Experimental investigations on ultimate bearing capacity of peat stabilized by a group of...

RESEARCH PAPER

Experimental investigations on ultimate bearing capacity of peatstabilized by a group of soil–cement column: a comparative study

Ali Dehghanbanadaki • Kamarudin Ahmad •

Nazri Ali

Received: 11 July 2013 / Accepted: 20 April 2014

� Springer-Verlag Berlin Heidelberg 2014

Abstract The aim of this paper was to determine the

ultimate vertical bearing capacity of rectangular rigid

footings resting on homogeneous peat stabilized by a group

of cement deep mixing (CDM) columns. For this purpose,

a series of physical modeling tests involving end-bearing

and floating CDM columns were performed. Three length/

depth ratios of 0.25, 0.5, and 0.75 and three area

improvement ratios of 13.1, 19.6, and 26.2 % were con-

sidered. Bearing capacity of the footings was studied using

different analytical procedures. The results indicated that

compared to unimproved peat, the average ultimate bearing

capacity (UBC) improvement of floating and end-bearing

CDM columns were 60 and 223 %, respectively. The

current study found that simple Brom’s method predicted

the UBC of the peat stabilized with floating CDM columns

with reasonable accuracy, but underestimated the UBC by

up to 25 % in the case of end-bearing CDM columns.

Published laboratory experiences of stabilizing soft soils

using soil–cement columns were also collated in this paper.

Keywords Bearing capacity � Cement column � Failure

patterns � Peat soils � Stabilization

Abbreviations

CDM Cement deep mixing

UBC Ultimate bearing capacity

OPC Ordinary Portland cement

List of symbols

B Footing width

l Column length

h Height of the box

cuc Undrained shear strength of the column

cus Undrained shear strength of the soil

quc Unconfined compression of the column

R UBC reduction factor

a Area improvement ratio

cc Compression index

cr Recompression index

k Constant coefficient (5.5)

qmin Lower bound of UBC

qmax Upper bound of UBC

1 Introduction

The past 30 years have seen increasingly rapid advances in

the field of soil improvement techniques. Among numerous

techniques for improving soft soils, cement deep mixing

(CDM) method has been used extensively to increase

bearing capacity and decrease settlement [15, 20, 25, 27,

34, 41, 49, 51]. This method is an economical process that

improves weak ground to enable it to resist low-to-mod-

erate loading conditions. This technique was originally

developed in Japan and Sweden and widely used for sta-

bilization of peat soils over the last three decades [16, 17,

23, 26, 34, 37, 39, 43, 45]. In practice, cement is injected

and mixed with the surrounding soil using a pumping

system, in the form of slurry (wet mixing) or powder (dry

mixing) to produce stronger and firmer ground namely

soil–cement columns [3, 4, 13, 31, 39]. Depending on the

A. Dehghanbanadaki (&) � K. Ahmad � N. Ali

Department of Civil Engineering, University Technology

Malaysia, 81310 Johor Bahru, Johor, Malaysia

e-mail: [email protected]

K. Ahmad


N. Ali


123

Acta Geotechnica

DOI 10.1007/s11440-014-0328-x

project specifications, CDM columns can be applied in

different configurations such as single, panel, block, and

grid. [19].

An extensive amount of theoretical and experimental

work has been reported on the bearing capacity of

improved soil by a group of CDM columns [5–7, 12, 14,

22, 28, 32, 33, 38, 42, 43, 46, 47]. Several physical mod-

eling tests were performed considering influential factors

such as undrained shear strength of the soil and the col-

umns, rigidity of the foundations, area improvement ratios,

and column preparations and installation techniques. All

physical modeling has focused only on soft clays, whereas

geotechnical characterizations of peat soil are considerably

different from inorganic soils. These soils are produced by

the disintegration and decomposition of plants under

waterlogged conditions and are problematic in terms of

their low strength, high compressibility, and high organic

content [24, 29, 35, 36, 44]. Peat soils can be classified

according to their particle size distribution, botanical ori-

gin, degree of decomposition, water content, and fiber

types. Von post [53] classified this soil into ten groups

ranging from H1 to H10. In this classification system, H1

indicates a fully fibrous and undecomposed soil, while H10

represents completely amorphous soil as shown in Table 1.

Due to its composition, the engineering properties of

peat are very site-dependent, exhibiting considerable

change over short distances and depths [23]. The hollow

cellular structure of these soils has been found to have an

adverse effect on its compressibility and bearing capacity.

The high organic content of peat tends to impede the

cementation process which attenuates the bearing capacity

of CDM columns [21]. Thus, peat is widely regarded as the

worst foundation soil for supporting man-made structures,

and it is not safe to anticipate the ultimate bearing capacity

(UBC) of CDM columns in peat the same as clays.

Therefore, the main objective of this paper was to deter-

mine the UBC of small-scale peat stabilized by a group of

end-bearing and floating CDM columns with different area

improvement ratios. The failure patterns and comparison of

UBC with different analytical methods are also examined.

2 Experimental procedure

2.1 Geotechnical characterizations of peat

Disturbed and undisturbed peat samples were collected

from Pontian (located in Johor—Malaysia) at a depth of

about 1 m. A series of field and experimental tests such as

classification, liquid limit, plastic limit, permeability,

consolidation, organic content, fiber content, and undrained

unconsolidated triaxial tests were conducted to determine

the geotechnical characterizations of the peat. These tests

were carried out using the American Society of Testing

Material committee (ASTM) [1] and the British Standard

(BS 1377, 1990) [8], and the results will be discussed in the

following sections.

In order to evaluate the undrained shear strength of the

soil, vane shear tests (VSTs) were performed at different

locations using 50 mm diameter and 100-mm-height field

vane in accordance with ASTM D 2573. This method is a

simple, rapid method for the determination of undrained

shear strength. Due to the fibrous structure of peat, VST

will often give misleading results and overestimate the

undrained shear strength of peat and should be interpreted

with great caution. In this study, a cone penetration appa-

ratus was used to determine the liquid limit of peat in

accordance with BS 1377, 1990, Part 2.

2.2 Soil preparation

A series of small-scale model tests were conducted using a

rigid rectangular tank 300 mm by 200 mm in area and

350 mm in depth. The tank was made of 20-mm-thick

acrylic plates. All sides of the box were tightly fixed to

prevent lateral movements during consolidation and load-

ing. In order to reduce the effect of side-wall friction,

lubricating oil was smeared on the inner side of the walls.

The size of the tank was large enough to accommodate

columns arrangement, so that there would be no interfer-

ence between the walls of the tank and the failure zone of

CDM columns. Prandtl [40] stated that the critical distance

needed from the edge of the footing depends on the width

of the footing and friction angle of soil. As a result, due to

the undrained condition of the peat (/ = 0), the critical

distance needed from the edge of the footing should at least

be equal to the width of the footing (B). Therefore, in this

study, this critical distance was selected as 1.5 B.

Table 1 Peat soil classification by Von Post [52]

Degree of

humification

Plant structure Decomposition

H1 Easily identified None

H2 Easily identified Insignificant

H3 Still identifiable Very slight

H4 Not easily identified Slight

H5 Recognizable but vague Moderate

H6 Indistinct Moderately

strong

H7 Faintly recognizable Strong

H8 Very indistinct Very strong

H9 Almost not

recognizable

Nearly complete

H10 No discernible Complete

Acta Geotechnica

123

The peat was air-dried under laboratory conditions. Only

peat passing a 2.00-mm sieve was mixed thoroughly at

natural water content of 495 % and was poured into the tank

for the consolidation process. A rigid rectangular steel

plate 198 mm in width, 298 mm in length, and 20 mm in

thickness was utilized for consolidation. A geotextile layer

was used for drainage at the bottom of the tank. The con-

solidation pressure was applied continuously and uniformly

by means of a pneumatic cylinder. The aim of consolidation

process was to achieve a homogenous peat with average

undrained shear strength of 10 kPa. Therefore, vertical

stress was applied gradually from 2 to 15 kPa over a period

of 4 days. The consolidation was under a two-way draining

condition. In the consolidation process, next stress incre-

ment was applied when the linear variable differential

transducers (LVDTs) showed no vertical displacement.

When the consolidation came to an end, a VST was per-

formed on the other tank at the same conditions at a depth of

50 mm below the surface to compare the undrained shear

strength of the peat with a desirable value of 10 kPa.

2.3 Model design

In practice, the diameter of the single CDM column varies

from 0.5 to 2.1 m, while the lengths range between 10 and

30 m [18]. The height of the model was decided by consid-

ering the geometry of CDM columns in practice. By

assuming the length and the diameter of the CDM columns as

12 and 1.5 m, respectively, the height of the model and the

diameter of the column were designed conveniently to be

200 and 25 mm, representing a linear scale factor of 1/60.

One of the most important factors in designing CDM

columns is area improvement ratio (a = area of the col-

umns to the total area) which depends on the project

specification. Practically, a range of 10–30 % for area

improvement ratio was proposed for common treatments

[16]. Hence, in this research, three area improvement ratios

of 13.1 % (4 columns), 19.6 % (6 columns), and 26.2 % (8

columns) were chosen for the CDM columns in the

experiments. The columns 25 mm in diameter were laid

out in a rectangular pattern. The center-to-center spacing in

a row was 40 mm, and the spacing between rows was 100,

66.7, and 50 mm.

2.4 Column installation method

Unlike the work conducted by Yin and Fang [28] and

Rashid [42] in which the CDM columns were constructed

and cured out of the soil, the CDM columns in this study

were installed and cured inside the soil using the continu-

ous replacement method. A thin open-ended steel pipe with

an internal diameter of 25 and 0.8 mm thickness was

pushed slowly into the peat at predetermined locations and

depths using wooden frames. A thin layer of grease was

applied on both the inner and outer surface of the steel pipe

prior to insertion to decrease the friction between the pipe

and the peat. Then, the peat-filled pipe was removed slowly

from the tank to create holes. Subsequently, the peat was

homogenized and mixed with cement (OPC) for 5 min at

the typical dosage of 300 kg/m3 by the mass of wet peat to

construct the cement–peat columns. Finally, the cement–

peat mixture was placed in the holes to make a composite

foundation. It should be noted that cement–peat mixture

was not pressed during its placement in the hole. The

procedures were repeated until all columns were completed

to the appropriate length. According to several laboratory

unconfined compression strength tests on cement–peat at

different curing times, it was revealed that the 14 days

undrained shear strength of the cement–peat was only

65 % of the strength at 28 days. Thus, it was decided to

cure the stabilized peat specimens for 28 days.

2.5 Loading procedure

In the loading stage, a rigid steel rectangular footing with a

length of 200 mm, a width of 75 mm, and a thickness of

20 mm was utilized to represent a rigid body resting on the

stabilized soil. In this study, the loading procedures were

conducted under the stress control conditions with an

increment of 1 kPa per minute. This approach allows direct

control of the stress. Displacement of the footing was

recorded by two LVDTs placed on opposite sides across

the center of the rectangular footing. The geometry of the

test setup and configuration of the CDM columns are

shown schematically in Fig. 1.

2.6 Testing program

A total of 13 physical modeling tests consisting of one

unimproved peat, three peats improved with end-bearing

CDM columns, and nine peat improved with floating CDM

columns tests were conducted as listed in Table 2. Analysis

was carried out to determine the UBC of the stabilized

ground under vertical loading. In addition, to increase the

reliability of measures, several tests comprised of uncon-

fined compression strength and VST were performed on the

other box under the same conditions before loading. In

addition, unconfined compression strength tests were con-

ducted separately on CDM columns based on BS 1377,

1990, Part 7: Section 7 to determine the undrained shear

strength of the column materials. In Table 2, n indicates the

number of CDM columns, TS-01 represented the unim-

proved soil while TS-02, TS-03, and TS-04 indicate soil

improved by end-bearing CDM columns, and TS-04 to TS-

13 denoted the soil stabilized by floating CDM columns. In

the tests with floating columns, the ratios of the length of

Acta Geotechnica

123

the CDM columns to the height of the soil (l/h) considered

were 0.25, 0.5, and 0.75.

3 Prediction of UBC

3.1 Experimental methods

The experimental evaluations of the UBC for both the

unimproved peat and peat improved by floating CDM

columns were determined based on classical double tan-

gent method. In this method, the UBC is obtained at the

intersection of two tangents, one at the beginning and the

other at the point of the plot when three successive equal

incremental loads result in increasing incremental settle-

ment in the log–log plot (vertical stress against displace-

ment graphs). On the other hand, the UBC of the peat

improved by end-bearing CDM columns was determined

based on the peak points (failure points) of the experi-

mental curves.

100 mm

50 mm

40 mm

75 mm 75 mm

25 mm

40 mm

66.67 mm

LVDT1LVDT2

Rigid plate Geotextile

CDM column

Footing

Datalogger

Drainage outlet

Load cell

(a)

(b)40 mm

200 mm

300 mm 300 mmB=75 mm

200 mm

Fig. 1 a Test setup. b Column configuration

Table 2 Test conditions and relevant parameters

Tests Test

condition

n l (mm) a(%)

l/h

TS-01 Unimproved 0 0 0 0

TS-02 End-bearing 4 300 13.1 1



TS-05 Floating 4 75 13.1 0.25

TS-06 Floating 6 150 19.6 0.25

TS-07 Floating 8 225 26.2 0.25

TS-08 Floating 4 75 13.1 0.5

TS-09 Floating 6 150 19.6 0.5

TS-10 Floating 8 225 26.2 0.5

TS-11 Floating 4 75 13.1 0.75

TS-12 Floating 6 150 19.6 0.75

TS-13 Floating 8 225 26.2 0.75

Acta Geotechnica

123

3.2 Analytical method

The majority of the existing analytical methods for the

determination of UBC of stabilized soil with a group of

cement columns were mainly dependent on the strength

properties of the columns. In this study, different analytical

methods were used to predict the UBC of the stabilized

peat. Preliminary work on the bearing capacity of improved

soil with column-like elements was undertaken by Broms

[10]. In the case of end-bearing CDM columns, the UBC

was determined based on two different methods suggested

by Broms [11] as shown in Eqs. (1) and (2).

qu ¼ 0:7quc:aþ k 1� að Þ:cus Broms� að Þ ð1Þ

qu ¼ 0:7quc:aþ k 1þ b=l

� �:cus Broms� bð Þ ð2Þ

where quc and cus are the unconfined compression strength

of the column and undrained shear strength of the soft soil,

respectively, while b and l are the dimensions of the

footing. On the basis of the experimental and theoretical

investigations conducted by Bergado et al. [2], k is taken as

5.5. In addition, a is the area improvement ratio. In the case

of soil improved with floating CDM columns, the UBC was

taken as the stress corresponding to a settlement equal to

20 % of the CDM column diameter based on Broms-c

method [9].

The results of this study were compared to a lower

bound (qmin) and upper bound (qmax) of the UBC of soft

soil improved by a group of CDM columns as established

by Boussida et al. [7] and Boussida and Porbaha [5, 6].

However, their evaluation of analysis had a number of

limitations. Their approach was based on the yield design

theory and assumed that the unimproved soil and the CDM

columns were deemed to have the same unit weight and

purely cohesive materials. According to their theory, the

lower and upper bound of UBC can be estimated by Eqs.

(3) and (4), respectively, and kc indicates the cohesion ratio

based on Eq. (5).

qmin ¼ cus 4þ 2a kc � 1ð Þf g ð3Þ

qmax ¼ cus 2ffiffiffi2pþ 2

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ a kc� 1ð Þ½ � 2þ a kc� 1ð Þ½ �

pn oð4Þ

kc ¼ cuc=cusð5Þ

in which cuc and cus are undrained shear strength of column

and soil, respectively, and a is area improvement ratio.

As mentioned by Terzaghi and Vesic [50, 52], the the-

oretical UBC of the rectangular footing rested on the

cohesion soil was considered based on Eq. (6). Further-

more, settlement and bearing capacity analysis of stabilized

soil using column-like elements are usually performed with

the assumption that the composite soil has mean weighted

shear strength as indicated in Eq. (7). Consequently, in this

research, a comparison in terms of UBC was performed by

introducing the reduction factor (R). This is defined as the

ratio of the UBC of different methods to the proposed

Terzaghi and Vesic method [50, 52] which was calculated

based on the homogenized cohesion. The reduction factor

has the potential to compare and anticipate the UBC of peat

improved by end-bearing CDM columns with the well-

known Terzaghi and Vesic [50, 52] equations and can be

expressed as Eq. (8).

qu ¼ 5:7c� 1þ 0:2b

l

� �ð6Þ

c� ¼ cuc:aþ 1� að Þ:cus ð7ÞR ¼ UBCðDifferent methodsÞ=UBCðHomogeneous methodÞ ð8Þ

where qu is UBC of improved soil based on Terzaghi and

Vesic method, c* indicates homogenized cohesion, and

b and l are the width and length of the rectangular footing.

cuc and cus are the undrained shear strength of column and

soil, respectively, and a is the area improvement ratio.

4 Results and discussion

4.1 Soil properties

Visual inspection showed that the peat was dark brown in

color with a pasty texture. The peat was considered as H3

according to the von Post System based on its degree of

humification. An average fiber content of 80 % indicated

that the peat could be classified as fibrous peat. Grain size

analysis of the peat was carried out according to BS 1377,

1990., Part 2, and the corresponding curve is displayed in

Fig. 2. The average geotechnical properties and chemical

composition of the cement used in this study are summa-

rized in Tables 3 and 4. It can be noted by the values

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10

Per

cent

fin

er (

%)

Particle size (mm)

Fig. 2 Particle size distribution curve of fibrous peat

Acta Geotechnica

123

reported in Table 3 that the soil was significantly com-

pressible with a high moisture content of 495 % indicating

that the peat had a very high water-holding capacity.

4.2 Model test results

As is well known, the UBC of improved soil by column-

like elements mainly depends on the following: (1) area

improvement ratio, (2) shear strength of soil and col-

umn, (3) end-bearing conditions, (4) column spacing,

and (5) boundary conditions. In this research, the UBC

of the reinforced soil is defined by a dimensionless ratio

of bearing capacity factor (BCF) as expressed by Eq.

(9):

BCF ¼ qu=cusð9Þ

In the Eq. (9), qu is the UBC of stabilized peat, and cus is

the undrained shear strength of the soil. It should be noted

that the peat in this study was in a nearly undrained

condition since the loading was fast and drainage valves

were closed during loading. Table 5 presents the results of

the undrained shear strength of the soil (cus) which was

determined by in situ VSTs, and correspondingly, the

undrained shear strength of CDM columns (cuc) was

directly obtained from unconfined compression strength

tests on stabilized peat specimens.

As shown in Table 5, it is apparent that the undrained

shear strength of the peat varied between 8.8 and 10.3 kPa,

while the undrained shear strength of the CDM columns

was in the range of 78.3–89.3 kPa. In earlier works on the

deep mixing method, many researchers have found that the

cohesion of cement–clay was very high (up to hundred

times or more) compared to the undrained cohesion of soft

clay [19, 28, 42]. However, the results of this study showed

that the undrained shear strength of cement–peat was only

nine times that of unimproved peat. The lower shear

strength of the cement–peat mixture compared with a

cement–clay mixture is due to the decrease in the effi-

ciency of the reactions with the cement and less solid

particles in peat. This is because the physicochemical

properties of peat soils are significantly different from the

clays. Furthermore, higher water-to-cement ratio and high

organic matter in peat tend to retard hydration and reac-

tions of chemical stabilization process, which decreases the

undrained shear strength of cement–peat.

To eliminate the scale effect, the vertical displacement

of the footing was normalized by the width of the footing

following Boussida et al. [7] which presented physical

Table 3 General characterization of the peat

Item and standards Results (average)

Classification (ASTM 5715-00) Fibrous

Classification (Von post) H3

Moisture content % (BS 1377, 1990., Part 2) 495

Liquid limit (BS 1377, 1990., Part 2) 260

Plastic limit (BS 1377, 1990., Part 2) NP

pH (BS 1377, 1990., Part 3) 4.1

Organic content (%) (BS 1377, 1990., Part 3) 91

Fiber content (%) (ASTM, 1997-91) 80

Specific gravity (BS 1377, 1990., Part 2) 1.38

In situ unit weight (kN/m3) 10

Permeability (m/day) (BS 1377, 1990., Part 6) 0.89

cc (Compression index) (ASTM D 2435-70) 3

cr (Recompression index) (ASTM D 2435-70) 0.251

cu—VST (ASTM D—2573) (kPa) 11

cu—UCT (BS 1377, 1990., Part 7) (kPa) 10

cu—UU (BS 1377, 1990., Part 7) (kPa) 12

VST vane shear test, UCT unconfined compression test, UU uncon-

solidation undrained test, NP none plastic

Table 4 Chemical composition of the cement used in this study [54]

Chemical compositions Content %

SiO2 21

Al2O3 5.3

Fe2O3 3.3

CaO 68.6

MgO 1.1

SO3 \0.01

Na2O \0.01

K2O \0.01

Table 5 Shear strength parameters of prepared soil

Tests cus (kPa) cuc (kPa) quc (kPa)

Ts-01 9.1 – –

Ts-02 8.80 79.60 159.2

Ts-03 9.40 89.30 178.6

Ts-04 9.80 83.40 166.8

Ts-05 9.50 85.80 171.6

Ts-06 9.80 82.30 164.6

Ts-07 9.10 80.70 161.4

Ts-08 10.10 88.40 176.8

Ts-09 10.30 88.30 176.6

Ts-10 9.70 82.80 165.6

Ts-11 9.50 78.60 157.2

Ts-12 9.70 79.40 158.8

Ts-13 9.40 78.30 156.6

cuc and cus are undrained shear strength of column and soil, respec-

tively, and quc is unconfined compression strength of column

Acta Geotechnica

123

modeling tests for bearing capacity analysis of the cement

columns. So, in this section, the results of each test were

plotted in terms of vertical stress against normalized dis-

placement to the width of the footing. It is evident from

Fig. 3 that stress–displacement response of unimproved

peat (TS-01) was very similar to that of behavior of soft

clays. A similar observation had been reported previously

by Rashid [42] who studied the deformation of soft clays

under vertical loading using a rectangular rigid foundation.

Figures 3, 4, and 5 show the comparisons between

stress–displacement responses of the unimproved peat with

the tests reinforced with the end-bearing and floating CDM

columns at different area improvement ratios. As can be

seen, in the stabilized peat with floating CDM columns, all

trends showed a ductile behavior. However, by increasing

the CDM column length, the differences between the trends

of the peat improved by floating CDM columns and

unimproved peat became greater, which was due to higher

skin interactions between the CDM columns and sur-

rounding peat. On the other hand, in the case of stabilized

peat using end-bearing CDM columns, the UBC increased

with increasing area improvement ratio because it

increased the stiffness of the columns.

The UBC of the unimproved peat resulted in an average

value of 38 kPa using double tangent method. It was

obvious that in the tests with end-bearing CDM columns,

the relationship between vertical load and normalized

displacement before failure was almost linear, which was

completely different to floating CDM columns. In the end-

bearing tests, the UBC achieved by the experimental

method were 77.7, 87.6, and 91.9 kPa for TS-02, TS-03,

and TS-04 with BCF of 8.83, 9.32, and 9.38, respectively.

These values were the peak points of the graph. Beyond

peak points, when the entire composite ground failed, extra

stress could not be tolerated, and consequently, the trends

decreased significantly [28, 46–48].

Compared to unimproved peat, it was revealed that the

UBC of peat stabilized with end-bearing CDM columns

increased up to 200, 229 and 240 % using area improve-

ment ratios of 13.1, 19.6, and 26.2 %, respectively, while

in the case of peat reinforced with floating CDM columns,

the average increase in the UBC was 60 % (average of 9

tests). Moreover, it was observed that TS-02 (a = 13.1 %),

TS-03 (a = 19.6 %), and TS-04 (a = 26.2 %) failed at 4,

2.5, and 1.9 % normalized vertical displacement of the

footing, respectively. It showed that as the area improve-

ment ratio increases, the vertical displacement for failure

decreases. One possible reason is that when the area

improvement ratio increases, the composite soil tends to be

stiffer and can endure more stress in less vertical dis-

placement. Consequently, the vertical displacement for

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100

Dis

plac

emen

t/w

idth

of

foot

ing

Vertical stress(kPa)

TS-01 (l/h = 0)

TS-02 (l/h = 1)

TS-05 (l/h = 0.25)

TS-08 (l/h = 0.5)

TS-11 (l/h = 0.75)

Fig. 3 The relationship between vertical stress and displacement to

the width of footing (a = 13.1 %)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100

Dis

plac

emen

t/w

idth

of

foot

ing


TS-01(l/h = 0)

TS-03 (l/h = 1)

TS-06 (l/h = 0.25)

TS-09 (l/h = 0.5)

TS-12 (l/h = 0.75)



0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 20 40 60 80 100 120

Dis

plac

emen

t/w

idth

of

foot

ing


TS-01(l/h = 0)

TS-04 (l/h = 1)

TS-07 (l/h = 0.25)

TS-10 (l/h = 0.5)

TS-13 (l/h = 0.75)



Acta Geotechnica

123

failure declines. The average range of 2.8 % of normalized

vertical displacement of the footing at failure moment

confirms the studies conducted by Boussida and Porbaha

[5]. They stated that in the case of soft clay improved by

end-bearing CDM columns, the vertical displacement for

failure was\10 % of the normalized vertical displacement

of the footing.

As expected, in the case of peat improved by floating

CDM columns (TS-05 to TS-13), the required normalized

vertical displacement of the footing at the point of failure was

significantly higher compared to the end-bearing cases. This

was attributed to the fact that in the case of peat reinforced

with end-bearing CDM columns, the slope of the stress-

normalized displacement graphs was higher compared to

floating CDM columns, which showed failure at lower dis-

placements. In addition, it was clear that up to 10–15 %

(average = 14 %) of the normalized vertical displacements

of the footing, the UBC did not depend on area improvement

ratios. This was because even using floating CDM columns,

the trend of stabilized soil remained ductile, the same as

unimproved peat. Thus, in the case of peat treated by floating

CDM columns, raising the UBC, up to 10 to 15 % normal-

ized vertical displacement of the footing, was needed

regardless of the area improvement ratio.

Figure 6 shows the comparison between the BCF

obtained from experimental work with different analytical

methods in the case of end-bearing CDM columns.

Boussida and Porbaha [5, 6] declared that the prediction by

Brom’s method tends to underestimate the bearing capacity

if the cohesion ratio, kc [based on Eq. (5)], is [30. In this

study, the range of cohesion ratios of the tests improved by

end-bearing CDM columns was between 8.5 and 9 con-

firming the theory stated by Boussida and Porbaha [5, 6].

According to Fig. 6, the BCF achieved by experiments was

not exactly in the range of lower and upper bounds of the

UBC proposed by Boussida et al. [7] and Boussida and

Porbaha [5, 6]. The differences between the BCF calcu-

lated based on experimental and Broms-(b) method were

14.2, 8.5, and 3.6 % for the TS-02, TS-03, and TS-04,

respectively. Therefore, in the case of treated peat with

end-bearing columns, the findings of UBC of the current

study were consistent with Broms-(b) method. Thus, it can

be concluded that based on all mentioned methods, by

increasing the area improvement ratio, the BCF of stabi-

lized soil increased, which was attributed to higher stiffness

of soil using end-bearing CDM columns.

Figure 7 compares the BCF of the peat stabilized by

floating CDM columns with Broms-(c) method. The min-

imum and maximum of experimental BCF of 5.63 and 7.94

for peat improved by floating CDM columns were attrib-

uted to TS-05 and TS-13, respectively. As indicated in

Fig. 7, the average relative differences of BCF between

Broms-(c) and experimental method were insignificant.

Therefore, it is proven here that Broms-(c) method pre-

dicted the UBC and BCF of the treated peat with floating

CDM columns with high accuracy.

In this section, in order to interpret the reduction factor

(R), different comparisons were made regarding the UBC

of three methods, including (1) experimental, (2) Broms-

(a), and (3) Broms-(b). Figure 8 depicts the variation of

reduction factors of the UBC in the case of end-bearing

CDM columns based on Eq. (8). As discussed earlier, these

factors can anticipate the UBC of peat improved by end-

bearing CDM columns from the well-known Terzaghi and

Vesic methods [50, 52] using the homogenized cohesion. It

6

6.5

7

7.5

8

8.5

9

9.5

10

= 13.1% = 19.6% = 26.2%

Bea

ring

cap

acit

y fa

ctor

(B

CF

)

Experimental Broms - (a) Broms - (b)

Boussida & Porbaha (lower bound) Boussida & Porbaha (upper bound)

Fig. 6 BCF of peat improved by end-bearing CDM columns

4

4.5

5

5.5

6

6.5

7

7.5

8

Bea

ring

cap

acit

y fa

ctor

(B

CF

)

Experimental (double tangent)

Broms - (c)

Fig. 7 BCF of improved soil by floating CDM columns

Acta Geotechnica

123

is obvious that in comparison with all the calculated

methods, the experimental results of this study were the

closest to the anticipated method proposed by Terzaghi and

Vesic. As shown in Fig. 8, by increasing the area

improvement ratio, the reduction factor (R) decreased in all

the methods which indicated that the differences between

the calculated UBC and well-known Terzaghi and Vesic

method increased. Thus, it is not safe to determine the UBC

of the peat stabilized with the group of soil–cement col-

umns based on the Terzaghi and Vesic method [50, 52]

based on homogenized cohesion.

4.3 Failure patterns

After each test was completed, the deformed ground sur-

face was inspected. During all the tests, it was observed

that the extent of soil bulging on the sides of the rectan-

gular footing was very small, indicating that the boundary

effect on the results was likely to be insignificant. Figure 9

shows a schematic deformed profile conditions attributed

to (1) unimproved (TS-01), (2) improved peat with floating

(TS-05), and (3) end-bearing (TS-04) CDM columns at the

point of failure. As can be seen, in the TS-01, the deformed

surface around the footing was completely symmetric and

punching shear failure was observed. As shown in Fig. 9,

the necessary vertical displacement for the failure of the

peat reinforced by end-bearing CDM columns was less

than that of floating and unimproved (Fig. 10). Further-

more, in the tests using end-bearing columns (TS-02, TS-

03, TS-04), small unsymmetric heaves around 2 mm were

generated around the footing, while progressive cracks

were observed during loading and the length and width of

the cracks increased at higher stress. Figure 11 shows the

failure patterns and progressive cracks in the TS-04.

Table 6 shows a comparison between parameters of this

study with other scientific researches in the case of failure

patterns of end-bearing CDM columns. Parameters such as

undrained shear strength of soil and column and failure

patterns of composite ground are tabulated. In the first

comparison, most of the parameters in the TS-03 were the

same as that performed by Rashid [42], while failure pat-

terns were somewhat different (Fig. 12). From these data,

we can see that study performed by Rashid [42] resulted in

a combination of shearing and bending failure for the CDM

columns. There are several possible explanations for these

differences:

These dissimilarities were because of differences in the

nature of the soil. The geotechnical characterizations of

peat and CDM columns (peat ? cement) were different to

that of clay.

Differences in applying stress to the stabilized area can

affect the failure pattern. As stated before, in this study,

vertical stress was applied to the stabilized area in the stress

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

TS-02 ( = 13.1%) TS-03 ( = 19.6%) TS-04 ( = 26.2%)

Red

ucti

on f

acto

r of

the

UB

C (

R)

R- Experimental

R- Broms - (a)

R- Broms - (b)

Fig. 8 UBC reduction factor (R)

285

290

295

300

305

310

315

-125 -100 -75 -50 -25 0 25 50 75 100 125

Hie

ght

leve

l (m

m)

Distance from center line (mm)

Vertical stress

TS-01

TS-04

TS-05

Fig. 9 Schematic deformed profile at failure (note: dimensions of the

footing are not to scale)

Punching shear along the footing

Fig. 10 Failure of unimproved peat (TS-01)

Acta Geotechnica

123

control of 1 kPa/min, while in the tests conducted by

Rashid [42], strain control of 6.20 mm/min was used.

Sample preparation and column installation of these

methods were completely different. In his research, the

clay was prepared using an OCR (over consolidation ratio)

of 10 and the columns were constructed and cured outside

of the soil. Consequently, even with approximately the

same area improvement ratio and soil and column

undrained shear strength, a different failure pattern can be

expected.

In the second comparison in the case of end-bearing

columns, the results of this study showed great differences

from that performed by Kitazume et al. [30, 32]. One of the

key parameters governing the behavior of soft soils

improved by soil–cement columns is the column–soil

stiffness ratio (shear strength of columns to the soil). In

their physical modeling, the area improvement ratio was

79 % and the stiffness ratio was around 95. Apparently, the

stiffness of the CDM columns was too high, and so the

columns acted the same as the piles and the failure pattern

was related completely to the failure of the columns.

Moreover, as in this study, Kitazume et al. [30, 32] indi-

cated that peak vertical stress values were observed at

\10 % of normalized displacement of the footing.

The third comparison is that of Yin and Fang [28] who

performed a plane strain, physical modeling of clay

improved by end-bearing CDM columns using an area

improvement ratio of 12.6 % (nine columns). The column

preparation and installation technique were similar to those

of Kitazume et al. [30, 32], and the properties of soil are

shown in Table 6. Yin and Fang [28] prepared and cured

the column outside of the soil and inserted the columns

using PVC pipes. Finally, they generated column failure

using a rigid plate of 300 mm by 300 and 30 mm in

thickness with strain control of 1 mm/min. The most

important differences in the present model compared to

others were the undrained shear strength of the columns.

As for this research, brittle failure was also observed from

the stress-normalized displacement curve; however, Yin

and Fang [28] introduced a wedge-shaped block failure

pattern for the CDM columns.

Unlike the brittle behavior of end-bearing CDM col-

umns, the failure trends of nine tests stabilized by floating

CDM columns exhibited ductile behavior. In these tests,

the footing displaced the underlying improved peat almost

symmetrically, and no heave was observed around the

footing. In the tests reinforced with short CDM column

lengths (l/h = 0.25), due to smaller improvement com-

pared to long CDM column lengths, the columns moved

downward with the peat without any breakage. One the

other hand, in long column lengths (l/h = 0.5 and l/h = 0.

75), the CDM columns yielded at shear in a point around

20–25 % of the column length measured from the bottom

of the footing.

Progressive cracks along the footing at failure

Fig. 11 Point of failure of peat improved by end-bearing CDM

columns (TS-04)

Table 6 Comparison between parameters of this study with other

scientific research in the case of failure patterns of end-bearing

columns

Parameters This study

(TS-03)

Rashid

[42]

Kitazume

et al. [30, 32]

Yin and

Fang [28]

a % 19.6 17.3 79 12.6

/� 0 0 0 0

cus (kPa) 9.4 6.9 4 3

cuc (kPa) 89.3 86.75 379 425

Failure

pattern

Shear Shear and

bending

Shear and

bending

wedge-

shaped

a is area improvement ratio, / is friction angle of the soil, and cuc and

cus are undrained shear strength of column and soil, respectively

Punching shear along the footing

No heave

No heave

Fig. 12 Failure of peat improved by floating CDM columns (TS-13)

Acta Geotechnica

123

In the case of floating columns, two comparisons are

performed between parameters of this study including TS-

10 and TS-13 with two tests including Test 1 and Test 2

performed by Rashid [42], and the details are summarized

in Table 7. As can be seen, the failure patterns of this study

were compared to two different tests performed by Rashid

[42]. It should be noticed that all tests were in the

undrained condition with the same area improvement ratio

of about 26 %. As mentioned before, CDM columns in the

TS-10 and TS-13 failed in a shear pattern at a point around

20–25 % of the column length measured from the bottom

of the footing. As indicated in Table 7, the most important

difference between TS-10 and TS-13 (this research) and

Test 1 and Test 2 performed by Rashid [42] was undrained

shear strength of the columns. In Test 1, due to the lower

undrained shear strength of the columns, which was

36 kPa, the columns failed in shear. On the other hand, in

Test 2, the undrained shear strength of the columns

increased to 121.8 kPa which has caused bending failure

instead of shear failure. Consequently, it can be concluded

that by increasing the undrained shear strength of the col-

umns, because of higher stiffness of the columns, the

failure pattern changes from shear to bending failure.

5 Concluding remarks

The main aim of this paper was to point out the effects of

cement deep mixed columns on the UBC of peat soils in

floating and end-bearing CDM conditions. The following

conclusions can be drawn from the present study:

Both floating and end-bearing CDM columns enhanced

the UBC of soft soil. In the case of floating columns, the

UBC was improved by an average increase of 60 %, while

end-bearing columns increased the UBC of soft peat from

200 to 240 %. Therefore, it was demonstrated that end-

bearing CDM columns were more dominant than floating

columns.

The analytical method for calculating the UBC of peat

stabilized with end-bearing CDM columns based on

homogenous cohesion, introduced by Terzaghi, overesti-

mated the UBC significantly.

In the case of stabilization tests with floating columns,

the results of the UBC were compatible with Brom’s

method.

The failure patterns of unimproved peat and peat sta-

bilized with floating CDM columns exhibited punching

shear, while in the case of peat improved by end-bearing

CDM columns, progressive cracks and small heave were

observed around the footing.

It was observed that the peat improved by end-bearing

CDM columns (three tests) failed at \4 % of normalized

vertical displacement of the footing. Whereas, in the case

of peat improved by floating CDM columns (nine tests), the

average vertical displacement needed for the failure was

14 % of normalized vertical displacement of the footing.

It should be pointed out that many factors can influence

the UBC and failure pattern of stabilized ground with a

group of CDM columns such as strength properties of the

CDM columns, configuration of the columns, end-bearing

conditions, and area improvement ratios. However, while

this physical modeling simulation is considered capable of

reproducing the main features of stabilized peat deposit

with CDM columns, caution must be applied with small

sample sizes as the findings might not be transferable to

field conditions. Thus, real-scale physical model simulation

tests should be carried out to observe the actual perfor-

mances of stabilized peat by a group of CDM columns.

References

1. ASTM D 2974 (2000) Standard test method for moisture, ash,

and organic matter of peat and other organic soils. Book of

ASTM standards. ASTM, Philadelphia

2. Bergado DT, Anderson LR, Miura N, Balasubramaniam AS

(1994) Lime/cement deep mixing method. Improvement tech-

niques of soft ground in subsiding and lowland environments.

A.A. Balkelma, Rotterdam, pp 99–130

3. Black JA, Sivakumar V, Madhav MR, McCabe B (2006) An

improved experimental test setup to study the performance of

granular columns. Geotech Test J 29(3):193–199

4. Black JA, Sivakumar V, Madhav MR, Hamill GA (2007) Rein-

forced stone columns in weak deposits-laboratory model study.

J Geotech Geoenviron 133(9):1154–1161

5. Boussida M, Porbaha A (2004) Ultimate bearing capacity of soft

clays reinforced by a group of columns-application to a deep

mixing technique. Soils Found 44(3):91–101

6. Boussida M, Porbaha A (2004b) Bearing capacity of foundations

resting on soft ground improved by soil cement columns. In:

International Conference on Geotechnical Engineering (ICGE

2004), pp 173–180

7. Boussida M, Jelali B, Porbaha A (2009) Limit analysis of rigid

foundations on floating columns. Int J Numer Anal Methods

9(3):89–101

8. British Standard Institution BS (1990) Methods of test for soils

for civil engineering purposes. British Standard Institution,

London

Table 7 Comparison between parameters of this study with other

scientific research in the case of failure patterns of floating columns

Parameters This study

(TS-10)

Rashid [42]

Test 1

This study

(TS-13)

Rashid [42]

Test 2

a % 26.2 26 26.2 26

/� 0 0 0 0

cus (kPa) 9.1 6.5 9.4 6.4

cuc (kPa) 82.8 36 78.3 121.8

Failure

pattern

Shear Shear Shear Bending

Acta Geotechnica

123

9. Broms BB (1964) Lateral resistance of piles in cohesive soils.

J Soil Mech Found Div ASCE 90(2):27–63

10. Broms BB (1982) Lime columns in theory and practice. In:

Proceedings of International Conference of Soil Mechanics,

Mexico, pp 149–165

11. Broms BB (2000) Lime and lime/columns. Summary and visions.

In: Proceedings of the 4th International Conference on Ground

Improvement Geosystems, vol 1, pp 43–93

12. Broms BB (2001) Discussion—centrifuge model tests on failure

envelope of column type deep mixing method improved ground.

Soils Found 41(4):103–107

13. Broms BB (2002) Stabilization of soil with lime columns. In:

Foundation engineering handbook. Kluwer Academic Publisher

14. Broms BB, Boman P (1979) Stabilisation of soil with lime col-

umns. Ground Eng 12(4):23–32

15. Bruce D (2001) An introduction to the deep mixing methods as

used in geotechnical applications, vol III. The verification and

properties of treated ground. Report No. FHWA-RD-99-167, US

Department of Transportation, Federal Highway Administration

16. Bruce DA, Bruce ME (2001) Practitioner’s guide to the deep

mixing method. Ground Improv 5(3):95–100

17. Bruce DA (2002) An introduction to deep mixing methods as

used in geotechnical applications, vol III. The verification and

properties of treated ground FHWA-RD:99-167

18. Coastal Development Institute of Technology (2002) The deep

mixing method-principle, design and construction. A.A. Balk-

elma, Netherland

19. EuroSoilStab (2002) Development of design and construction

methods to stabilize soft organic soils: design guide soft soil

stabilization, industrial and materials technologies programme

(Brite- EuRam III), European Commission, CT97-0351, Project

No. BE 96-3177, pp 15–60

20. Han J, Zhou H, Tand Ye F (2002) State-of-practice review of

deep soil mixing techniques in China. Transportation Research

Record No. 1808, Soil Mechanics, pp 49–57

21. Hebib S, Farrell ER (2003) Some experiences on the stabilization of

Irish peats. Can Geotech J 40(1):107–120. doi:10.1139/T02-091

22. Horpibulsuk S, Miura N, Koga H, Nagaraj TS (2004) Analysis of

strength development in deep mixing: a field study. Ground

Improv 8(2):59–68

23. Huat BBK, Kazemian S, Prasad A, Barghchi M (2011) A study of

the compressibility behavior of peat stabilized by DMM: model

and FE analysis. Int J Phys Sci 6(1):196–204

24. ICE Manual of Geotechnical Engineering (2012) Institution of

Civil Engineers. Chapter 35 Organics/peat soils ICE manual

25. Janz M, Johansson SE (2001) The function of different types of

binder in content with deep stabilization Swedish Deep Stabil-

ization Research Centre, Report No 9, Linkoping

26. Japanese Geotechnical Society Standard (2000) Practice for

making and curing stabilized soil specimens without compaction.

vol 5, Chapter 7, (JGS 0821-2000)

27. Karstunen M (1999) Alternative ways of modelling embankments

on deep-stabilized soil. In: Proceedings of the International

Conference on Dry Mix Methods for Deep Soil Stabilization,

pp 221–228

28. Yin J-H, Fang Z (2010) Physical modelling of a footing on soft

soil ground with deep cement mixed soil columns under vertical

loading. Mar Georesour Geotechnol 28:173–188

29. Kazemian S, Huat B, Prasad A, Barghchi M (2011) Study of peat

media on stabilization of peat by traditional binder. IJPS

6(3):476–481

30. Kitazume MT, Miyajima I, Karastanev K (1996) Bearing

capacity of improved ground with column type DMM. In:

Yonekura T, Shibazaki B (eds) Grouting and deep mixing, vol 1,

pp 503–508

31. Kitazume M, Yamamoto M (1998) Stability of group column

type DMM ground. Report of Port and Harbour Institute, vol

37(2), pp 3–28

32. Kitazume M, Yamamoto M, Udaka Y (1999) Vertical bearing

capacity of column type DMM ground with low improvement

ratios. In: Bredenberg H, Broms BB (eds) Dry mixing methods

for deep soil stabilization, pp 245–250

33. Kitazume M, Okano K, Miyajima S (2000) Centrifuge model

tests on failure envelope of column type deep mixing method

improved ground. Soils Found 40(4):43–55

34. Krenn H, Karstunen M (2009) Numerical modelling of deep

mixed columns below embankment constructed on soft soil.

Geotechnics of soft soils focus on ground improvement,

pp 159–164

35. Mesri G, Ajlouni M (2007) Engineering properties of fibrous

peats. J Geotech Geoenviron 133(7):850–866

36. Moore PD (1989) The ecology of peat-forming processes. Int J

Coal Geol 12(1–4):89–103

37. Okumura T (1996) Deep mixing method of Japan. Grouting and

deep mixing. In: Proceedings of the 2nd International Conference

on Ground Improvement Geosystems. Balkema, Rotterdam,

pp 879–887

38. Omine K, Ochiai H, Bolton MD (1999). Homogenization

method for numerical analysis of improved ground with cement-

treated soil columns. In: Proceedings of the International Con-

ference on Dry Dry Mix Methods for Deep Soil Stabilization,

pp 161–168

39. Porbaha A (1998) State of the art in deep mixing technology: part

I. Basic concepts and overview. Ground Improv 2(2):81–92

40. Prandtl L (1921) Uber Die Eindringungsfestigkeit Plastischer

Baustoffe Und Die Festigkeit Von Schneiden. Zeitschrift fur

angewandte Mathematik und Mechanik 1(1):15–20

41. Puppala A, Madhyannapu R, Nazarian S,Yuan D, Hoyos L (2007)

Deep soil mixing technology for mitigation of pavement rough-

ness. Texas Department of Transportation Research and

Technology

42. Rashid A (2011) Behavior of weak soils reinforced with soil

columns formed by deep mixing method. In: Phd Thesis. Uni-

versity of Sheffield

43. Rathmayer H (1996) Deep mixing methods for soft soil

improvement in the Nordic Countries. In: Proceedings the 2nd

International Conference on Ground Improvement Geosystems,

Grouting and Deep Mixing, 14–17 May, Tokyo, 2, pp 869–877

44. Stanek W, Worley IA (1983) A terminology of virgin peat and

peat lands. In: Proceedings of International Symposium on PeatUtilization, Bemidji State University, Bemidji, Minn, pp 75–104

45. Tan TS, Goh TL, Yong KY (2002) Properties of Singapore

marine clays improved by cement mixing. Geotech Test J

25(4):422–433

46. Terashi M, Tanaka H (1981a) Ground improvement by in situ

deep mixing method. In: Proceedings of 10th International

Conference Soil Mechanics and Foundation Engineering, Stock-

holm, pp 777–780

47. Terashi M, Tanaka H (1981b) Settlement analysis for deep mixing

method. In: Proceedings of 10th International Conference Soil

Mechanics and Foundation Engineering, Stockholm, pp 955–960

48. Terashi M, Tanaka H (1983) Bearing capacity and consolidation

of the improved ground by a group of treated soil columns.

Report of the Port and Harbour Research Institute, vol 22, No. 2,

pp 214–266

49. Terashi M (2005) Keynote lecture: design of deep mixing in

infrastructure applications. In: Proceedings of International

Conference on Deep Mixing. Best Practice and Recent Advance,

pp 25–45

50. Terzagi K (1943) Theoretical soil mechanics. Wiley, New York

Acta Geotechnica

123

http://dx.doi.org/10.1139/T02-091

51. Topolnicki M (2004) In situ soil mixing. In: Moseley MP, Kirsch

K (eds) Ground improvement. Spon Press, New York,

pp 331–428

52. Vesic AS (1973) Analysis of ultimate load of shallow founda-

tions. J Soil Mech Found Div ASCE 99(SM1):45–73

53. Von Post L (1922) SGU peat inventory and some preliminary

results, pp 1–27

54. YTL product data sheet (2008) Chemical compositions of the

cement. YTL Cement Marketing Sdn Bhd, Kuala Lumpur

Acta Geotechnica

123

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