BYDr. V. BALAKUMARSENIOR CONSULTANTSIMPLEX INFRASTRUCTURES LIMITEDCHENNAI 600 008

DESIGN STUDIO IIT MADRAS - CHENNAI

In the design of foundation system for relatively heavy structures that cannot tolerate large settlements, balancing the performance and cost, had always been a challenge for the foundation designers. This objective of generating an economical and safe foundation system has recognized the fact that most structures can tolerate a certain amount of settlement. Consequent to this by sustained research, successful attempts were made to use the piles as settlement reducers for the raft . INTRODUCTION17 AUG 2015

17 AUG 2015*By tradition whenever the bearing capacity or the settlement or both are problems deep piles were thought of a foundation system. Even when bearing capacity is not a problem the procedure remained the same. While the traditional design is safe, satisfying all the serviceability requirements, it does not satisfy the economics. The presence of raft and its capability of transferring the load is completely ignored.INTRODUCTION

The piled raft foundation system, as it is named, provides a skilful geotechnical concept for the design of foundation for structures which are sensitive to large settlements. The piled raft foundation system has been extensively used to support tall and heavily loaded structures in a successful manner permitting larger settlements close to the permissible value.The combined piled raft foundation system utilizes the pile group for control of settlements with the piles providing most of the stiffness at the service loads, while the raft elements provide the additional capacity at ultimate load levels.

It has now been fully recognized that in foundation design the economics lies in controlling the settlement, rather than eliminating it. All the codes and guidelines for foundation design throughout the world accept that there is a permissible settlement for structures depending on their serviceability requirements. In the last three decades a number of structures have been successfully supported on piled raft foundation system. Some of them have also been monitored by instrumentation and the results have been used for further development.

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Piled raft utilises the piled support for control of settlement with the piles providing the required stiffness under serviceability loads and the raft providing the required additional capacity at ultimate loading.. Hence the design has to consider not only the capacity of the pile elements and the raft elements but also the combined capacity and and the interaction under the serviceability loading conditions.

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17, Aug ,2015* Conditions Favorable for Piled Raft & Advantages

Soil profiles consisting of relatively stiff clays.Soil profiles consisting of relatively dense sands. Advantages of piled raftThe serviceability of the foundation system is enhanced by the reduction in the settlement.Improvement on the load carrying capacity by the process of load sharing between the raft and the pile, andReduction in internal stress and bending moment of the raft by proper design of the pile layout.

Unfavorable for Piled RaftSoil profiles containing soft clays near the surface.Soil profile containing loose sands near the surface. This has been over come by compacting the sand.Soil profiles which contain soft compressible layers at relatively shallow depths.Soil profiles which are likely to undergo consolidation settlements.Soil profiles which are likely to undergo swelling movements due to external causes.However this situation is changing with the advent of new ground improvement techniques and materials like geo-synthetics and so on.17,Aug2015*

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17,Aug,2015*It is a fact that the installation of piles compact sand.Hence the piled raft on sand gains an additional advantage in that the piles compact the sand during installation to enhance its state of compaction to help the raft share a higher load. It is to be noted that permissible settlement for sand is less than clay. The applicability of the piled raft on sand has been established this foundation system becomes very useful.Piled raft on sand - a note

OBJECTIVEAlthough the existing design methods fulfil the serviceability requirements, there is a need for determining the ultimate limit state of the piled raft so as to establish adequate factor of safety against failure that will ensure stability. This factor is often ignored as in majority of the cases the serviceability limit state of the superstructure becomes the deciding factor. As such a simpler method of designing the piled raft and the estimation of the ultimate bearing capacity of the piled raft system may be more useful. This presentation makes an attempt to explain the design process based on EQUIVALENT PIER THEORY and the encouragement of Prof.HARRY.G. POULOS IS ACKNOWLEDGED WITH GRATITUDE

THE DESIGN ISSUESThe design of piled raft foundation system has to consider a number of issues which includes:The ultimate vertical, lateral and the reaction due to moment loadsMaximum settlement Differential settlement Permissible settlementLoads and moments for the design of raft and the pile.On many occasions it is considered that only vertical load is given high importance but the other loading such as wind ,earthquake etc are also to be given equal importance.

THE BASIC DESIGN PRINCIPLESConventional approach

Here the piles are designed to take the entire structural load

Creep piling

The The piles are designed to operate at a particular level of working load wherein significant creep will occur; typically 70% to 80% of the working load capacity. Sufficient piles are included to reduce the contact pressure between the raft and the soil below the pre-consolidation pressure of the soil.

Differential settlement control

Here the piles are located strategically at the centre to reduce the differential settlement rather than the total settlement.

But the extreme method will be to make the piles take 100% of the ultimate load wherein the piles are designed only as a settlement reducer.

THE DESIGN PROCEDUREThe preliminary stage of design - mainly a feasibility study

The second stage is to assess the location of the piles and the general characteristics of the pile

Final detailed design to obtain the optimum number of the pile location layout and the distribution of settlement and the pile loads.

VARIOUS DESIGN METHODSStrip on springPlate on springBoundary element methodFinite element methodCombination of boundary element and finite element method

Some of the characteristics of the design

1) Number of piles 2) Nature of loading namely uniformly loaded or concentrated3) Raft thickness4) Load levels

MAIN FEATURES OF RESEARCH SO FAR FOR DESIGNIncreasing the number of piles may not always produce any additional advantages The raft thickness does not influence the overall total settlement but influences the differential settlementLocating the piles in a strategic manner to suit the settlement reduction required.

DATA FOR DESIGNSoil investigation report,Column layout and the loading.Permissible settlement

DESIGN OUTPUTPile layout with diameter spacing and length,Raft size and thicknessPile capacitySettlement reduction achieved.

17,Aug , 2015*DISCUSSION ON THE BEHAVIOUR OF PILED RAFTThe behaviour of piled raft need to be understood clearly before getting into the design aspect. Since the economics of the design is governed by the pile group behaviour and the interaction of the various elements generating the load sharing behaviour This is done here by means of the results from 1g model tests carried out the the author as apart of his Ph.D programme. Although the study has been carried out with sand as bed material the concept remains the same for clay also provided it has some bearing capacity for the raft to share the load

The variation of angle of internal friction with unit weight is presented in the figure.

However the results here presents are related to medium dense sand.17,August 2015*

SMALL SCALE MODEL STUDIES

1G tests were conducted on circular model piled raft

raft diameter = 200mm Thickness (t)= 8mm pile diameter = 10mmpile raft area ratio = 9.25, 6.25, 4.25Length of the model pile = 160mm17,Aug,2015*

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Characteristic response of Plain raft and piled raft (medium sand)17,Aug,2015*

Comparison of load-settlement response of free standing pile group and pile group of piled raft17,Aug,2015*

The characterized load settlement response for the piled raft with three different pile raft area ratios are presented.

The three phase behaviour is seen to be identical both in the case of field and laboratory model. 17,Aug,2015*

THE VARIATION OF STIFFNESS (N/mm)17,Aug,2015*

Area ratio %Stiffness at various phasesPhase OAPhase ABPhase BC9.2529004202806.2526003902204.251600340170

The design of piles depends up on the load shared by the pile group. The load sharing ratio has been defined as pr = Qpr - Qr Q prWhere apr = Load sharing ratio at any given settlementQr = Load taken by the plain raft for any given settlement at which apr is computedQpr = Load taken by the piled raft for the same settlementThe prvalue has been plotted against settlement for the various cases studied. Also the effect of various parameters on the pr value has been plotted for two different settlement levels.17,AUG 2015*

Variation of pr with settlement for various densitiesSettlement Vs LS ratio PR for various length24.01.2015*

24.01.2015*Area Ratio

This confirms that the pile group initially shares more load and gradually with the increase in settlement commences providing the stiffness to the raft to take higher load at any particular settlement; at higher settlement level, the pile group adds the required capacity for the raft to take a higher load compared to the unpiled raft. 17Aug2015*

Non-dimensional plots for various lengths

Non-dimensional plot for piled raft with different pile spacing

It is observed that the behaviour exhibited a hyperbolic trend. . It has been established that the hyperbolic load settlement response can be expressed in terms of Kondtner type hyperbolic functions when the inverse of stiffness (settlement / load, i.e. w/p) is plotted against settlement (w).

Hyperbolic plots for various pile lengths (circular)

Although Chins method [8] tends to over predict the ultimate load, the linear functions represent the pile performance reasonably well.

If the pile group and continuum can be considered as an equivalent pier then the pier can be treated as a single pile

Chins graph circular piled raft variation in length

DESIGN EXAMPLE*

*Elevation of Palace RegencyBuilding, Chennai

BASIC DESIGN

The applied load is shared by the piles and the raft equally.Piles have to be dominantly floating.The settlement level must be such that the piles must mobilize friction entirely.The factor of safety against block feature was computed by F = Pw + N Pi P The raft was instrumented with settlement gauges loaded in such a way that the settlement pile can be reached in both the deviation at main three different limits. The lay out of piles and the settlement gauges are presented in the layout. *

BEHAVIOUR OF PROTOTYPE PILED RAFT1. Purasavalkam2. Name : Palace Regency3. Details : Twelve storied building with basement residential and commercial for the basement and first two floors.4. Maximum column load : 2875kN5. Minimum column load : 1055kNThe soil profile has been presented below.Foundation system : 93 piles 600mm dia capped with 600mm thick raft.Pile termination layer : medium dense to dense sand, N- value around 40.*

Layout of piles and settlement markers*

SECTIONAL ELEVATION WITH GEOTECHNICAL DATA*

Construction of pile and raft in progress at Palace Regency site, Chennai*

DETAILED ANALYSES

Finite Element Simulation and Meshing of Piled RaftSettlement Contour*

Observed Settlement Vs Computed Value at Various SectionThe computed settlement is higher in the edges and smaller in the center. This is mainly due to the fact that in reality, the edges had a retaining wall which was adding to the rigidity. Also the elastic analysis does not take into account the structural rigidity in this case.*

Raft contact stress along grid GContact stress at specific points of the raft*

Contact stress between the rows of piles in transverse sections*

The Head Load Tip Load Distribution with the column load*

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Time dependent load settlement curvesPercentage Load Taken by the Raft at Various Stages of Construction period*

EQUIVALENT PIER THEORY

Frank etal., (1991) have studied the load settlement response of two piles forming a part of a bridge foundation, and had established that their behaviour can be predicted by conducting the pressure meter test. Their prediction of pile behaviour is based on a tri-linear relationship for the skin friction mobilisation based on the pressure meter tests The model they had used is given in Figure.

EVALUATION OF STRESS STRAIN CHARACTERESTICS FROM FIELD TESTS

The load settlement response predicted by the pressure meter with the shaft friction mobilisation can be compared with the equivalent pier analyses to validate the in-situ Es value over the length of the pile (pile group) and the shaft friction over the length of the pile group. This will also establish the shaft stress distribution at any given settlement level and the in-situ Es value which can be used in the detailed analyses.

LOAD SETTLEMENT RESPONSE AND STRESS STRAIN RESPONSE FROM PRESSUREMETER STUDY.

Tri-linear Model, Frank et al (1991)

The following figures present the shaft stress distribution over the length of the pile by pressuremeter test results and from the numerical analyses of the 1g model tests. it is seen that the trend of the shaft stress distribution obtained from both the cases agree closely,indicating that the tri-linear model assumed in the analyses of the pressuremeter results and the actual behaviour of piled raft obtained from the 1g model are identical.This establishes the fact that if the pile group of piled raft can be idealised as a single large pier, then the procedure adopted by Frank et al(1991) can be used to predict the behaviour of pile group of piled raft foundations. RESULTS OF FIELD STUDY

Comparison of Theoretical and Experimental Load Distributions for Test Piles (Roger Frank et al.,) )1991)

17 Aug 2015* PILE SUPPORTED RAFT BEHAVIOURA pile supported raft, not designed as piled raft, can also behave as a piled raft piled raft and becomes effective to minimize both total and differential settlement.It improves the bearing capacity and brings the internal stress level and bending moment with in the raft. The observational study of a case history is explained

Study of the General Soil Profile The general soil profile comprises of alternate seams of soft clay medium dense to dense sand and stiff clay. From the economy point of view fixed length piles required to generate the capacity was proposed. The importance was given more to field test in evaluating the shear parameter for design. So static cone penetrometer was performed. 17 Aug 2015*

Figure presents typical Cone Penetration test data.17Aug 2015*

DETAILS OF PILES AND RAFT

450mm diameter driven cast in-situ pilesAxial capacity=65 tSpacing=3D Length from G.L=10m6m of the pile passes through sand layer. No. of piles=437 nos.Raft thickness 400mm. Pile - raft area ratio 0.09Hydro test load 17.9m of standing water.17 Aug 2015*

17Aug2015*

RESULTS OF FIELD STUDYLoad settlement response curve for two similar tanks are presented in the Figure. In order to study the behaviour, the pile group is considered as a single unit and load taken by the pile group was evaluated as a pier. By Davissons method the pile group as a pier capacity was found to be 11000 kN and 8000kN.17 Aug 2015*

Figure presents the characterized load settlement response of one of the ammonia tanks. This indicates a three phase behaviour. Phase 1 is marked as OA, Phase 2 is marked as AB ; and Phase 3 is marked as BC. It is seen that the rate of change in the stiffness is gradual compared to stage OA and in the phase BC it is rapid. It is this behaviour that is compared with the response in the 1g model studies on the circular piled raft.Typical characterized Load Settlement Response Ammonia Tank17,AUG 2015*

17,AUG 2015*SR and apr at different of load from hydro test for typical Ammonia Tank.Settlement reduction and load sharing at different load levels for 1g model tests. Area ratio 0.05

% of LoadingSRSRapr2595905090847582771007372

% of LoadingSR (%)apr (%)2572405060307554291005027

The following table presents a comparison of the SR and apr at different settlement levels.It is seen that SR and apr reduces rapidly in the initial stages but the reduction rate reduces at higher load level. Comparing the progression of SR and apr values estimated in table that follows, it can be seen that the behaviour of model piled rafts and the pile supported raft are more or less identical. 17,AUG 2015*

SR and apr at different of load from hydro test for typical Ammonia TankSettlement reduction and load sharing at different settlement levels for model piled raft17,AUG 2015*

% of LoadingSRapr2595905092847582771007372

% of LoadingSRapr2572405060307554291005027

Considering the load settlement response and the stress strain response obtained from pressuremeter test the first step in the design process is to treat the pile group as an equivalent pier.Poulos (2001) has shown that while studying the settlement behaviour of the pile group, that if the pile group with the soil prism can be considered as a single pier, then the procedure applied for a single pile behaviour can be used for the prediction of the load settlement response of the equivalent pier numerically.DESIGN MODEL

The equivalent pier modulus is defined as Eeq = ES+ (Ep - Es) At/AgWhere in,EEQ Equivalent pier modulus, ES Elastic modulus of the soil obtained from the pressure meter test, At - total cross sectional area of the piles, Ag gross area of the pierThe applicability of equivalent pier model for the study on piled raft behavior has eelier studied by Horikoshi (1995) but to a limitted extent.Conti

NUMERICAL MODELING

The results for the small scale model tests conducted on the piled raft were validated using FEA package ANSYS. The ANSYS is a total research package containing more than 200 elements. This facilitates the handling of different problems in engineering. For studying this interaction problem static analysis from structural model is adopted. Material models: Linear elastic and MISO model.Elements used: Plane42 and Solid45 (3 degrees of freedom)*

3D ANALYSIS

The model is presented solid 45 elements have been used. Properties of circular piled raftProperties of bed material*

RaftPileErrDiaThicknessDiaLengthArea ratio200mm8mm10mm160mm5.2%3000 N/mm20.33

MaterialEssState of compactionPoorly graded sand15.5kN/m337.5035 N/mm20.30Medium dense

Comparison of load settlement response between ANSYS (linear) and model test results for circular piled raftAs the settlement increases beyond the critical settlement, the linear analysis predicts much higher stiffness.*

Axisymmetric model and mesh used in ANSYS analysesThe axi symmetric analysis of circular piled raft has been carried out for the 21 pile radial configuration. In the analysis each concentric ring of pile is taken as continuous annulus with an overall stiffness equal to the sum of the stiffness of the individual piles The problem was taken as large deformation problem and plane 42 elements have been used.*

Settlement contour for the load of 8.70kN for circular piled raft in medium dense sand*

Finite element mesh of a circular piled raft (Quarter model) used in ANSYS analysisMaterial Model = MISOElements = Solid 45Settlement contour for a circular piled raft for the load of 8.1 kN*

Comparison of load-settlement behaviour between ANSYS and model test data (Circular Raft)*

Raft contact stress at typical locations of the raft for the load of 2.1kN (settlement =1.80mm)Average Raft Stress = 33% of the Applied Load*

Vertical stress at typical locations of the raft for the load of 8.10 kN (settlement = 17.80mm)Average Raft Stress = 65% of the Applied Load*

Pile Head stress for the load of 8.1 kN(settlement =17.8mm)Figure presents the head stress distribution. The stresses are vertical and varies from inner pile to outer pile. The outer pile carries more stress. However the increase is not proportional to the applied load due to the non linear behavior of the system.Stresses in pile tips for the load of 8.1 kN(settlement =17.8mm)Figure presents the tip stressses at the final stage.The tip stress is only 9% to 19% of the applied load indicating that the major portion of the load is taken by friction.*

Variation of stress along the shaft of typical piles along the centre line of raft for 8.10kN*

Load distribution between raft and pile of the piled raft at different settlement levels in terms of loadTypically the load shared by the pile group reduces from 65% to 35%.*

Comparison of settlement vs pr for experimental and numerical studies*

SQUARE PILED RAFT*

Properties of square piled raftEr and r are the same as circular piled raft

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Raft sizeRaft thicknessPile diaSpacingLengthArea ratio200mm8m10m4d160m4.9%200mm8m10m6d160m2.25%

Quarter model and finite element mesh adopted for square piled raft in ANSYS nonlinear analysisSettlement contour for the load of 8.70kN(settlement 18.90mm)*

Comparison of load- settlement response between ANSYS and test data for square piled raft with 4d pile spacing*

Vertical stress in the square piled raft with piles at 4d spacing for the load of 8.7kN*

Pile head stress for the load of 8.70kN (4d pile spacing)Pile tip stress for the load of 8.7kN (4d pile spacing)*

Variation of stress over the length of pile of square piled raft for the load of 8.7kN (No. of piles 25 at 4d spacing)*

Load share between raft and piles of square piled raft (25 piles at 4d spacing) in terms of load*

Comparison of settlement vs pr for experimental and numerical studies*

It is seen that the contact pressure was found to be uniform and the load sharing ratio was found to be increasing with settlement as such from the elastic analysis. Head load, tip load distribution was found to be such that it establishes the ductile behavior of the pile group. It was also found that the load sharing was 57% for raft and 43% for piles indicating that the design and performance of the piled raft was in commensurate the third generation piled raft. Although the piles were placed below the column, there was an effective load distribution. Although the initial assumption was 50% for raft and pile the final distribution of 57% for raft and 43% for piles indicates a very close agreement.*

CONCLUSIONS

* It is seen that the design of piled raft although appears to be complicated, a systematic design approach makes the entire process very simple. The present developments advancements in the computational tools like finite element analyses, optimisation principles like ANN, Genetic algorithm Ant colony theory etc has enhanced the confidence in the designers that any geotechnical problem can solved if not precisely but to an acceptable level. By far the piled raft has become an alternate by choice to the deep piles. It offers a lot of scope for research also. This is a field oriented problem and the accuracy of design largely depends upon the accuracy with which the data is obtained. This is the most difficult task and geotechnical engineers you have to strive to develop in-situ testing so that your design data will be accurate or atleast acceptable.

The load settlement response of the plain raft and the piled raft are similar irrespective of the parameters associated with the raft, piles and the bed. The response is characterized as three phased comprised of elastic and elasto plastic strain hardening behavior.The three phased response of the piled raft exhibited elastic response till the settlement level of around 1% of the dimension of the raft which is more or less equal to the critical settlement of the free standing pile group.The stiffness of the piled raft was much higher than the plain raft in the initial stages of settlement and as the settlement increased the stiffness approached the value of the plain raft stiffness.Although the stiffness of the piled raft increased with the length, diameter and the number of piles (Pile raft area ratio, AR), pile length of 0.8 times the size of the raft was not having any pronounced effect on the behavior; similarly the d/t ratio beyond unity and pile raft area ratio beyond 5 to 6% did not have any effect on the SR or load sharing ratio.CONCLUSIONS

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