field behaviour of stiffened deep cement mixing piles
TRANSCRIPT
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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
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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)
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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)
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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)
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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
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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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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
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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)
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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
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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
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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
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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
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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)
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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
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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
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(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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