3Th-22

Proceedings of Indian Geotechnical Conference December 22-242013 Roorkee

NUMERICAL ANALYSIS OF PILED-RAFT FOUNDATION UNDER VERTICAL LOAD IN STONE COLUMN IMPROVED SOIL

R Bhowmik MTech Student CSIR-CBRI Roorkee riyabhowmik89gmailcom M Samanta Scientist CSIR- CBRI Roorkeemanojit_samantarediffmailcom ABSTRACT Three dimensional finite element analysis of piled-raft foundation under vertical load in stone column improved soft clay has been carried out considering pile-soil slip interface model Elastic-perfectly plastic Mohr-Coulomb failure criteria for soft clay and stone column are taken in analysis Axial force on pile bending moment total settlement of raft and proportion of load carried by raft and pile for different intensity of vertical load on piled-raft area replacement ratio and slenderness ratio of stone column have been investigated Analysis shows that improving soft clay by stone column reduces the total settlement and maximum bending moment of the raft in piled-raft The axial forces in pile are also reduced due to better raft-soil interaction Proportion of total load carried by raft increases with increasing area replacement ratio and slenderness ratio of stone column under piled-raft INTRODUCTION Piled-raft concept has been most notably applied to high-rise buildings all over the world and increasingly being recognized as an effective and economical foundation system for high-rise buildings bridges and heavy industrial plants [13 20] Piled-raft system works through the combined action of three bearing elements ie raft piles and subsoil [9 11 and 19] The basic concept of this system is that the foundation contains only those numbers of piles which are required to reduce the settlements to tolerable values and improve the bearing capacity adequately [6 17 and 18] This foundation system acts most efficiently when raft carries a significant amount of load but total differential settlement exceeds the permissible values Addition of few numbers of piles in strategic location beneath raft reduces the settlement besides providing additional bearing capacity and stiffness to foundation [6] Various researchers studied pile-soil pile-pile raft-soil and raft-pile interaction of piled-raft foundation through analytical numerical and laboratory model study and proposed various design methodologies [7 8 9 10 12 14 19 and 21 Soil profile consisting stiff clay and dense sand are favorable situations for piled- raft due to better raft-soil bearing interaction [18] Soil profile consisting of soft clay loose sand soft compressible and swelling layer at relatively shallow depth will not

be effective for piled-raft due to poor raft-soil bearing interaction [18] In-situ strengthening of softloose soil by different ground improvement method will improve the raft-soil bearing interaction in piled-raft system Various researchers have tried to improve the efficacy of piled-raft system in unfavorable soil profile by incorporating different ground-improvement methods such as stone columns and grid-form deep mixing walls [15 24 and 25] Liang et al (2003) investigated the response of piled-raft foundation on soft soil strengthened by short granular piles made of flexible materials through three dimensional finite element analysis The authors termed this long-short hybrid piled raft foundation where long piles are rigid and short piles are flexible as lsquoComposite piled raft foundationrsquo Sand cushion was used beneath the raft and its effect on the bearing behaviour of piles and soil was investigated by varying its thickness It was inferred that cushion increases the efficacy of the employed ground improvement method by redistributing the stress between piles and soil Axial stress in long pile reduces for increasing sand cushion thickness up to an optimum limit The inferences were corroborated with a case history of seven-storey building founded on composite piled-raft foundation in the coastal part of China Zhao et al (2006) carried out settlement calculation of long-short composite piled-raft foundation based on the shear-deformation method The soil was

R Bhowmik M Samanta

assumed to behave as an elasto-plastic material and Mylonakis-Gazetas model was incorporated to model pile-pile and pile-soil interaction The proposed methodology was validated through an example of published case history Wang et al (2010) carried out laboratory model study on performance of composite piled-raft foundation under vertical loading with sand column lime column and steel pipe pile as vertical reinforcing elements It was concluded that a composite foundation with a combination of long rigid piles and short granularflexible columns has a higher bearing capacity than the composite foundation with only granularflexible columns with the same conditions of soil ground and loading Yamashita and Yamada (2009) and Yamashita et al (2011) reported in-situ observations of piled-rafts combined with grid-form deep cement mixing walls for seven-storey and twelve-storey buildings on soft cohesive and liquefiable sand From the long-term observations of settlements and load shared by piles and raft it was concluded that composite piled-raft foundation system is effective in controlling settlements and tilt of the foundation laid on unfavorable soil profile Among a variety of methods used for ground improvement stone columns are widely used for improving very softsoftloose soil Study on response of composite piled-raft foundation in stone column improved soft soil is very limited A detailed numerical work was carried out to investigate the raft-soil and pilendashsoil interaction in stone column improved soft soil Three dimensional analysis of composite piled-raft system had been done using finite element based package Plaxis 3D Foundation A drained condition of soft clay around stone column had been taken in the present study Stone column were considered as replacement type and construction effect was not modeled in present study This paper describes details of the numerical works carried out using finite element package comparison of raft-soil and pile-soil interaction in improved and unimproved ground and efficacy of stone column method to improve the performance of composite piled- raft on soft soil

NUMERICAL MODELLING Finite Element Mesh amp Boundary Conditions Raft-soil and pile-soil interaction were investigated by carrying out three dimensional finite element analysis using software package Plaxis 3D Foundation Homogenous soft clay was taken for present study and piles in the piled-raft system were taken as floating pile The diameter of the piles was taken as 05 m and the length as 20 m A square raft of width 15 m and thickness 1 m was considered Replacement type stone columns between the piles in piled-raft system were employed to improve the strength and stiffness of the soft clay The connection between the pile and raft in the piled-raft system was considered as rigid whereas the stone columns were unconnected to the raft The schematic diagram of the considered hypothetical problem is shown in Figure 1 The pile-soil interface was considered as slip while the raftndashsoil interface was considered as smooth The numerical domain of the model was fixed from trial calculations during which the boundaries were increasingly extended till the stresses and displacements of the piled-raft were unaffected by further increase in the size of the domain From the analysis side boundaries were fixed at a distance of 10 B from all around the raft where B is the width of the raft The bottom boundary was set at a depth of 2L from the tip of the piles where L is depth of pile Vertical boundaries with their normal at x and z direction were fixed Bottom boundaries were fixed in all directions and the lsquoground surfacersquo was free in all directions Figure 2 shows the typical 3D mesh with boundary fixities used in the analyses The uniformly distributed vertical load was applied on the top of the raft surface after the initial equilibrium was reached The construction effects of the piles were not considered in present analysis The piles were kept at stress-free state at the beginning of the analysis [13]

Numerical analysis of piled-raft foundation under vertical load in stone column improved soil

Fig 1 Sketch of Composite Piled Raft System

Fig 2 3D Mesh with Boundary Conditions Constitutive Modelling For the present analysis soft soil stone column and sand cushion were modeled using Mohr-Coulomb model This model was chosen to represent behaviour of soil as it can predict the failure behaviour of soil accurately even though it utilizes those soil parameters which can be obtained from basic tests in laboratory The elasto-plastic Mohr-Coulomb model is based on linear elastic-perfectly plastic stress-strain curve (Figure 3) The plastic behaviour is governed by six yield functions (fij) and six plastic potential functions (gij) [5] The yield functions are functions of stress and strain and are dependent on soil parameters cohesion (c) and friction angle (Oslash) Plastic yielding

is related to the condition fij = 0 This condition can be represented as hexagonal cone in principal stress space as shown in Figure 4 As long as the stress states are within the yield surface the material behaves as linear elastic (f lt 0) The most attractive feature of this model is that it employs only five soil parameters namely Youngrsquos modulus (E) Poissonrsquos ratio (micro) cohesion (c) friction angle (Oslash) and dilatancy angle (Ψ) to model the behaviour of soil These properties are assumed to remain constant even at the occurrence of material hardening or softening after the onset of yielding

Fig 3 Linear Elastic-Perfectly Plastic Model

Fig 4 Mohr-Coulomb Yield Surface [5]

The soil continuum was discretised by 15 noded wedge elements in the 3D mesh [4] The element has three translational degrees of freedom in three perpendicular directions Raft was modeled by 6-noded triangular elements with 6 degrees of freedom at each node The pile was created as solid circular pile composed of same volume elements as soil with 6 degrees of freedom at each node [4] The raft and pile were considered to be linear-elastic The soil-pile interface is modeled by 16

CUSHION MAT

STONE COLUMN

R Bhowmik M Samanta

noded quadrilateral interface elements as shown in Figure 5 The eight pair of nodes is compatible with the quadrilateral side of soil and pile element in vertical direction [4] Each node has three translational degrees of freedom (ux uy and uz) allowing simulation of slipping and gapping between soil and pile The thickness of this interface element is zero but a virtual thickness is employed to calculate the stiffness properties of the interface The stiffness and strength of this interface is determined by the parameter Rinter This parameter relates the stiffness of the interface and the soil through the following relations tan (Oslashi) = Rinter tan (Oslashs) (1) Ei = Rinter 2Es (2) Gi = Rinter 2Gs (3) Where subscript s and i denote the soil and the interface parameters respectively A default value of 045 of Poissonrsquos ratio for interfaces was used in the analysis

Fig 5 Nodes () and Stress Points (x) in 16-noded

Interface Element [4] Validation The validation of finite element model in PLAXIS was done by comparing with the load-displacement results of piled-raft foundation published by Lee et al (2010) ABAQUS a finite element based package was used to study the three-dimensional bearing behavior of a piled-raft on soft clay by Lee et al (2010) The load displacement curves of piled-raft of floating pile group (3 x 3) of 05 m pile diameter and 16 m in length and square raft of size 10 m (thickness 1 m) was used for validation for both slip and no-slip condition between pile and soil The pile head was rigidly connected to raft For slip condition between the pile and soil an

interface friction co-efficient 03 was used The interface between the soil and raft was considered to be smooth The pile soil interface was simulated by 2D quadratic 18 node elements in ABAQUS The raft and piles were modeled with an isotropic elastic material simulated by 27 noded 2nd order hexahedral elements The relevant properties of pile raft soft clay and rock used in the analysis listed in Table 1 Figure 6 compare the results between the Lee et al (2010) and present PLAXIS analysis The load-settlement curve obtained from PLAXIS matches well with the previous results for both the conditions The small discrepancies arise due to difference in modeling the soil pile and raft

Table 1 Material Properties Used Mat-erial

Model Ersquo (MPa)

crsquo (kPa)

Φrsquo (ordm)

microrsquo K0 γt (kNm3)

Pile Elastic 12500 - - 025 001 25 Raft Elastic 30000 - - 02 001 25

Soft Clay

Mohr-Coulomb

5 3 20 03 065 18

Rock Mohr-Coulomb

500 01 45 03 05 19

Fig 6 Comparison of Abaqus amp Plaxis Result

0 0 02 04 06 08

MODEL-ABAQUS-NO SLIP CONDITION MODEL-PLAXIS-NO-SLIP CONDITION MODEL-ABAQUS-SLIP CONDITION MODEL-PLAXIS-SLIP CONDITION

Parametric Study A series of numerical analyses on piled-rafts (PR) in unimproved and improved ground were performed The diameter of the piles was taken as 05 m and length as 20 m (floating) The raft was considered as square with width 15 m and thickness 1 m The stone columns were arranged in square pattern A layer of sand cushion of 03m thickness was provided between stone column and raft for stress redistribution between soil and pile The soil properties used for clay stones and sand cushion were decided from the range of values provided in Bowles (1998) which were further verified from literature (Shahu et al 2011 Ambily et al 2007) The values used are listed in Table 2 The parameters and its range chosen for parametric study are presented in Table 3 In all the studies performed here only long-term (drained) response was investigated The consolidation effects were hence neglected and the soil parameters employed were drained shear strength parameters The values of drained Youngrsquos modulus and drained shear strength parameters were kept constant for the total depth of the soil layer The properties of pile and raft were based on a typical pile and raft made of M25 grade of concrete Efficacy of stone column to improve the performance of composite piled-raft foundation was evaluated by comparing raft-soil and pile-soil interaction in improved and unimproved ground These were evaluated in terms of bending moment and settlement of raft axial force distribution in pile and proportion of load shared by piles for the range of parameters considered in the analysis

Table 2 Properties of Soil used

Property Materials

Soft clay Stone Sand Cushion

E (kPa) 5000 55000 20000

micro 035 03 03

Oslash 0 43 30

Ψ 0 10 40 γbulk (kN 16 1662 155

c (kPa) 20 0 0

Rinter 09 0932 058

Table 3 List of Parameters chosen for parametric

Soft clay

Imp Grnd

Ar LsDs

Loading Condn Vertical

04 V 06 V 1V

Unimp Grnd

--- ---

A set of analysis was done to see the influence of decreasing spacing between piles and subsequently increasing the number of piles in piled-raft foundation in unimproved ground From Figure 7 it was observed that there is an optimum number of piles for a given load level beyond which increasing the number of pile has negligible effect on improving the behaviour of the piled-raft foundation From the results it was inferred that for chosen piled-raft geometry and material properties the optimum number of piles is around 16 (4 times 4 pile configuration) beyond which there is not much improvement achieved in settlement or ultimate bearing capacity of the foundation This configuration had been chosen for all further parametric studies Figure 8 shows the plan view of this chosen configuration This figure also explains the location of the piles considered for depicting results in parametric study

R Bhowmik M Samanta

Fig 7 Percent load Shared vs Number of Piles

Fig 8 Plan View of Piled Raft with 4 times 4 pile

configuration Results and Discussions Effect of Area Replacement Ratio (Ar) All the results reported in this section are obtained from numerical studies done on piled raft foundation of 4 times 4 pile configuration with stone column of length 10 m The stone columns were arranged in square pattern for its ease in fitting in between the pile arrangement The effect of introducing this ground improvement method was examined under vertical load level of 06V V being the ultimate bearing capacity of unpiled raft UBC of unpiled raft had been taken as load corresponding to settlement of 10 of width of raft Figure 9 shows the variation in load shared by piles with change in area replacement ratio Load shared by the piles in soft soil reduced significantly due to increase in the stiffness of the soft soil after installation of stone column This overall improvement in the soil media by stone column influenced the overall response of

piled-raft foundation system positively as can be seen from bending moment profile of raft in Figure 10 The profile shows the moment along A-Line (Fig 8) A decrease of approximately 18 in maximum moment of raft was observed for an area replacement ratio of 35 Figure 11 amp Figure 12 show the axial force distribution of interior and corner pile along the depth for an area replacement ratio of 15 and 35 A 28-40 decrease in axial force on corner and interior pile was observed for an area replacement ratio of 35 It can be inferred that a higher area replacement yields better results than the initial area replacement ratio of 15

Fig 9 Load Shared vs Area Replacement Ratio

Fig 10 Bending Moment Profile of Raft

0 20 40 60

Nos of Piles

--06 V

-10 -05 00 05 10

PR-4-4-AR-0 PR-4-4-AR-15 PR-4-4-AR-35

A-Line

Interior Pile

Corner Pile

0 10 20 30 40

Fig 11 Distribution of Axial Force at Corner Pile

Fig 12 Distribution of Axial Force at Interior Pile Effect of Length of Stone Column (LsDs) The LsDs ratio of stone column was varied from 5 10 and 15 for an area replacement ratio of 35 The results were compared with piled-raft in unimproved ground Figure 13 shows the variation in percent load carried by the piles This proportion gets reduced due to greater stiffness of the soil-stone column media with increasing pile length A decrease in load shared by pile was observed up to LsDs ratio of 10 beyond which it has marginal effect Figure 14 shows the displacement profile of the raft Decrease in maximum settlement of composite piled-raft was observed with increasing area replacement ratio A reduction of approximately 6 was achieved in settlement profile for LsDs of 15 Thus it can be inferred that increasing the length of stone columns are effective

in improving the overall settlement of composite piled raft foundation Figure 15 shows the distribution of axial force of corner pile along depth The axial force in the pile decreases with increasing LsDs ratio of stone column A sharp decrease in axial force in pile was observed for an LsDs ratio of 5 Beyond LsDs ratio of 10 the rate of improvement becomes insignificant

Fig 13 Percent Load Shared by Pile vs LsDs of Stone Column

Fig 14 Settlement Profile of Raft

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS

PR-4-4-Ar-15 PR-4-4-Ar-35

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-Ar-15 PR-4-4-Ar-35

0 5 10 15

-078 -076 -074 -072 -070 -068 -066 -064 -062 -060

-10 -05 00 05 10

PR-4-4-CUS

PR-4-4-L-10

PR-4-4-L-15

R Bhowmik M Samanta

Effect of Increasing Load Level The change in response of piled-raft foundation with increase in the vertical load level was also investigated Applied vertical load levels where 04 V 06 V and 10 V LsDs ratio of stone column was kept 10 with an area replacement ratio of 35 The focus was to investigate the change in the load carrying behaviour of bearing elements of composite piled-raft foundation system with increasing vertical load level As seen from Fig 16 load shared by piles decreases with increase in applied load It implies that the capacity of piles got mobilized from the initial load level of 04 V itself But the capacity of raft was getting mobilized slowly with increment in the applied load So at higher load levels raft became more active in sharing the load Figure 17 shows the variation in axial force at the head of corner pile with increasing load level for different area-replacement ratio of stone columns As observed before a stiffer soil media due to presence of stone columns reduces the axial force at piles

Fig 16 Percent Load Shared by Pile vs Applied Load Level

Fig 17 Axial Force at Head of Pile vs Applied

Load Level

Conclusions Results of a detailed 3D numerical study carried out on composite piled-raft foundation in soft clay improved by replacement type stone column had been presented in this paper Effect of area replacement ratio and slenderness ratio of stone columns on raft-soil and pile-soil interaction had been investigated and presented A study on the influence of increasing applied load level on the overall response had also been done and presented here Replacement type of stone column method was found to be beneficial in improving soft soil which in turn improved the response of composite piled-raft foundation The following conclusions may be drawn from the numerical study bull Application of stone column to improve poor

soil media reduces the load shared by the piles This proportion decreases with increasing area replacement ratio of the stone column

bull Stone columns are effective in improving the overall settlement of composite piled raft foundation

bull Bending moments in raft of piled raft are reduced when soil is improved with stone columns

bull Higher area replacement ratio of stone columns yields better results

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-L-5 PR-4-4-L-10 PR-4-4-L-15

-900 -800 -700 -600 -500 -400 -300 -200 -100

0 02 04 06 08 1

Ar-0 Ar-1545 Ar-35

04 06 08 10

V Page 8 of 10

bull The axial force distribution in piles reduces significantly when stone columns were introduced in the soil media and the response gets improved with increase in the area replacement by stone columns

bull While increasing the length of stone column it was found that there is an optimum length of stone column for given soil parameters to get the maximum improvement in overall response

bull Load shared by the piles and settlement of the raft decreased with increasing the length of the stone column

bull When the length of the stone columns was increased there was a significant improvement reached in axial load distribution in piles but as inferred before the effect of optimum length was also visible here

bull The participation of piles and raft for carrying the applied load changes with increasing load levels load shared by piles decreases with increase in applied load while the involvement of raft increases simultaneously It implies that piles come into action at its full capacity from initial load level itself But the capacity of raft mobilizes gradually with increment in the applied load So at higher load levels raft becomes more active in sharing the load

Notations B = Width of the raft m L = Length of pile m D = Diameter of pile m Ls = Length of stone column m Ds = Diameter of stone column m Savg = Average settlement of piled-raft m = (2 scenter + scorner ) 3 scenter = Settlement at center of raft scorner = Settlement at corner of raft S = Settlement at the considered point on raft of piled raft m P = Total load on piled-raft kNm2 V = Ultimate bearing capacity (UBC) of unpiled- raft for soil profile considered kNm2 Ar = Area Replacement Ratio X = Distance from the center of raft m Y = Distance from the head of pile m M11 = Bending Moment about z-axis kN-mm

N = Axial load at head of pile kN E = Youngrsquos modulus (kPa) micro = Poissonrsquos ratio Oslash = Angle of internal friction Ψ = Dilatancy angle γbulk = Bulk unit weight kNm3 c = Cohesion kPa Rinter = Interface coefficient REFERENCES 1 Ambily A P and Gandhi S R (2007)

ldquoBehavior of stone columns based on experimental and FEM analysisrdquo Journal of Geotechnical and Geoenvironmental engineering American Society of Civil Engineers 133(4) 405ndash415

2 Bowles JE (1988) ldquoFoundation Analysis and Designrdquo McGraw-Hill Inc

3 Brinkgreve R B J and Swolfs W M (2007) ldquoPlaxis 3D Foundation Version 2 Reference Manualrdquo PLAXIS bv Netherland

4 Brinkgreve R B J and Swolfs W M (2007) ldquoPlaxis 3D Foundation Version 2 Scientific Manualrdquo PLAXIS bv Netherland

5 Brinkgreve R B J and Swolfs W M (2007) ldquoPlaxis 3D Foundation Version 2 Material Models Manualrdquo PLAXIS bv Netherland

6 Burland JB (1995) ldquoPiles as settlement reducers Keynote addressrdquo 18th Italian Congress on Soil Mechanics Pavia Italy

7 Clancy P and Randolph MF (1996) ldquoSimple design tools for piled raft foundationsrdquo Geacuteotechnique 46(2) 313-328

8 Clancy P and Randolph MF (1993) ldquoAn approximate analysis procedure for piled raft foundationsrdquo International Journal for Numerical and Analytical Methods in Geomechanics 17 849-869

9 Clancy P and Randolph MF (1993) ldquoAnalysis and Design of Piled raft Foundationsrdquo Aust Geomechs J 1 1-21

10 Cooke R W (1986) ldquoPiled Raft Foundation on Stiff Clays-A Contribution to Design Philosophyrdquo Geacuteotechnique 36(2) 169-203

11 Davis EH and Poulos HG (1972) ldquoThe Analysis of Piled Raft Systemsrdquo Aust Geomechs J 2 21-27

R Bhowmik M Samanta

12 De Sanctis L and Mandolini A (2006) ldquoBearing Capacity of Piled Rafts on Soft Clay Soilsrdquo Journal Of Geotechnical And Geoenvironmental Engineering ASCE 132(12)

13 Katzenbach R Bachmann G Boled-Mekasha G Ramm H (2005) ldquoCombined Pile raft foundation(CPRF) An appropriate solution for the foundations of high-rise buildingsrdquo Slovak Journal of Civil Engineering 19-29

14 Lee J H Kim Y Jeong S (2010) ldquoThree-dimensional analysis of bearing behavior of piled raft on soft clayrdquo Computers and Geotechnics 103ndash114

15 Liang FY Chen LZ and Shi XG (2003) ldquoNumerical analysis of Composite piled raft with cushion subjected to vertical loadrdquo Computers and Geotechnics 30 443-453

16 Poulos HG (1993) ldquoPiled Rafts in Swelling or Consolidating Soilsrdquo Journal of Geotechnical and Geoenvironmental Engineering ASCE 119(2) 374-380

17 Poulos HG (2001a) ldquoMethods of analysis of piled raft foundationsrdquo A report prepared on behalf of Technical Committee TC18 on Piled Foundations

18 Poulos HG (2001b) ldquoPiled raft foundations design and applicationsrdquo Geacuteotechnique 51(2) 95-113

19 Poulos HG amp Davis EH (1980) ldquoPile Foundation Analysis and designrdquo Wiley New York

20 Poulos HG Small JC and Chow H (2011) ldquoPiled Raft Foundations for Tall Buildingsrdquo Geotechnical Engineering Journal of the SEAGS amp AGSSEA 42(2)

21 Randolph MF (1994) ldquoDesign Methods for Pile group and piled raftsrdquo Proceedings 13th ICSMFE New Delhi 5 61-82

22 Shahu J T and Reddy Y R (2011) ldquoClayey soil reinforced with stone column group model tests and analysesrdquo Journal of Geotechnical and Geoenvironmental Engineering American Society of Civil Engineers 137(12) 1265ndash1274

23 Wang X Z Zheng JJ and Yin JH (2010) ldquoOn composite foundation with different vertical reinforcing elements under vertical loading a physical model testing studyrdquo Journal of Zhejiang University-Science A (Applied Physics amp Engineering) 11(2) 80-87

24 Yamashita K and Yamada T (2009) ldquoSettlement and load sharing of a piled raft with ground improvement on soft groundrdquo Proceedings of the 17th International Conference on Soil Mechanics and Geotechnical Engineering Vol 2 1236-1239 doi103233978-1-60750-031-5-1236

25 Yamashita K Hamada J and Takeshi Yamada (2011) ldquoField Measurements on Piled Rafts with Grid-Form Deep Mixing Walls on Soft Groundrdquo Geotechnical Engineering Journal of the SEAGS amp AGSSEA Vol 42 No2

26 Zhao M H Zhang L and Yang M (2006) ldquoSettlement calculation for long-short composite piled raft foundationrdquo J Cent South Univ Technology

INTRODUCTION

R Bhowmik M Samanta

assumed to behave as an elasto-plastic material and Mylonakis-Gazetas model was incorporated to model pile-pile and pile-soil interaction The proposed methodology was validated through an example of published case history Wang et al (2010) carried out laboratory model study on performance of composite piled-raft foundation under vertical loading with sand column lime column and steel pipe pile as vertical reinforcing elements It was concluded that a composite foundation with a combination of long rigid piles and short granularflexible columns has a higher bearing capacity than the composite foundation with only granularflexible columns with the same conditions of soil ground and loading Yamashita and Yamada (2009) and Yamashita et al (2011) reported in-situ observations of piled-rafts combined with grid-form deep cement mixing walls for seven-storey and twelve-storey buildings on soft cohesive and liquefiable sand From the long-term observations of settlements and load shared by piles and raft it was concluded that composite piled-raft foundation system is effective in controlling settlements and tilt of the foundation laid on unfavorable soil profile Among a variety of methods used for ground improvement stone columns are widely used for improving very softsoftloose soil Study on response of composite piled-raft foundation in stone column improved soft soil is very limited A detailed numerical work was carried out to investigate the raft-soil and pilendashsoil interaction in stone column improved soft soil Three dimensional analysis of composite piled-raft system had been done using finite element based package Plaxis 3D Foundation A drained condition of soft clay around stone column had been taken in the present study Stone column were considered as replacement type and construction effect was not modeled in present study This paper describes details of the numerical works carried out using finite element package comparison of raft-soil and pile-soil interaction in improved and unimproved ground and efficacy of stone column method to improve the performance of composite piled- raft on soft soil

NUMERICAL MODELLING Finite Element Mesh amp Boundary Conditions Raft-soil and pile-soil interaction were investigated by carrying out three dimensional finite element analysis using software package Plaxis 3D Foundation Homogenous soft clay was taken for present study and piles in the piled-raft system were taken as floating pile The diameter of the piles was taken as 05 m and the length as 20 m A square raft of width 15 m and thickness 1 m was considered Replacement type stone columns between the piles in piled-raft system were employed to improve the strength and stiffness of the soft clay The connection between the pile and raft in the piled-raft system was considered as rigid whereas the stone columns were unconnected to the raft The schematic diagram of the considered hypothetical problem is shown in Figure 1 The pile-soil interface was considered as slip while the raftndashsoil interface was considered as smooth The numerical domain of the model was fixed from trial calculations during which the boundaries were increasingly extended till the stresses and displacements of the piled-raft were unaffected by further increase in the size of the domain From the analysis side boundaries were fixed at a distance of 10 B from all around the raft where B is the width of the raft The bottom boundary was set at a depth of 2L from the tip of the piles where L is depth of pile Vertical boundaries with their normal at x and z direction were fixed Bottom boundaries were fixed in all directions and the lsquoground surfacersquo was free in all directions Figure 2 shows the typical 3D mesh with boundary fixities used in the analyses The uniformly distributed vertical load was applied on the top of the raft surface after the initial equilibrium was reached The construction effects of the piles were not considered in present analysis The piles were kept at stress-free state at the beginning of the analysis [13]

CUSHION MAT

STONE COLUMN

R Bhowmik M Samanta

Model Ersquo (MPa)

crsquo (kPa)

Φrsquo (ordm)

Soft Clay

Mohr-Coulomb

5 3 20 03 065 18

Rock Mohr-Coulomb

500 01 45 03 05 19

0 0 02 04 06 08

Property Materials

E (kPa) 5000 55000 20000

micro 035 03 03

Oslash 0 43 30

Ψ 0 10 40 γbulk (kN 16 1662 155

c (kPa) 20 0 0

Rinter 09 0932 058

Soft clay

Imp Grnd

Ar LsDs

04 V 06 V 1V

Unimp Grnd

--- ---

R Bhowmik M Samanta

0 20 40 60

Nos of Piles

--06 V

-10 -05 00 05 10

PR-4-4-AR-0 PR-4-4-AR-15 PR-4-4-AR-35

A-Line

Interior Pile

Corner Pile

0 10 20 30 40

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS

PR-4-4-Ar-15 PR-4-4-Ar-35

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-Ar-15 PR-4-4-Ar-35

0 5 10 15

-078 -076 -074 -072 -070 -068 -066 -064 -062 -060

-10 -05 00 05 10

PR-4-4-CUS

PR-4-4-L-10

PR-4-4-L-15

R Bhowmik M Samanta

Load Level

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-L-5 PR-4-4-L-10 PR-4-4-L-15

-900 -800 -700 -600 -500 -400 -300 -200 -100

0 02 04 06 08 1

Ar-0 Ar-1545 Ar-35

04 06 08 10

V Page 8 of 10

R Bhowmik M Samanta

INTRODUCTION

CUSHION MAT

STONE COLUMN

R Bhowmik M Samanta

Model Ersquo (MPa)

crsquo (kPa)

Φrsquo (ordm)

Soft Clay

Mohr-Coulomb

5 3 20 03 065 18

Rock Mohr-Coulomb

500 01 45 03 05 19

0 0 02 04 06 08

Property Materials

E (kPa) 5000 55000 20000

micro 035 03 03

Oslash 0 43 30

Ψ 0 10 40 γbulk (kN 16 1662 155

c (kPa) 20 0 0

Rinter 09 0932 058

Soft clay

Imp Grnd

Ar LsDs

04 V 06 V 1V

Unimp Grnd

--- ---

R Bhowmik M Samanta

0 20 40 60

Nos of Piles

--06 V

-10 -05 00 05 10

PR-4-4-AR-0 PR-4-4-AR-15 PR-4-4-AR-35

A-Line

Interior Pile

Corner Pile

0 10 20 30 40

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS

PR-4-4-Ar-15 PR-4-4-Ar-35

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-Ar-15 PR-4-4-Ar-35

0 5 10 15

-078 -076 -074 -072 -070 -068 -066 -064 -062 -060

-10 -05 00 05 10

PR-4-4-CUS

PR-4-4-L-10

PR-4-4-L-15

R Bhowmik M Samanta

Load Level

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-L-5 PR-4-4-L-10 PR-4-4-L-15

-900 -800 -700 -600 -500 -400 -300 -200 -100

0 02 04 06 08 1

Ar-0 Ar-1545 Ar-35

04 06 08 10

V Page 8 of 10

R Bhowmik M Samanta

INTRODUCTION

R Bhowmik M Samanta

Model Ersquo (MPa)

crsquo (kPa)

Φrsquo (ordm)

Soft Clay

Mohr-Coulomb

5 3 20 03 065 18

Rock Mohr-Coulomb

500 01 45 03 05 19

0 0 02 04 06 08

Property Materials

E (kPa) 5000 55000 20000

micro 035 03 03

Oslash 0 43 30

Ψ 0 10 40 γbulk (kN 16 1662 155

c (kPa) 20 0 0

Rinter 09 0932 058

Soft clay

Imp Grnd

Ar LsDs

04 V 06 V 1V

Unimp Grnd

--- ---

R Bhowmik M Samanta

0 20 40 60

Nos of Piles

--06 V

-10 -05 00 05 10

PR-4-4-AR-0 PR-4-4-AR-15 PR-4-4-AR-35

A-Line

Interior Pile

Corner Pile

0 10 20 30 40

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS

PR-4-4-Ar-15 PR-4-4-Ar-35

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-Ar-15 PR-4-4-Ar-35

0 5 10 15

-078 -076 -074 -072 -070 -068 -066 -064 -062 -060

-10 -05 00 05 10

PR-4-4-CUS

PR-4-4-L-10

PR-4-4-L-15

R Bhowmik M Samanta

Load Level

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-L-5 PR-4-4-L-10 PR-4-4-L-15

-900 -800 -700 -600 -500 -400 -300 -200 -100

0 02 04 06 08 1

Ar-0 Ar-1545 Ar-35

04 06 08 10

V Page 8 of 10

R Bhowmik M Samanta

INTRODUCTION

Property Materials

E (kPa) 5000 55000 20000

micro 035 03 03

Oslash 0 43 30

Ψ 0 10 40 γbulk (kN 16 1662 155

c (kPa) 20 0 0

Rinter 09 0932 058

Soft clay

Imp Grnd

Ar LsDs

04 V 06 V 1V

Unimp Grnd

--- ---

R Bhowmik M Samanta

0 20 40 60

Nos of Piles

--06 V

-10 -05 00 05 10

PR-4-4-AR-0 PR-4-4-AR-15 PR-4-4-AR-35

A-Line

Interior Pile

Corner Pile

0 10 20 30 40

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS

PR-4-4-Ar-15 PR-4-4-Ar-35

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-Ar-15 PR-4-4-Ar-35

0 5 10 15

-078 -076 -074 -072 -070 -068 -066 -064 -062 -060

-10 -05 00 05 10

PR-4-4-CUS

PR-4-4-L-10

PR-4-4-L-15

R Bhowmik M Samanta

Load Level

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-L-5 PR-4-4-L-10 PR-4-4-L-15

-900 -800 -700 -600 -500 -400 -300 -200 -100

0 02 04 06 08 1

Ar-0 Ar-1545 Ar-35

04 06 08 10

V Page 8 of 10

R Bhowmik M Samanta

INTRODUCTION

R Bhowmik M Samanta

0 20 40 60

Nos of Piles

--06 V

-10 -05 00 05 10

PR-4-4-AR-0 PR-4-4-AR-15 PR-4-4-AR-35

A-Line

Interior Pile

Corner Pile

0 10 20 30 40

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS

PR-4-4-Ar-15 PR-4-4-Ar-35

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-Ar-15 PR-4-4-Ar-35

0 5 10 15

-078 -076 -074 -072 -070 -068 -066 -064 -062 -060

-10 -05 00 05 10

PR-4-4-CUS

PR-4-4-L-10

PR-4-4-L-15

R Bhowmik M Samanta

Load Level

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-L-5 PR-4-4-L-10 PR-4-4-L-15

-900 -800 -700 -600 -500 -400 -300 -200 -100

0 02 04 06 08 1

Ar-0 Ar-1545 Ar-35

04 06 08 10

V Page 8 of 10

R Bhowmik M Samanta

INTRODUCTION

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS

PR-4-4-Ar-15 PR-4-4-Ar-35

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-Ar-15 PR-4-4-Ar-35

0 5 10 15

-078 -076 -074 -072 -070 -068 -066 -064 -062 -060

-10 -05 00 05 10

PR-4-4-CUS

PR-4-4-L-10

PR-4-4-L-15

R Bhowmik M Samanta

Load Level

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-L-5 PR-4-4-L-10 PR-4-4-L-15

-900 -800 -700 -600 -500 -400 -300 -200 -100

0 02 04 06 08 1

Ar-0 Ar-1545 Ar-35

04 06 08 10

V Page 8 of 10

R Bhowmik M Samanta

INTRODUCTION

R Bhowmik M Samanta

Load Level

-10 -09 -08 -07 -06 -05 -04 -03 -02 -01 00

-800 -600 -400 -200 0

N (kN)

PR-4-4-CUS PR-4-4-L-5 PR-4-4-L-10 PR-4-4-L-15

-900 -800 -700 -600 -500 -400 -300 -200 -100

0 02 04 06 08 1

Ar-0 Ar-1545 Ar-35

04 06 08 10

V Page 8 of 10

R Bhowmik M Samanta

INTRODUCTION

R Bhowmik M Samanta

INTRODUCTION

R Bhowmik M Samanta

INTRODUCTION

Documents

3Th-22