 illustrates the results from the predicted horizontal displacement and that
measured in the field. The results are compared on different days but they are close enough to
show a small deviation between them. The predicted horizontal displacement is bigger than that
measured in the field. This deviation can be due to the soil stiffness provided by the geotextile
coated sand columns.
63
M Fna)2002 - 2IW3
A Case Study on the Performance of Embankments on Treated Soft Ground
0
-2
-C
-6
-
~
--
-8
-A
0
-10 -
-
-+-
-,-
Day 50 (Predicted)
Day 80 (Predicted)
Day 840 (Predicted)
Day 57 (Measured)
Day 82 (Measured)
Day 850 (Measured)
12
0.00
0.05
0.10
0.15
0.20
0.25
0.30
Horizontal Displacement(m)
Figure 5-6: Horizontal displacement predicted by Plaxis
Figure 5-7 gives the relationship between the maximum horizontal displacement and the
maximum settlement in the high section of the embankment (HW4).
Figure 5-8 shows the
predicted pore pressure dissipation. The largest pore pressure dissipation in the case of sand
columns also occurs at EL. -7.5, but it is not too large due to the rigidity of the sand columns.
64
A Case Study
on the
Performance
Embankments Cas
on Treat
SotGon
d
S vothPefracofEbnmnsoTraeSotGud
~~~
A~~~~~~~~~
~ ~ of~
n1
InV 2003
NA~~~~
2002
MEn
0.25 E
0
0.20
0
CO
0.15 -
0
N
0.10 -
-- -- ------
- - -
- - -
0
a:
E
--
0.05
-
-
E
0.00 0.0
a)
I
0.2
0.4
0.6
0.8
I
1.0
Maximum Settlement (m)
Figure 5-7: Relationship between the maximum horizontal displacement and maximum settlement in HW4
20
15
-A-
E)
-U-
U)
CD
L)
0
0)
Ln
10
*
I
m---.in Vi
EL. -5.0 m (Pre dicte d)
EL. -7.5m (Pre dicte d)
EL. -2.5 m (Predicted)
5
aO
0
10
100
1000
10000
Time (Days)
Figure 5-8: Predicted excess pore pressure in three locations beneath the high embankment (HW4)
65
M.Eng 2002 - 2003
6
Conclusion
In the Netheilands the western part of the country consists of soft, very compressible soils --
peat and soft clay. In this area, the organization HSL-South is involved in the construction of a
new high-speed railway link between Amsterdam and Brussels. Short construction time and
stringent requirements on long-term settlements are important issues in the design of the
construction of embankments overlying soft soils.
Since conventional railway embankments
would not be appropriate regarding these requirements, the No-Recess project was created to test
methods for stabilizing soft ground behavior. The five ground improvement trials demonstrate
successful applications of recently developed geotechnical techniques.
This thesis analyzes one unimproved embankment test and two of the five reinforced
embankment trials in order to provide a basis for evaluating the performance of the five ground
improvement solutions. Calculations and engineering judgment were employed to develop the
proper parameters for the analysis of the Hoeksche Waard site conditions. It was clear from the
one-dimensional calculations that the embankment would have lateral spread, therefore a
computer program was necessary to calculate the two-dimensional analysis.
The results of the settlement, horizontal displacement, and pore pressure dissipation were
compared between the three embankments calculated by Plaxis model and the two improved
embankment trials used in the HW site. The results from the Plaxis model fit well in most of the
measurements done in the field. The pore pressure dissipation in HW4 is not too high due to the
rigidity of the sand columns, which prevent the soft layer from compressing, thereby preventing
the water from escaping so easily.
The large settlement achieved with the conventional embankment (wick drains) is due to the
vertical drains, which accelerate the dissipation of the pore pressure.
66
Therefore, the large
A Case Study on the Performance of Embankments on Treated Soft Ground
- 2003
MEn
.Fn 2002
202-20
amount of settlement is achieved much faster than with the other ground improvement options.
In the case of the geotextile coated sand columns, the settlement achieved was much smaller than
that with wick drains. The geotextile casing limits the movement of the sand, making the sand
stiffer than the surrounding soil, which allows the transfer of the embankment load to the bearing
layer.
In conclusion, the results achieved may help to validate the methods used in the design
project. No-Recess ground improvement options are offered to develop better ground
improvement techniques in soft soils, where construction and residual settlement are important
issues.
67
A Case Study on the Performance of Embankments on Treated Soft Ground
M.Eng 2002 - 2003
References
[1] Andrawes, K. Z. et al. (1983). "The behavior of a geotextile reinforced sand loaded by a
strip footing." Improvement of Ground,vol.1, A.A.Balkema, 329-334.
[2] Balasubramaniam, A.S. et al. (1994). "Embankment on sand columns: Comparison between
model and
field." Predictions versus Performance in Geotechnical Engineering,
A.A.Balkema, 167-173.
[3] Barends et al. (1999). "Geotechnical Engineering for Transportation Infrastructure", A.A.
Balkema.
[4] Batereau, C. et al. (1983). "Improvement of ground through the application of geotextiles."
Improvement of Ground,vol.2, A.A.Balkema, 459-462.
[5] Bredenberg, Hakan et al. (1999). "The performance of stabilized soil columns in two Dutch
testsites." Dry Mix Methodsfor Deep Soil Stabilization,A.A.Balkema, 239-244.
[6] Charles J. A. (1997). "Treatment of compressible foundation soils." Ground Improvement
Geosystems, Thomas Telford Publishing, 109-119.
[7] Craig W. H. and Z. A. Al-Khafaji (1997). "Reduction of soft clay settlement by compacted
sand columns." GroundImprovement Geosystems, Thomas Telford Publishing, 218-224.
[8] CUR (1996). Building on Soft Soils, A.A.Balkema.
[9] CUR (2001). EvaluatieNo-Recess - testbanen Hoeksche Waard, Data CD.
[10] Edil, T.B. (1983). "Improvement of peat: a case history." Improvement of Ground, vol.2,
A.A.Balkema, 739-744.
[11] Holm, G. (1983). "Lime columns under embankments - a full scale test." Improvement of
Ground, vol.3, A.A.Balkema, 909-912.
68
M.Enji 2002 -_2003
[12] Ilander, A. et al. (1999). "Eurosoilstab - Kivviko test embankment - Construction and
research." Dry Mix Methods for Deep Soil Stabilization, A.A.Balkema, 347-354.
[13] Jamiolkowski, M. B. et al. (1983). "Behavior of oil tanks on soft cohesive ground improved
by vertical drains." Improvement of Ground,vol.2, A.A.Balkema, 627-632.
[14] Jansen, H. L. (1997). "Eurosoilstab Test Site's Gravendeel NL Subsoil Characterization and
Soil Parameters." Fugro Ingenieursbureau B.V.
[15] Ladd, Charles (1991). The Twenty-Second Terzaghi Lecture, American Society of Civil
Engineers.
[16] Lambe, T. William and Robert V. Whitman (1969). Soil Mechanics, John Wiley & Sons,
Inc.
[17] Masterbroek (1998). "Soil Investigation No-Recess test site Hoeksche Waard report." Fugro
Ingenieursbureau B.V.
[18] Nods, Max (2002). "Put a sock on it." Ground Engineering
[19] Ovesen, N. K. et al. (1983). "Centrifuge tests of embankments reinforced with geotextiles on
soft clay." Improvement of Ground, vol.1, A.A.Balkema, 393-398.
[20] Terzaghi, et al. (1996). Soil Mechanics and engineering Practice. John Wiley & Sons, Inc.
[21] Wu C. S. and C. Y. Shen (1997). "Improvement of soft soil by geosynthetic-encapsulated
granular column." Ground Improvement Geosystems, Thomas Telford Publishing, 211-217.
69