Field Behaviour of Stiffened Deep Cement Mixing Piles

7/28/2019 Field Behaviour of Stiffened Deep Cement Mixing Piles

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Proceedings of the Institution of Civil Engineers

Ground Improvement

Pages 117 doi: 10.1680/grim.900027

Paper 900027

Received 27/08/2009 Accepted 08/06/2010

Keywords: columns/embankments/foundations

Institution of Civil Engineers & 2011

Ground Improvement

Field behaviour of stiffened deep cement

mixing piles

Jamsawang, Bergado and Voottipruex

Field behaviour of stiffeneddeep cement mixing pilesj1 Pittaya Jamsawang

Lecturer, Department of Civil Engineering, King Mongkuts Universityof Technology North Bangkok, Bangkok, Thailand

j2 Dennes T. BergadoProfessor, School of Engineering and Technology, Asian Institute ofTechnology, Klongluang, Pathumthani, Thailand

j3 Panich VoottipruexAssociate Professor, Faculty of Technical Education, King MongkutsUniversity of Technology North Bangkok, Bangkok, Thailand

j1 j2 j3

Full-scale pile load tests were performed on soft Bangkok clay improved by stiffened deep cement mixing (SDCM)

piles and deep cement mixing (DCM) piles installed by jet-mixing to compare their performance. The SDCM pile is a

DCM pile with a precast reinforced concrete core pile inserted in the middle. A series of full-scale tests consisting of

axial compression, lateral and pullout interface between the concrete core pile and surrounding DCM material were

performed. The length of the concrete core pile influenced both the ultimate axial bearing capacity and the

settlement of the SDCM piles more than its section area. Furthermore, the section area of the concrete core pile

affected both the lateral ultimate bearing capacity and the lateral displacements of SDCM piles significantly.

Moreover, the SDCM piles with area ratio (Acore/ADCM) of 0.17 and length ratio (Lcore/LDCM) of 0.85 increased the axial

and lateral ultimate bearing capacities so that they were as much as 2.2 and 15 times higher than the corresponding

values of DCM piles, respectively. The flexural strength of the DCM pile obtained from the laboratory was 16% of its

unconfined compressive strength whereas that obtained from full-scale lateral load tests was much lower at 47% of

its unconfined compressive strength. The strength reduction factor, Rinter, at the interface between the concrete core

pile and DCM pile in the field averaged 0.40, which agreed with the data from laboratory tests of 0.380.46.

NotationAcore section area of concrete core pile (m

2)

Acore/ADCM area ratioADCM section area of deep cement mixing pile (m2)

cDCM undrained shear strength of deep cement mixing

pile (kPa)

csoil cohesion of soil (kPa)

cu,end undrained cohesion of the soil at the bottom end

of the pile (kPa)

cu i undrained cohesion of soil layer i (kPa)

DDCM diameter of deep cement mixing pile (m)

E50 modulus of elasticity of deep cement mixing pile

(kPa)

e eccentric distance (m)

f9c compressive strength of prestressed concrete pile

(MPa)

fy tensile strength of steel

Gs specific gravity

Hi soil layer thickness (m)

IDCM moment of inertia of deep cement mixing pile (m4)

Lcore length of concrete core pile (m)

Lcore/LDCM length ratio

LDCM length of deep cement mixing pile (m)MR modulus of rupture (kN/m2)

Mult ultimate bending moment (kN-m)

Nc bearing capacity factor

Pmax effective maximum past pressure

Po effective overburden pressure

Pult ultimate lateral load (kN)

Qult ultimate axial bearing capacity (kN)

Qpileult ultimate axial bearing capacity in case of pile

failure (kN)

Qsoilult ultimate axial bearing capacity in case of soil

failure (kN)

qu unconfined compressive strength of deep cement

mixing pile (kPa)

Rinter strength reduction factor for interface

Su undrained shear strength

Tult ultimate tensile load (kN)

Wn water content

1


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ycrack depth of crack location (m) adhesion factor of the interface of deep cement

mixing pile

h,crit total lateral pressure acting on the pile at critical

section (kPa)

nter interface shear strength (kPa)

1. IntroductionGround improvement by deep cement mixing (DCM) piles has

been widely used to improve the engineering properties of soft

clay layers. The DCM piles can effectively reduce settlements of

full-scale embankments (Bergado et al., 1999; Lai et al., 2006).

The DCM piles also have low strength and stiffness, especially

flexural strength (Petchgate et al., 2003a, 2003b, 2004), and may

lead to low axial and lateral ultimate bearing capacities and large

deformations. Consequently, a DCM pile is not suitable for

carrying high compression and lateral loads. Liu et al. (2007)

introduced geogrid-reinforced and cast-in-place concrete piles to

support embankments on soft clay. Dong et al. (2004) stated that

a concrete or cast-in-situ pile is deemed uneconomical as a

friction pile for embankment support because much of the

strength of the pile materials has not been utilised when thesurrounding soft ground fails. Hence, a new composite pile has

been introduced. It consists of an DCM with a concrete core pile

inserted in the middle and is called a stiffened deep cement

mixing (SDCM) pile. The concrete core pile with higher strength

and stiffness serves to resist the compressive and flexural stresses

on the pile shaft and carries most of the load which is, in turn,

transmitted to the DCM pile through their interfaces. Previously,

the field pile load test on a DCM pile in soft Bangkok clay under

axial compression and lateral loads had been studied by many

researchers such as Petchgate et al. (2003a, 2003b, 2004), as

shown in Figure 1 and the behaviour of a DCM pile under

embankment loading involving axial and lateral loads had been

studied by many researchers such as Chen (1990), Honjo et al.

(1991), Bergado et al. (1999), Lai et al. (2006), and others. A

series of pile load tests were conducted to investigate the behav-

iour of SDCM piles in China by Wu et al. (2005) and Zheng and

Gu (2005). Most of the test results were concerned with only the

axial bearing capacities of the SDCM piles. Jamsawang et al.

(2008) studied and simulated the settlement behaviour of a

composite foundation consisting of an SDCM pile and untreated

Problems of DCM pile

Petchgate ., (2003b)et al

00

10

25

60

90

Medium stiff clay

Backfill clay

Weathered clay

Soft clayS 16 t /mu

2

DCM pile 05 m

C

Q

Q

DCM2

u(pile fail)

u(soil fail)

of 30 t/m was expected

14 t

10 t (controls)

Undrained shear strength: t /m2

0 20 40 60

D

epth:m

0

1

2

3

4

5

6

PL1P2LP3LP4LP5LP6L

Bearing capacity

Pile failure Soil failure

Max. load in case of pile failure

Measured max. load

16141210

86420

Load:t

1 2 3 4 5 6

Pile failure Soil failure

DCM DCM

Qc

Qf

CDCM

Qpileult DCM h,crit u,DCMA (3 ) q Q Q Qsoilult f c

Figure 1. Low quality of DCM piles on soft Bangkok clay

(Bergado et al., 1999; Lai et al., 2006)

2

Ground Improvement Field behaviour of stiffened deep cement

mixing piles



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soil in the laboratory. However, full-scale pile load tests onSDCM piles under vertical and lateral loading as well as pullout

interface test have not yet been studied. To continue the previous

research, a series of full-scale tests consisting of SDCM piles and

DCM piles in soft Bangkok clay under axial compression load,

lateral load and pullout interface test were conducted to compare

their performance. Thus, the scope of the present paper is to

present the data related to the aforementioned full-scale tests on

soft Bangkok clay improved by SDCM piles and DCM piles

installed by the jet-mixing method and the effects of the cross-

sectional areas and lengths of the concrete core piles on the

bearing capacities and settlements of SDCM piles.

2. Site characterisationThe test site is located at the campus of the Asian Institute of

Technology (AIT) which is 40 km north of Bangkok, Thailand.

The soil profile and soil properties of the subsoil in the upper-

most three layers at the AIT campus are presented in Figure 2.

The uppermost 10 m of the soil profile can be divided into three

layers. The weathered crust forms the uppermost layer having a

thickness of 2.0 m and this is underlain by a soft clay layer which

extends down to about 8.0 m depth. The undrained shear strength

obtained from field vane test of the soft clay was 16 to 17 kPa. A

medium stiff clay layer was found to be underlying the soft clay

layer at 8 to 10 m depth having an undrained shear strength ofmore than 30 kPa. The underlying stiff clay layer extended from

10 to 15 m depth.

3. Concrete core pileEach SDCM pile was constructed by inserting a prestressed

concrete core pile in the middle of the DCM pile with 0.6 m

diameter (Figure 3(a)). The DCM pile had a diameter of 0.6 m.

The prestressed concrete pile was selected to behave as a stiff

core because it has high strength and stiffness and it was cheaper

than a steel pile. The concrete core piles (Figure 3(b)) consisted

of 0.18 m 3 0.18 m and 0.22 m 3 0.22 m square cross-sections

with 4.0 and 6.0 m lengths. The corresponding area ratio (Acore /

ADCM), defined as the cross-sectional area of the core pile over

the cross-sectional area of the DCM pile, and the length ratio

(Lcore/LDCM), defined as the length of core pile over the length of

DCM pile, were 0.11 and 0.17 as well as 0.57 and 0.85,

respectively.

4. Test pile installationThe DCM piles were constructed in situ by a jet-mixing method

employing a jet pressure of 22 MPa. Both SDCM and DCM piles

were installed at 2.0 m spacing. The water/cement (w/c) ratio of

the cement slurry and the cement content employed for the

construction of deep mixing were 1.5 and 150 kg/m3 of soil,

respectively. Each deep mixing pile has a diameter of 0.6 m and a

length of 7.0 m, penetrating down to the bottom of the soft clay

layer. Each SDCM pile was constructed by inserting a prestressed

concrete core pile in the middle of DCM pile. The concrete corepile was inserted after the deep cement mixing was completed

but while the DCM was still soft and not yet cured. During the

Weathered crust

Soft clay

Medium stiff clay

0

1

2

3

4

5

6

7

8

9

10

Depth:m

14 16 18 20

Unit weight: kN/m3

Unit weight

Gs

26 265 27

Gs

0 40 80 120

PL WN LL

PL, W , LLN

0 20 40 60

Corrected S : kPafrom vane shear test

u

Po Pmax

0 50 100 150

Po and Pmax

0 1 2 3 4 5 6

OCR

Figure 2. Subsoil profile and relevant parameters (PL, plastic limit;

LL, liquid limit; OCR, overconsolidation ratio)

3


mixing piles



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curing period, the concrete core pile was anchored at the ground

surface to prevent it from sinking. The deep mixing piles were

allowed to cure until about 80 days. Subsequently, a series of full-

scale load tests on SDCM and DCM piles under axial compres-

sion load and lateral load were performed to determine their

ultimate bearing capacities and lateral resistances. In addition,pullout interface tests between concrete core and DCM piles were

also conducted to determine the interface resistance between the

concrete core and the DCM pile. The layout of the test piles is

shown in Figure 4.

5. Unconfined compression tests on deepcement mixing core samples

To obtain engineering properties of the DCM pile in the test

site, three DCM piles were constructed (Figure 4) so that core

samples could be extracted for unconfined compression tests in

the laboratory in order to determine unconfined compressive

strength, qu, and modulus of elasticity corresponding to 50%

unconfined compressive strength, E50. Unconfined compressive

tests were performed on 50 mm diameter by 100 mm height

samples. The values are scattered over the entire depth without

any clear trend of the influence of the depth on the values of

unconfined compressive strength and modulus of elasticity

(Figure 5(a)). The values of unconfined compressive strength

ranged from 500 to 1500 kPa with the average value of 900 kPa

while the modulus of elasticity ranged from 50 000 to

150 000 kPa with an average value of 90 000 kPa indicating that

E50 101qu as shown in Figure 5(b). It can be seen that the

correlation ratio of E50/qu obtained from field coring samplesranged from 60 to 150.

6. Pullout interface and flexural strengthtests in the laboratory

The pullout interface tests were conducted to determine the

strength reduction factor for interfaces (Rinter) in accordance with

that defined by Brinkgreve and Broere (2006) as

Rinter inter=cDCM1:

where inter is interface shear strength between the DCM and

concrete core pile and cDCM is the undrained shear strength of the

DCM pile.

Specimens were prepared by pouring cement-admixed clay into a

PVC mould, and inserting a cement core pile at the centre as

ADCM

Acore

Concretecore pile

DCM pile

Soft claylayer

Medium or stiff clay layer

Lcore

LDCM

018

022

022

018

Prestressed concrete core pile

Concrete 35 MPaf c

8 4 mm stands1750 MPa

fy

3 mm stirrupsspacing varied

Concrete 35 MPaf c

8 4 mm stands1750 MPa

fy

3 mm stirrupsspacing varied

Figure 3. (a) Schematic diagram of SDCM pile; (b) details of

prestressed concrete core piles (dimensions in m)

4


mixing piles



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shown in Figure 6(a). The size of the concrete core pile was

17 mm diameter and the sizes of the DCM were 50 and 100 mm

corresponding to area ratios of 0.12 and 0.03, respectively. The

results are shown in Figure 6(a), in terms of peak interface shear

strength plotted against the cohesion of DCM material obtained

from unconfined compression tests, and demonstrate that the

strength reduction factors varied from 0.38 to 0.46. To evaluate

the flexural strength of DCM pile correlated to unconfined

compressive strength of core samples, flexural strength tests on

cement-admixed clay specimens were performed. The use of a

simple beam with three-point loading was conducted in accor-

dance with ASTM D 1635-00 (ASTM, 2000) in the laboratory.

The specimen dimensions were 100 mm 3100 mm in cross-

section and 500 mm in length. The cement contents were varied,

namely 0.3516, 15 and 20% by weight. The correlations of the

test results are shown in Figure 6(b) showing that flexural

20 20 20 20 20 2020 20 20

2 0

20

DCM-L1 DCM-L2 SDCM-L1 SDCM-L2 SDCM-L3 SDCM-L4 SDCM-L5 SDCM-L6 SDCM-L7 SDCM-L8

DCM-C1 DCM- 2C SDCM- 1C SDCM- 2C SDCM- 3C SDCM- 4C SDCM- 5C SDCM- 6C SDCM- 7C SDCM- 8C

Coring 1 Coring 2 Coring 3 SDCM-P1 SDCM-P2 SDCM-P3 SDCM-P4

SDCM pile DCM pile SDCM pile with inclinometer DCM pile with inclinometer

Figure 4. Pile load test layout

Weathered crust

Soft clayDCM

pile

Medium stiff clay

0

1

2

3

4

5

6

7

8

9

10

Depth:m

Coring-1Coring-2Coring-3

0 1000 2000

Unconfined compressivestrength, : kPa

(a)qu

0 100000 200000

Modulus of elasticity: kPaE50

Coring-1Coring-2Coring-3

240000

200000

80000

40000

160000

120000

0

Modulusofelasticity,

:kPa

E50

0 400 800 1200 1600 2000 2400

Unconfined compressive strength, : kPa(b)

qu

E

R

50 u2

101q

05024

E

q

50

u

150

E

q

50

u60

Figure 5. Field test results on DCM piles: (a) engineering

properties of DCM piles; (b) relationship between E50 and qu of

DCM pile

5


mixing piles



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strength or modulus of rupture of DCM material corresponds to

16% of unconfined compressive strength. The flexural strength of

DCM and concrete core piles is plotted in Figures 7(a) and (b).

7. Equipment and procedures of full-scaleload tests

7.1 Axial compression load test

Figure 8 shows the schematic set-up for applying axial compres-

sion loads to the test pile using a hydraulic jack acting against a

platform. The platform was comprised of steel sheets and con-

crete boxes having a total weight of 500 kN supported by upper

cross H-beams. The reaction beam or test beam was support by

lower cross H-beams supported by a concrete box that distributed

the total weight of the platform to the surrounding soil. The

vertical load was applied to the test pile through a 600 kN

capacity hydraulic jack. A 500 kN capacity proving ring wasinserted between the jack and the reaction beam to measure the

applied load. The ball bearing was inserted between the proving

and the reaction beam to ensure the vertical direction of the

applied loads from the hydraulic jack. The vertical settlement of

the test pile under the applied load was measured using two dial

gauges, which were connected to two reference beams placed on

both sides of the jack. The axial compression tests were

performed in accordance with ASTM D-1143 (ASTM, 1994a).

The load was applied in increments of 10 kN. Each load

increment was maintained for 5 min. The load increments were

applied until a continuous increase of the vertical displacements

occurred under a slight or further no increase in load.

7.2 Lateral load test

Figure 9 shows the schematic set-up for applying the lateral loads

to the SDCM pile using a hydraulic jack acting against the sides

of an excavation. The base excavation was performed around the

300

250

200

150

100

50

0

Interfaceshearstrength,

:kPa

inter

0 100 200 300 400 500 600

Undrained shear strength, : kPa

(a)

cDCM

A Acore DCM/ 012

A Acore DCM/ 03

Pullout load

100mm

inter

Dead load

DCM pile

Concrete

core inter

DCM

046

c

inter

DCM

038

c

300

250

200

150

100

50

0

Modulusofrupture,

:kPa

M

R

0 300 600 900 1200

Unconfined compressive strength, : kPa

(b)

qu

Load

Test specimen

100 100 100 100 100

100

M

q

R

u

016

Unit: mm

Figure 6. Laboratory test results: (a) relationship between

interface shear strength and undrained shear strength; (b)

relationship between modulus of rupture and unconfined

compressive strength

180

160

140

120

100

80

60

40

20

0

Flexuralstrength:kPa

0 5 10 15 20 25

Cement content: %

(a)

14000

12000

10000

8000

6000

4000

2000Flexuralstrength:kPa

Core size: m

(b)

0018 018 022 022

Figure 7. (a) Flexural strength of DCM piles; (b) flexural strength

of concrete core piles

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mixing piles



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test pile at depth of 1.5 m from the original ground level to

provide enough area at the side of the excavation for the

necessary reactive capacity to the maximum anticipated lateral

test loads. A concrete pile cap 0.4 m high was placed on the pile

to prevent local failure on the pile head and the load was applied

at 0.3 m from the base excavation level. A hydraulic jack with a

capacity of 600 kN was used to apply lateral loads. The loads

were read from a proving ring with capacity 100 kN and the

lateral displacement at the load application level was read from

one dial gauge connected to the reference beams. A ball bearing

was inserted between the proving and the reaction beam to adjust

the horizontal direction of the applied loads from the hydraulic

jack. Thick timber sheets were used as support to distribute the

load from the hydraulic jack to the side of the excavation. The

lateral load tests were performed in accordance with ASTM D-

3966 (ASTM, 1994b). The load was applied in increments of 0.5

and 1 kN for the DCM and the SDCM piles respectively. Each

load increment was maintained for 10 min. The load was applied

until continuous lateral displacements occurred at a slight or no

increase in load.

Concreteboxes

Concreteboxes

Concreteboxes

Concreteboxes

Concreteboxes

Steel sheets

Upper cross-beams

Supportbeams

Reactionbeam

Supportbeams

Lower cross-beams

Concrete boxsupports

100

Prestressed concrete pile

Steel test plate

Hydraulic jack

GL.

Ball bearing Proving ring

Dial gauge

Referencebeam

Test SDCM pile

Concrete boxsupports

Figure 8. Schematic set-up for pile under axial compression load

7


mixing piles



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7.3 Pullout interface testFigure 10 shows the schematic set-up for applying axial tensile

loads to the test pile using a hydraulic jack with a capacity of

600 kN acting between the test beam and reaction frame. The

length of the cores embedded in the SDCM was 1 m with another

1 m protruding out of the SDCM to perform the pullout tests.

In the preparation for the test, clay cement over the top 1 m of

the SDCM was removed leaving 1 m of the core embedded in the

DCM pile. A steel rod was connected to the test pile and the

reaction frame in order to pull the prestressed concrete core pile

from the DCM. The test beam was supported by a concrete box

that distributed the total load to the surrounding soil. The vertical

settlement of the test pile under the applied load was measured

using two dial gauges, which were connected to two reference

beams placed on both sides of the jack. The pullout interface

tests were performed in accordance with ASTM D-3689 ( ASTM,

GL.

150m

Dial gauge

Reference beam

030Pile cap

Prestress concrete pile

Test pile

Ballbearing

Bearing test plate

150m

Timber support 15 15 m

Steel plateHydraulic jackProving ring

Figure 9. Schematic set-up for pile under lateral load

Ball bearing

Proving ring

Hydraulic jack

Reaction frame

Reaction beams

Concretesupport

ConcretesupportSteel rod

Dial gaugesReferencebeam

100

100

100

DCM pile

Prestressed concrete pile

Figure 10. Schematic set-up for pullout test (dimensions in m)

8


mixing piles



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1994c). The load was applied in increments of 5 kN. Each loadincrement was maintained for 5 min. The load was applied until

continuous vertical displacements occurred at a slight or no

increase in load.

8. Test results and discussions

8.1 Axial compression load tests

Two axial compression tests on DCM piles (DCM-C1 and DCM-

C2) and eight axial compression tests on SDCM piles (SDCM-C1

to SDCM-C8) varying the length and the cross-sectional area of

the prestressed concrete core pile are listed in Table 1. Figure 11

shows the axial compression load plotted against settlement forall ten test piles. The ultimate bearing capacities of all test piles

are tabulated in Table 1 and they were determined by the slope

tangent method at the point of intersection of the initial and final

tangents to the loadsettlement curve, as suggested by Butler and

Hoy (1977).

8.1.1 Axial compression test on DCM piles

The ultimate bearing capacities of DCM piles, DCM-C1 and

DCM-C2, were 220 and 140 kN, respectively. For the DCM-C1,

the load was applied until continuous vertical displacements

occurred between loads of 240 to 250 kN and there was no

increase in load beyond a load of 250 kN. After the test, the

failure mode of DCM C-1 was observed by excavation to 1 m

depth from the top of the pile head. Cracks were observed around

the pile head due to the high stress concentration at the pile head.

Similarly, the pile DCM-C2 failed suddenly after increasing the

axial load from 160 to 170 kN. The test procedure was stopped

and the failure mode of DCM C-2 was observed at a depth of

about 0.50 m from the top of the pile head due to low shear

strength and the poor quality of this part of the DCM pile. The

large difference in ultimate bearing capacities of DCM-C1 and

DCM-C2, as much as 80 kN, confirmed the poor quality that

commonly occurred in DCM piles resulting in low bearingcapacity (Petchgate et al., 2003a).

8.1.2 Axial compression pile load test on SDCM piles

As shown in Figure 11, the ultimate bearing capacities of the

SDCM piles with 0.18 m 3 0.18 m square section and 4 m long

concrete core piles (SDCM-C7 and SDCM-C8) were 270 and

260 kN, respectively. The average ultimate bearing was 265 kN,

which was 1.2 and 1.9 times higher than the values for DCM-C1

and DCM-C2, respectively. Similarly, the ultimate bearing capa-

cities of the SDCM piles with 0.22 m 3 0.22 m square section

and 4 m long concrete core piles (SDCM-C3 and SDCM-C4)

were 280 and 270 kN, respectively, being 1.3 and 2

.0 times higher

than those of DCM-C1 and DCM-C2, respectively. Thus, the

insertion of the concrete core pile into the DCM pile increased

the bearing capacity. Moreover, the ultimate bearing capacity of

SDCM piles with 0.22 m 3 0.22 m concrete core pile was slightly

higher by 10 kN in comparison with the corresponding SDCM

piles with a 0.18 m 3 0.18 m, 4 m long concrete core pile.

Furthermore, settlements for the SDCM piles were less than those

for the DCM piles at the same load. This implies that a concrete

core pile can increase the stiffness of a DCM pile and offer more

linear behaviour and reduced settlements.

The ultimate bearing capacities of the SDCM piles with

0.22 m 3 0.22 m square section, 6 m long concrete core piles

(SDCM-C1 and SDCM-C2) were 320 and 310 kN, respectively.

The average ultimate bearing capacity of the SDCM pile was

315 kN which was 2.2 and 1.4 times greater than those of DCM-

C1 and DCM-C2, respectively. The ultimate bearing capacity of

the SDCM, 6 m long core pile was greater than that of the 4 m

long core pile by as much as 35 kN. Moreover, the settlements

for the SDCM pile with a 6 m core pile were less than those for

the SDCM pile with a 4 m core pile at the same load. This

implies that a longer concrete core can add more stiffness than a

shorter core pile and can transfer the load more efficiently than a

Number LDCM: m DDCM: m Lcore: m Core size: (m 3 m) Lcore/LDCM Acore/ADCM Qult: kN

DCM-C1 7.0 0.60 140

DCM-C2 7.0 0.60 220

SDCM-C1 7.0 0.60 6.0 0.22 3 0.22 0.85 0.17 320

SDCM-C2 7.0 0.60 6.0 0.22 3 0.22 0.85 0.17 310

SDCM-C3 7.0 0.60 6.0 0.18 3 0.18 0.85 0.11 300

SDCM-C4 7.0 0.60 6.0 0.18 3 0.18 0.85 0.11 300

SDCM-C5 7.0 0.60 4.0 0.22 3 0.22 0.57 0.17 280

SDCM-C6 7.0 0.60 4.0 0.22 3 0.22 0.57 0.17 270

SDCM-C7 7.0 0.60 4.0 0.18 3 0.18 0.57 0.11 270

SDCM-C8 7.0 0.60 4.0 0.18 3 0.18 0.57 0.11 260

Table 1. Comparison of ultimate bearing capacities (Qult) in axial

compression tests

9


mixing piles



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shorter core pile from the top part to the bottom part by

transferring the load directly to the DCM material.

Figure 12 shows the vertical bearing capacities of SDCM piles

with varying area (Acore/ADCM) and length (Lcore/LDCM) ratios in

comparison with that of DCM piles. Figure 12 demonstrates that

the length ratio influenced the vertical bearing capacity more than

the area ratio. Referring to Figure 11, the length of the concretecore pile affected both the ultimate bearing capacity and

settlement and was more dominant than the cross-sectional area

of the concrete core pile.

8.1.3 Modes of failure in axial load tests

Lorenzo (2005) suggested that the ultimate bearing capacity of an

individual DCM pile can be obtained depending on the mode of

failure using the following relationship

Qsoilult DDCMX

Hi cui

" #

4DDCM

2 cu,endNc soil failure 2:

Qpileult

4DDCM

23h,crit qu pile failure 3:

where (Hicu i) is the summation of the product of soil layer

thickness (Hi) and the corresponding undrained cohesion (cu i) of

all soil layers within the depth of deep mixing pile installation;

is the adhesion factor of the interface of deep mixing pile which

can be taken as 1.0; cu,end is the undrained cohesion of the soil at

the bottom end of the pile; Nc can be taken as 9.0; qu is the

0

10

20

30

40

50

60

Settlement:mm

0 50 100 150 200 250 300 350 400

Axial compression load: kN

6 m long core pile

4 m longcore pile

000

100

200

800

1000

Weatheredcrust

Soft clay

Medium stiff clay

Stiff clay

SDCM-C1 (022 0 22 60) SDCM-C2 (022 0 22 60) SDCM-C3 (018 0 18 60) SDCM-C4 (018 0 18 60) SDCM-C5 (022 0 22 40) SDCM-C6 (022 0 22 40) SDCM-C7 (018 0 18 40) SDCM-C8 (018 0 18 40) DCM-C1DCM-C2

Q

Figure 11. Curves of axial load plotted against settlement from

field tests

350

300

250

200

150

100

50

0Verticalbearingcapacity:kN

00 02 04 06 08 10

L Lcore DCM/

022 022 m ( / 017) A Acore DCM

018 018 m ( / 011) A Acore DCM

Figure 12. Vertical bearing capacities of SDCM piles with varying

area (Acore/ADCM) and length (Lcore/LDCM) ratios compared with

DCM pile

10


mixing piles



11/17

unconfined compression strength of the deep cement mixing pile;and h,crit is the total lateral pressure acting on the pile at the

critical section. Based on the field tests, the failure took place in

the weathered crust layer at the shallow depth neglecting the

effect of overburden pressure so that Equation 3 can be written as

Qpileult

4DDCM

2qu pile failure 4:

The ultimate bearing capacity in the case of soil failure was

calculated at 320 kN whereas the ultimate bearing capacity in the

case of pile failure depended on the unconfined compressive

strength of DCM piles as shown in Figure 13. The ultimate

bearing capacities of DCM-C1 and DCM-C2 indicated that the

unconfined compressive strengths of DCM-C1 and DCM-C2 were

800 and 500 kPa, respectively, which did not reach the average

unconfined compressive strength of 900 kPa as shown in Figure

5(a). Moreover, in order to obtain the calculated ultimate bearing

capacity in the case of soil failure of 320 kN, the unconfined

compressive strength of the DCM pile should be greater than

1100 kPa. As shown in Table 1, the ultimate bearing capacities of

SDCM piles with concrete core lengths of 6 m ranged from 300

to 320 kPa, which agreed with the calculated ultimate bearing

capacity in the case of soil failure. For the SDCM piles with

concrete core lengths less than 6 m, the failure could be consid-ered as pile failure. From the observations after tests, no damage

took place in any of the concrete core piles inserted in the DCM

piles, implying that pile failure could occur in the DCM material

below the concrete core pile tip.

8.2 Lateral load tests

Two lateral load tests on DCM piles (DCM-L1 and DCM-L2)

and eight lateral load tests on SDCM piles (SDCM-L1 to DCM-

L8) were performed by varying the lengths and cross-sectional

areas of the concrete core piles as tabulated in Table 2. The

lateral load tests were performed to measure the ultimate lateral

bearing capacity and to obtain the relationship between lateral

load and displacement of the test piles. Moreover, the relationship

between the lateral displacement and the depth was obtained frominclinometer readings. Figure 14 shows the lateral loads plotted

against displacements for all ten test piles including DCM and

SDCM piles. The ultimate bearing capacities of all test piles are

tabulated in Table 2.

8.2.1 Lateral pile load test on DCM piles

The ultimate lateral loads of the DCM piles with 4 m long

concrete core (DCM-L1 and DCM-L2) were 3.5 a n d 2.5 kN,

respectively, with an average ultimate lateral load of 3.0 kN,

which was very low due to low flexural strength (Petchgate et al.,

2004; Terashi and Tanaka, 1981). Excavation after the test

400

350

300

250

200

150

100

50

0

Ultimatebearingcapacity,

:kN

Qult

Qult in case of soil failure 320 kN

Qultincaseofpilefailure

DCM-C1

DCM-C2

0 200 400 600 800 1000 1200 1400 1600

Unconfined compressive strength, : kPaqu

Figure 13. Relationship between ultimate bearing capacity of

DCM pile and unconfined compressive strength

Number LDCM: m DDCM: m Lcore: m Core size: (m 3 m) Lcore/LDCM Acore/ADCM Pult: kN

DCM-L1 7.0 0.60 3.5

DCM-L2 7.0 0.60 2.5

SDCM-L1 7.0 0.60 6.0 0.22 3 0.22 0.85 0.17 46

SDCM-L2 7.0 0.60 6.0 0.22 3 0.22 0.85 0.17 45

SDCM-L3 7.0 0.60 4.0 0.18 3 0.18 0.85 0.11 44

SDCM-L4 7.0 0.60 4.0 0.18 3 0.18 0.85 0.11 43

SDCM-L5 7.0 0.60 6.0 0.22 3 0.22 0.57 0.17 35

SDCM-L6 7.0 0.60 6.0 0.22 3 0.22 0.57 0.17 34

SDCM-L7 7.0 0

.60 4

.0 0

.18

30

.18 0

.57 0

.11 33

SDCM-L8 7.0 0.60 4.0 0.18 3 0.18 0.57 0.11 33

Table 2. Comparison of ultimate bearing capacities (Pult) in lateral

load tests

11


mixing piles



12/17

revealed cracks at 0.10 m below the excavated base in DCM-L1,

whereas DCM-L2 cracked at the level of the excavated base (see

Figure 17 later). This means that the failure mode of the two

DCM piles was pile failure due to bending moment induced by

lateral load.

8.2.2 Lateral pile load test on SDCM pilesThe ultimate lateral loads for the SDCM piles with 0.18 m 3

0.18 m section, 4 m long concrete core piles (SDCM-L7 and

SDCM-L8) were the same at 33 kN which was 11 times greater

than that of the DCM pile due to much higher flexural strength of

the concrete core pile. After the test, the clay surrounding SDCM

piles was excavated to observe the failure mode of the SDCM

piles. The crack locations were found at 0.46 and 0.50 m below

the excavated base for SDCM-L7 and SDCM-L8, respectively, as

shown later in Figure 17. The ultimate lateral loads for the

SDCM piles with 0.18 m 3 0.18 m section, 6 m long concrete

core piles (SDCM-L5 and SDCM-L6) were 35 and 34 kN,

respectively, having an average ultimate lateral load of 34.5 kN,

which was close to the average ultimate lateral load for SDCM

piles with 4 m long concrete core piles. The lateral loadlateral

displacement curves and the locations of crack for the SDCM

piles, 4 and 6 m long were similar. Consequently, increasing the

length of the core did not affect the ultimate lateral load and

displacement of the SDCM pile (Figure 15) much in contrast to

the SDCM pile under compression load.

The ultimate lateral loads for the SDCM piles with 0.22 m 3

0.22 m section and 4 m long concrete core piles (SDCM-L3 and

SDCM-L4) were 44 and 43 kN, respectively, and the average

ultimate lateral load was 43.5 kN, which was 14.5 times higher

than that for the DCM pile and also 1.3 times higher that that for

50

45

40

35

30

25

20

15

10

5

0

La

teralload:kN

0 5 10 15 20 25Lateral displacement: mm

000

150

200

800

1000

P 120

Weatheredcrust

Soft clay

Medium stiff clay

Stiff clay

022 022 m core pi le

018 018 m core pi le

SDCM-L1 (022 6)

SDCM-L2 (022 6)

SDCM-L3 (022 4)SDCM-L4 (022 4)SDCM-L5 (018 6)SDCM-L6 (0 6)18SDCM-L7 (0 4)18

SDCM-L8 (0 4)18DCM-L1

DCM-L2

Figure 14. Curves of lateral load plotted against lateraldisplacement from field tests

504540353025201510

50L

ateralbearingcapacity:kN

00 02 04 06 08 10L L

core DCM

/

022 022 m ( / 017) A Acore DCM018 018 m ( / 011) A Acore DCM

Figure 15. Lateral bearing capacities of SDCM piles with varying

area (Acore/ADCM) and length (Lcore/LDCM) ratios compared to DCM

pile

12


mixing piles



13/17

SDCM piles with 0.18 m 3 0.18 m cross-sectional area at thesame length due to its larger cross-sectional area and greater

moment resistance. As shown in Figure 15, the lateral bearing

capacities of SDCM piles were influenced more by the area ratio

(Acore/ADCM) in comparison with the length (Lcore /LDCM) ratio.

Figure 14 confirms this behaviour of SDCM piles

8.2.3 Curves of lateral displacement plotted against

depth

Figures 16(a), (b) and (c) show the curves of lateral displace-

ments plotted against depth (below the excavated base) for DCM-

L2, SDCM-L6 and SDCM-L2, respectively, which were obtained

from the inclinometer data. These plots are typical of the free-

head pile where the moment at the loading point is zero (Broms,

1964). According to the measured data for DCM-L2 as shown in

Figure 16(a), the lateral movement was measured at a load

interval of 0.5 kN until failure took place. The lateral displace-

ment approached zero at a depth of 0.5 m (1 3 DCM pile

diameter) below the excavated base. Thus, the lateral movement

was developed only within the shallow depth of 1 3 DCM pile

diameter.

According to the curves of lateral displacement plotted against

depth for SDCM-L6 as shown in Figure 16(b), the lateral move-

ment was measured at load intervals of 5 kN until failure. The

lateral displacement approached zero at a depth of 2.0 m(3 3 SDCM pile diameters) below the excavated base. Thus, the

lateral movement developed at depths of 3 3 SDCM pile

diameters and the maximum lateral movement occurred in the

excavated base. Similarly, in the curves of lateral displacement

plotted against depth for SDCM-L2 as shown in Figure 16(c), thelateral movement was measured at load intervals of 5 kN until

failure. The lateral displacement occurred at 2.0 m (3 3 SDCM

pile diameter) below the excavated base. Thus, the influence zone

of the surrounding clay on the DCM pile was only about

1 3 DCM pile diameter whereas that on the SDCM pile was up

to 3 3 SDCM pile diameters, demonstrating the influence of pile

stiffness on the depth of lateral displacements.

8.2.4 Damage characteristics of DCM and SDCM piles

The locations of the plastic hinge, indicating the maximum

bending moment in the DCM and SDCM piles, were at or below

the excavated level as inferred from the crack locations. All

failures in the DCM piles occurred at 0.10 m below the excavated

base and at the excavated base for DCM-L1 and DCM-L2,

respectively. The ultimate bending moments in the DCM piles

can be calculated from Figure 17. The modulus of rupture can

also be calculated from the following relationship

Mult Pult(e ycrack)5:

MR Mult(DDCM=2)

IDCM6:

where Mult is ultimate bending moment; Pult is ultimate lateral

load; e is the eccentric distance from the load application level to

the excavated base; ycrack is the depth of the crack location from

P 120

Weatheredcrust

Soft clay

Medium stiff clay

1

2

3

4

5

6

7

8

Depth:m

1

2

3

4

5

6

7

8

Depth:m

1

2

3

4

5

6

7

8

Depth:m

0 02 04 06Lateral displacement: mm Lateral displacement: mm Lateral displacement: mm

0 5 10 15 20 0 5 10 15 20

Lateral load, P

05 kN10 kN15 kN20 kN25 kN( ailure)F

Lateral load, P

10 kN20 kN30 kN35 kN( ailure)F

Lateral load, P

10 kN20 kN30 kN40 kN45 kN(Failure)

(a) (b) (c)

Excavated base

Figure 16. Curves of lateral displacement plotted against depth

from field tests: (a) DCM pile (DCM-L2); (b) SDCM pile with

concrete core pile 0.18 m 3 0.18 m 3 6 m (SDCM-L6); (c) SDCM

pile with concrete core pile 0.22 m 3 0.22 m 3 6m (SDCM-L2)

13


mixing piles



14/17

the excavated base; MR is the modulus of rupture; DDCM is the

diameter of the DCM pile; and IDCM is the moment of inertia of

the DCM pile.

The moduli of rupture from back-analysis were found to be 60

and 30 kPa for DCM-C1 and DCM-C2, respectively, correspond-

ing to 7 and 4% of the average unconfined compressive strength

of 900 kPa, which was lower than the test results from the

flexural test in the laboratory. The locations of the maximum

bending moment and crack location in the SDCM were deeper

than those for the DCM piles. After the test, inspection of

SDCM-L1 to SDCM-L8 piles revealed no damage above theexcavated base and the failure resulted from the breaking of the

DCM and cracking of the prestressed concrete core pile. The

locations of these cracks were found at deeper depths varying

from 0.4 0 t o 0.60 m below the excavated base as shown in

Figure 17.

In summary, the failure behaviour of all DCM piles in the lateral

load tests had the same characteristics. The DCM piles cracked

near the excavated base arising from the bending moment. In the

SDCM piles, the cracks were located further and deeper down its

length. This difference in the location of the cracks could result

from low stiffness and a poor-quality jet-grouting process in the

DCM piles that was a result of non-homogeneous soil cement

material. Consequently, the DCM pile could not transfer the

moment to deeper depths. In contrast, the SDCM piles had higher

stiffness resulting from the reinforcement by the concrete core

pile.

8.3 Pullout interface test

Four pullout interface tests were performed between the concrete

core piles and the surrounding DCM materials. Two pullout

interface tests were conducted on 0.22 m 3 0.22 m square sec-

tion, concrete core piles (SDCM-P1 and SDCM -P2) and another

two on 0.18 m 3 0.18 m square section, prestressed concrete core

piles (SDCM-P3 and SDCM-P4). The length of the cores

embedded in the SDCM was 1 m for all tests. Figure 18 shows

graphs of tension load plotted against vertical displacement. The

maximum tensile load-bearing capacities in the pullout interface

tests are tabulated in Table 3. The interface shear stress was

calculated by dividing the pullout resistance by the surface areaof the concrete core pile embedded in the DCM pile. The

ultimate tensile loads (Pult) were 165, 155, 135 and 120 kN for

the test piles SDCM-P1, SDCM-P2, SDCM-P3 and SDCM-P4,

respectively. The interface shear strengths of the pile were 188,

176, 188 and 167 kPa for, SDCM-P1, SDCM-P2, SDCM-P3 and

SDCM-P4, respectively, with an average value of 179 kPa.

Consequently, the strength reduction factor for interfaces (Rinter)

defined by Brinkgreve and Broere (2006) in Equation 1 was

calculated as 179/450 0.40. This value is within the range of

pullout interface test results on concrete core and cement-

admixed clay performed in the laboratory, namely 0.38 to 0.46.

The interface shear strengths between concrete core pile and

surrounding DCM for axial compression tests can be calculated

as 518, 778, 634 and 950 kN for 0.18 m 3 0.18 m square section,

4 and 6 m long as well as 0.22 m 3 0.22 m square section, 4 and

6 m long concrete core piles, respectively, which were much

GL. 000

150 Excavated base

Pult 35 kN

Pile top 100

DCM-L1 DCM-L2

ycrack 010

e 030

Pult 25 kN

ycrack 00

Pult 335 kN Pult 4346 kN

ycrack040050

ycrack045060

SDCM pile with018 018 section

and 48 m long

SDCM pile with022 022 section

and 48 m long

Figure 17. Mode of failure of DCM and SDCM piles under lateral

loading tests

14


mixing piles



15/17

greater than the axial ultimate bearing capacities. Therefore, noslippage occurred at the interfaces between the concrete core pile

and DCM material during the axial compression tests.

9. ConclusionsThe observed results of a series of full-scale pile load tests on

soft clay foundation improved by deep cement mixing (DCM)

and stiffened deep cement mixing (SDCM) piles are presented

herein. Based on the results, the following conclusions can be

drawn.

(a) By comparing the full-scale axial compression load tests on

the DCM and SDCM piles, the ultimate bearing capacity of

SDCM piles, 6 m long and having 0.18 m 3 0.18 m and

0.22 m 3 0.22 m cross-sectional area concrete core piles as

well as those with 4 m long and 0.18 m 3 0.18 m and

0.22 m 3 0.22 m cross-sectional area concrete core pile can

be improved by as much as 2.0 times in comparison with

DCM piles. The poor quality of the DCM piles is believed tobe an important factor responsible for their low strength

values.

(b) The length of concrete core pile significantly affected the

axial ultimate bearing capacity and axial settlement of the

SDCM piles. In contrast, the cross-sectional area of the

concrete core pile had only slight influence. The effective

value of the length ratio (Lcore/LDCM) ranged from 0.57 to

0.85.

(c) In order to obtain the ultimate bearing capacity in the case of

soil failure, the unconfined compressive strength of the DCM

pile should be greater than 1100 kPa. The ultimate bearing

capacities of SDCM piles with core lengths of 6 m could

reach the ultimate bearing capacity considering soil failure

whereas the SDCM piles with the core lengths shorter than

6 m failed by pile failure.

25

2

15

1

05

0

Verticaldisplacement:mm

0 20 40 60 80 100 120 140 160 180

Axial tensile load: kN

000

100

200

300

800

1000

T

Excavatedbase

Soft clay

Medium stiff clay

Stiff clay

SDCM-P1

SDCM-P2

SDCM-P3

SDCM-P4

Figure 18. Curves of tensile load plotted against vertical

displacement from field tests

Number DDCM: m Lcore: m Core size: (m3 m) Acore/ADCM Interface area: m2 Tult: kN inter: kPa

SDCM-P1 0.60 1.0 0.22 3 0.22 0.17 0.88 165 188

SDCM-P2 0.60 1.0 0.22 3 0.22 0.17 0.88 155 176

SDCM-P3 0.60 1.0 0.18 3 0.18 0.11 0.72 135 188

SDCM-P4 0.60 1.0 0.18 3 0.18 0.11 0.72 120 167

Table 3. Comparison of maximum tensile load-bearing capacities

(Tult) in pullout interface tests

15


mixing piles



16/17

(d) The lateral ultimate bearing capacities of SDCM pilesincreased by 11 to 15 times in comparison with the

corresponding values from DCM piles. The low lateral

resistance of the DCM piles was due to its low flexural

strength.

(e) The cross-sectional area of the concrete core piles

significantly affected the lateral ultimate bearing capacity and

lateral displacement of the SDCM piles. In contrast, the

length of the concrete core piles only has slight effects.

(f) The modulus of rupture of DCM piles in the field was lower

than in the laboratory, amounting to 4 to 7% of the field

unconfined compressive strength and up to 16% of the

laboratory unconfined compressive strength.

(g) The DCM piles cracked near the base of the excavated pit

arising from bending moment whereas in the SDCM the

cracks were located further and deeper down the length.

This difference in the location of the cracks resulted from

the low stiffness and the poor quality of the DCM piles,

which led to a non-homogeneous soilcement material.

Consequently, the DCM pile could not transfer the moment

to deeper depths. On the other hand, the SDCM pile which

had greater stiffness and relatively homogeneous

characteristics due to the presence of the concrete core pilecould transfer the moment load to deeper depths. The

influence zone of the surrounding clay on the DCM pile was

only about 1 3 DCM pile diameter whereas that on SDCM

pile was 3 3 SDCM pile diameter.

(h) The strength reduction factor for interfaces, Rinter, obtained

from the full-scale pullout interface test was 0.40, implying

that the shear strength at the interface between the concrete

core pile and the surrounding DCM material was strong

enough to prevent any slippage.

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Field Behaviour of Stiffened Deep Cement Mixing Piles

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