Prediction and Measurement of Settlement of Piled Raft Foundation Over Thick Soft Ground

7/27/2019 Prediction and Measurement of Settlement of Piled Raft Foundation Over Thick Soft Ground

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Prediction and Measurement of

Settlement of a Piled Raft Foundation

over Thick Soft Ground

Zehai ChengSchool of civil engineering and architecture, Zhejiang University of Science and

Technology, Chinae-mail: [email protected]

ABSTRACTThe piled raft has proved to be an economical foundation type compared to conventional pile

foundations. The application of piled rafts on soft clay has increased recently. The settlementof a boiler foundation has been observed over a period of more than 5 years. The maximum

settlement was 50mm and the differential settlement was 7.8mm. The final settlement was

derived based on the field data. The parameter of compressive modulus of the subsoilunderneath piles was discussed. A simplified pile-soil-raft interaction method has been

presented in this paper. That is, the pile is modelled as an elastic spring with the stiffness of

the corresponding pile. The raft is analyzed by the Finite Element Method (FEM) with the 16-

node degenerated element. The element is particularly suitable to model the raft with irregular

thickness and shape. The settlement of the boiler foundation has been predicted successfullyby the simplified method. The predicted results are in good agreement with the measured.

KEYWORDS: piled raft foundation, settlement, pile-soil-raft interaction, predictionand measurement

INTRODUCTION

The thick soft soil layer in Chinese coastal area varies from tens of meters to hundreds of

meters in depth. Piled raft or piled box foundations always have been applied to high-rising

buildings over it, always the piles cant penetrate through the soft soil layer, and there is a

compressible saturated layer under the piles. In order to assure the buildings safety and have goodeconomy effect, controlling total and differential settlements is significantly important in the

design of pile foundations. Many achievements have been obtained by researchers. Poulos1 put

forward the elastic theory method of interaction of piles and subsoil based on the Mindlins

solution. Small et al.2 estimated the rate of consolidation of piled rafts and calculated the

magnitude of differential deflections and the associated moments that develop in the raft with

time by making use of infinite elements. Chaudhary 3 investigated the application of a group of

1072 piles for controlling settlement of an important and sensitive structure founded on weak

rock. 2D axi-symmetrical and 3D finite element models are employed to model the 9.0 m thick
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and 76 m diameter circular raft, underlying soil/rock and a mix of annular and orthogonal pile

layout. Comodromos et al.4 proposed a design strategy for an optimized design of pile raft

foundations subjected to non-uniform vertical loading based on both experimental data and

nonlinear 3-D analysis. Lee et al.5 described a three-dimensional behavior of a piled raft on soft

clay based on a numerical study using a 3D finite element method. The proportion of load sharingof the raft and piles at the ultimate state and the relationship between the settlement and overall

factor of safety was evaluated. Zhao6 presented a design theory of piled raft (box) foundation in

the Shanghai high-rise buildings. Dong7 suggested a new method of calculating the settlement of

piled box (raft) foundations. A formula of calculating the mean and differential settlement was

derived by Yang8, which has taken the interaction among the piles, soil and raft into

consideration. Huang9 applied plastically bearing pile in multistoried buildings. Cao10 used the

semi-analytical method to analyze the pile soil interaction. Liu11 studied the deformation

properties of pile groups. Kim 12 optimized the pile foundation design to reduce the settlement by

changing the arrangement of piles. Chen 13 analyzed the settlement of a piled raft foundation with

different pile length by integral equation.

In this paper a simplified method of analysis capable of taking into account pile-soil-raftinteraction has been presented to predict the foundation settlement. The piles are simplified as

elastic springs, the stiffness of which are determined by the load and settlement curve of piles

under raft. A new kind of element, 16-node degenerated isoperimetric element, is employed to

study the behavior of raft, which can give out the raft deformation. The element is particularly

suitable to model the raft with irregular thickness and shape. Compared with other methods, the

presented method is rather easy and simple to estimate the pile stiffness and to model layered

soils.

The parameter of compressive modulus in deep-level soft substratum may make a great

difference on the analyzing of the foundation settlement. Due to the disturbing and stress

releasing of the soil sample, the modulus of compression tested in the high pressure consolidation

test dont coincide with the field situation and is of big discreteness. An empirical relationshipbetween the modulus of compression and the specific penetration resistance, the number of

standard penetration in Shanghai has been presented. The modulus of compression for

cohesionless soil should be given by the in-situ static sounding penetrated or SPT.

The piled raft foundation has been applied to the Waigaoqiao power plant 5# boiler. The

foundation settlement has been observed for more than 5 years, and the measured data indicates

that the simplified method of piled raft foundation may forecast the settlement successfully.

PROJECT PROFILE

The installed capacity of the second phase of Waigaoqiao power plant reached

2900MW~1000MW, it was planned to build two boiler plants, each one weighs 500 000 kN.

The load is transferred to the board and piles through 4 pillars. The maximum vertical load on one

pillar may reach 128 000 kN, and the horizontal load is 18 000 kN. The laterally and

longitudinally distance between pillars may be 31.5m and 30.5m. The differential settlement of

the foundation is strict limited due to the heavy load, which means the overall inclination is less

than 1/500, and the maximum total settlement is less than 1.5cm. The size of the foundation is

shown in Figure 1. The area belongs to estuarine sediment. The soft soil layer varies from 150m

to 400m in depth, which mainly consist of muddy soil, silty clay and fine sand layer. Therefore


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the foundation design faced serious challenge because of heavy load, high standard, complicated

environment, low bearing capacity and high compression of soft soil layer.

Figure 1: Plan for boiler foundation board

The plane of the boiler foundation plate is a rectangle, 61 m47 m4.7 m, and 1.5m deep

buried in the ground. The strengthen grade of the concrete is C30. Steel pipe piles are chosen

according to the result of pile testing, which is 69m long, the pile bottom reached 2 layer. The

vertical limit bearing capacity of single pile is 9000kN, while the horizontal ultimate bearing

capacity is 300kN. The simplified analysis method of pile-soil-raft interaction is applied to design

the foundation in the engineering. The total number of piles is 306.

PREDICTION AND MEASUREMENT OF SETTLEMENT

Analysis of the stiffness of single pile in pile groupsThe settlement at the pile head is made up with two parts: the compression of the pile and the

settlement at the pile tip. The static load settlement curve of a single pile is shown in Figure 2,

which suggested that the settlement of the single pile has the characteristics as following14:

When the load on the top of the pile P< (0.5~0.6)Qu. (Qu. is the ultimate load), the settlement

increases linearly with the increase of load, and the stiffness remains approximately a constant

value.

Compared to the settlement at the top of the pile, the settlement at the tip of the pile beforereaching the ultimate load is relatively small, as shown in Figure 3, When P= 0.5Qu, the

settlement at the pile tip is 0.5mm, the settlement at the pile head is 12mm, the settlement at the

pile tip only occupies 4% of the total settlement. Thus, the settlement of an independent loaded

single pile at the pile head mainly consists of compression of the pile.

1#

Pile4

#

3#

27500

7500

412509750 9250

30500

31500

7750

39500

10500

2#


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Figure 2: P-S curve for the steel pipe pile Figure 3: Settlement ratio for the steel pipe pile

However, completely different from the independent single pile which is loaded alone, due to

the superimposed stresses, the settlement at the pile tip in pile groups is far larger than that in the

independent single pile. The settlement of pile groups is made up with the compression of the pile

and the settlement at the bottom of the pile. The stiffness of pile kin pile groups can be defined as

the following equation.

k

k

kS

PK = (1)

wherek

P is the load on the top of pile k,k

S is the settlement at the top of pile k. The settlement of

the single pile in pile groups may be indicated as kkk SS += , where k denotes the compressionof pile kand

kS is the settlement of substratum at the position of pile k.

k may be got by the P-S

curve whilek

P is available. Butk

S cannot be got only byk

P , because it is caused by the piles in

the group.

After the review and analysis of a large quantity of literature, the result indicates that the

force on the pile head is around average forcen

PP = , so the assumption may be given as:

Pn

PP

k==

(2)

wherePdenotes total load on the foundation plate and n is the number of total piles.

Substituting equation (2) into equation (1), we can obtain the following equation:

)()( nPPS

nPK

kk

k+

= (3)

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1 1.2

(P/Qu)

(%)

Settlementratio(

Load ratio

Pile compression/pile tip settlement

Pile tip settlement/pile top settlement

0

20

40

60

80

100

120

140

0 5000 10000 15000

P(kN)

(mm)

4000 8000 12000

P (kN)

Settlement(mm)

Pile top settlement

Pile tip settlement


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wherek

K is stiffness of single pile kin the group. According to the mentioned equation, the load

settlement curve of each pile in group may be obtained. Figure 4 shows the comparison of the

settlement of single pile at the pile head and at the pile tip respectively between in independent

single pile and in pile groups. It is obvious that the stiffness of a single pile in pile groups is much

smaller than that in independent loaded single pile. The P-S curves are different owing to

different pile location.

Figure 4: P~S curves in independent single pile and in pile groups

Analysis of the Settlement of Substratum

Layerwise summation method is widely used due to the complicated stress distribution in the

underlying stratum. Shanghai code of design building structure (DBJ08-11-59)15 set that for close

order-march piles, foundation cushion cap, pile groups and subsoil may be regarded as a deep

foundation and take no account of stress distribution around the pile, from the pile tip to the planethat subsidiary stress equals 0.2 times weight stress when calculate the final settlement.

Boussinesqs solution is used to figure out the subsidiary stress. Under the weight and additional

pressure, the modulus of compression of the soil in substratum is applied. The final settlement of

the pile foundation in any position may be obtained by the following equation:

SSs

= (4)

where Sdenotes the pile foundation final settlement, S is the settlement of the pile foundation

figured out by layerwise summation method,s

is the empirical coefficient of calculating the

settlement.s

may be given through the observed data and experience from similar projects or

check table 1 if there's no data. The depth of the pile injecting into bearing stratum in this project

is 69m.s

can be obtained through two points inner interpolation.

Table 1: Empirical coefficient for settlement calculation

Injecting depth of pile (m)


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Discussion of the Parameter of Compression

The modulus of compression tends to be different from the practical conditions and of big

discreteness. There are two reasons to explain why, the first is the disturbing of the soil sample

and the second is the change of the stress state. The disturbance degree is concerned to the

properties of the sample. Clay tends to remain stationary with little disturbing due to cohesion.

Sandy soil has little or even no cohesion, which has made it difficult to make undisturbed soil

samples. Besides, after the samples are taken out, the stress is released completely, though the

consolidation has been reformed in the experiment, but the sample cannot recover to it used to be,

which would also influence the compression.

After a sum of calculating of the settlement, it is concluded that in Shanghai, there exists an

empirical relationship between the specific penetration resistance in single bridge probe, point

resistance force in double bridge probe, the number of standard penetration and modulus of

compression as following16:

8.23 +=ss

PE (5)

6.51.2 += cs qE (6)

7.356.0 += NEs (7)

wheres

P denotes the specific penetration resistance in single bridge probe,c

q is the point

resistance force in double bridge probe, N is the number of standard penetration.

As shown in Table 2, the tests indicated that 2 silty clay intercalated with silt is 66m deep,the weight stress is about 520kPa, there is a big gap between Es andEs1-2,Es is 1.6 times ofEs1-2.

2 medium sand is 77m deep, which weight stress is about 620KPa, Es is 3.2 times larger than

Es1-2. It approaches to the modulus of compression estimated in the experiment, which is about

31.7~38.2MPa. Therefore, when the layer is shallow, the stress level of weight stress and

subsidiary stress is about 100~200kPa, Es is in agreement with Es1-2. While the layer is deep

enough, the stress level is far above 100~200kPa, Es is obviously larger than Es1-2, and the

difference increases with the increase of the layer depth.

Table 2: Calculation of compressive modulus

soil layer average depth(m)

Ps(MPa)

qc(MPa)

N Es1-2(MPa)

Es(MPa)

Eps,s(MPa)

Eqc,s(MPa)

EN,s(MPa)

3-1 21 1.00 0.93 3.6 4.21 4.30 4.25 7.5 5.7

2 66 3.27 1.68 16 6.29 10.0 12.61 9.13 14.6

2 77 11.93 12.7 50 7.43 25.0 38.24 32.8 31.7

Where Eps,s is compressive modulus estimated by the specific penetration resistance in single

bridge probe, Eqc,s is compressive modulus estimated by the point resistance force in double

bridge probe,EN,s is compressive modulus estimated by SPT.


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Vol. 16 [2011], Bund. A 131

The change of specific penetration resistance in single bridge probe and number of standard

penetration with the depth is shown in Figure 5. The specific penetration resistance and the

number of standard penetration can reflect the change of the layer accurately. In the rich of silt

and sand, the specific penetration resistance and the number of standard penetration is far higher

than other soil layers. Figure 6 demonstrates the result of estimating the modulus of compressionof different layers by in-situ test. It is concluded that the modulus of compression for the clay

with different methods is close. For the cohesionless soil the compressive modulus estimated by

the specific penetration resistance is concordant with that by SPT, however, the result estimated

by high pressure consolidation test is apparently low. It indicates that the disturbing of

cohesionless soil make a difference as the pressure is high. In brief, the modulus of compressionfor cohesionless soil should be obtained by in-situ specific penetration resistance or the standard

penetration test. The underlying stratum in this project is 2 medium sand, the modulus

calculated by specific penetration resistance and standard penetration test is 35MPa.

Prediction and measurement of settlement of the piledraft foundation

Finite-element Analysis of Raft

In order to know detail of the behavior of raft, i.e. inner force and deformation of raft, finite

element method always is preferred. Quadrangular or triangular isoperimetric elements based onthe Kirchhoffs and Reisseners plate theory are mostly used in the analysis of raft. The most

difficulties of the elements were that it could not be used directly to analyze the raft with irregular

thickness and no horizontal stiffness of piles and soil could be considered. Ling et al. 17 developed

a new plate element, with 16 nodes 40 degrees of freedom. Routine element of plate is

constructed based on the Kirchhoffs or Reisseners plate theory. Contrary to the routine element

of plate, this element was degenerated from the 16 nodes isoparametric solid element. The 16

nodes isoparametric solid element is constructed from three dimensional elastomer. The stiffness

matrix of 16 nodes isoparametric solid element ][K is expressed as:

0 10 20 30 40

(MPa)

m

0

10

20

30

40

50

60

70

80

Compressive modulus (MPa)

Depthofsoillayer(m)

By SPT

By static cone penetration test

N, Ps (5MPa)

0

10

20

30

40

50

60

70

80

0 20 40 60

K404 K450 K405J431 J433

DepthofSoillayer(m)

Figure 5: Change of specific penetration resistance

and number of SPT blow with depth

Figure 6: Compressive modulus based on

in-situ soil test


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=1

1

1

1

1

1

]][[][][ dddJBDBK T 8

Where J =Jacobi matrix, [D] =Coefficient of elasticity matrix. For isotropic material, [D] mayexpressed like this:

=

44

44

44

331313

131112

131211

][

d

d

d

ddd

ddd

ddd

D (9)

For three-dimensional theory of elasticity, where

)21)(1(

)1(3311

+

==

Edd ,

)21)(1(1312

+==

Edd ,

)1(244

+=

Ed

Eis Young modulus, is Poisson's ratio. The board element's [D] may be given by making

the hypothesis of the board in the stress-strain relation,

211 1 =

Ed ,

212 1

=

Ed ,

233 1

=

Ed , 0

13=d ,

)1(2 244

+=

Ed .

The element has 48 degrees of freedom, assuming the upper and downward nodesdisplacement is the same and there is no load on the surface of the board. The degrees of freedom

reduce to 24 and the element is three-dimensional degenerated isoparametric element.

Fundamental Equation

Figure 7: Simplified model for pile soil raft interaction

The simplified model for pile soil raft interaction is shown in Figure 7. The raft's equilibrium

equation may be given after finite element discretization:

{F}

Raft {K} {}

{Kf}


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Vol. 16 [2011], Bund. A 133

}{}]){[]([ FKKf

=+

(10)

where [K] is the matrix of the stiffness of nodes, including the horizontal one and the vertical one,[Kf] is the matrix of the stiffness of piles and soil, which is related to time due to the soft

underlying stratum, {} is displacement vector of nodes, {F} is load vector of the raft. [Kf]denotes a diagonal matrix, the elements on the diagonal is the pile's stiffness:

(11)

wherei

K is the stiffness of pile i.

By solving the equation (10), the deformation of the raft is available. The result is shown in

Figure 8 and Table 3.

Figure 8: Isoline map of foundation raft deformation


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Vol. 16 [2011], Bund. A 134

The maximum settlement of the foundation is 76.06mm, the maximum differential settlement

is 12.99mm, the maximum gradient is 1/2117, and the maximum settlement out plane is 2.17mm.

The predicted settlement is within the request on engineering.

The observing of the settlement of the Waigaoqiao Power Plant 5# boiler foundation haslasted for 5 years. The curve of the foundation settlement and time is shown in Figure 9. The

measurement shows that the maximum settlement is 50mm and the maximum differential

settlement is 7.8mm. The final settlement can be derived according to the measured data. Using

hyperbola method, the maximum final settlement of the foundation is 74.2mm, the maximum

differential settlement is 16.3mm, the maximum gradient is 1/1819, and the maximum settlement

out plane is 2.0mm shown in Table 3. The research indicates that the prediction of the final

settlement of the foundation which takes advantage of the simplified method is in good agreement

with the measured one.

Table 3: Comparison between prediction and measurement of the foundation settlementIndex Predicted final

settlementMeasured

valueFinal derived result by

measured value

1# column settlement (mm) 76.06 46.42 72.89

2# column settlement (mm) 63.07 42.57 57.77

3# column settlement (mm) 75.10 45.40 66.484# column settlement (mm) 64.28 50.37 74.20

1#~2# column non-uniform

settlement(mm)

12.99 15.12

1#~2# column gradient (1/n) 1/2117 1/1819

1#~3

#column non-uniformsettlement(mm)

0.96 6.41



settlement (mm)

11.78 1.31


2#~3# column non-uniformsettlement (mm)

12.03 8.71



settlement (mm)

1.21 16.3

2#~4

#column gradient (1/n) 1/26033 1/1871


settlement (mm)

10.82 7.72


Maximum out of plane (mm) 2.17 2.06


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Vol. 16 [2011], Bund. A 135

Figure 9: Comparison of the predicted result with the measured

CONCLUSIONSThe controlling of total and differential settlements is a key to design the pile-raft foundation

in super high buildings. This paper presented a simplified but helpful analysis method,

considering the interaction of soil-pile-raft but avoiding the complicated calculation. The

settlement of the piles under raft was divided into two parts, and estimated respectively. Thestiffness of piles was easily decided with the obtained settlement of piles under raft. The 16 nodes

degenerated element, which is suitable for the raft with irregular thickness, was employed to

analyze the behavior of the raft. The compressive modulus of the underlying stratum was

discussed. The disturbing of cohesionless soil may be influenced greatly during sampling and the

modulus of compression tends to be low in high pressure consolidation tests. It suggested that themodulus of compression should be given by the specific penetration test in situ or the standard

penetration tests. The settlement of the foundation in a typical engineering has been measured

completely. The simplified method of pile-soil-raft interaction has been successfully applied to

the design of the foundation. The prediction of the final settlement of the foundation agrees with

the measured well.

ACKNOWLEDGEMENTS

This work was financially supported by the Zhejiang Natural Science Foundation

(Y1100116) and the Zhejiang Major Scientific and Technological Innovations Group Project

(2009R50050).

REFERENCES1. Poulos. H. G. and Davis, E. H. (1980) Pile Foundation Analysis and Design, New

York.

2. Small J.C. and Liu H.L.S. (2008) Time-settlement Behaviour of Piled Raft FoundationsUsing Infinite Elements Computers and Geotechnics, 35 (2) , 187-195

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

Settlement(mm)

Settlement observation point

1/29/2002

4/29/2002

7/25/2002

10/30/2002

1/23/2003

3/5/2003

2/14/2004

3/7/2005

12/14/2005

Predicted final settlement

Derived final settlement

4#1

# 3

#2

#


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Vol. 16 [2011], Bund. A 136

3. Chaudhary M.T.A.(2007) FEM Modelling of a Large Piled Raft for Settlement Controlin Weak Rock Engineering Structures, 29 (11) , 29012907

4. Comodromos E.M., Papadopoulou M.C. and Rentzeperis I.K. (2009) Pile FoundationAnalysis and Design Using Experimental Data and 3-D Numerical Analysis, Computersand Geotechnics, 36(5), 819836

5. Lee J.H., Kim Y.H., Jeong S.S. (2010) Three-dimensional Analysis of Bearing Behaviorof Piled Raft on Soft Clay, Computers and Geotechnics, 37(1-2), 103114

6. Zhao X.H. (1999) Design Theory of Shanghai High-rise Buildings Pile-raft and Pile-boxFoundation, Tongji University Press

7. Dong J.G. and Zhao X.H. (1996) Settlement Calculation Method of Box (raft) PileFoundation, Chinese Journal of Geotechnical Engineering, 18(1), 80-84.

8. Yang M. and Zhang J.F. (1998) Practical Formulas of the Settlement of Pile Foundationin Soft Clay, Building Structure, 6 (7), 14-20

9. Huang S.M, Wang D.M. and Pei J. (1992) Design Method of Controlled-settlementComposite Piled Foundation, Industrial Construction9(7)41-44

10.Cao Z.Y. (1993) Theory and Application of Structure and Medium, Hehai UniversPress

11.Liu J.L, Huang Q., Li H. and Gao W.S. (1995) Deformation Behaviour and SettlementCalculation of Pile Group under VerticaI Load, Chinese Journal of Geotechnical

Engineering17 (6), 1-13

12.Kim K.N., Lee S.H., Kim K.S., Chung C.K., Kim M.M. and Lee H.S. (2001) OptimalPile Arrangement for Minimizing Differential Settlements in Piled Raft Foundations,

Computers and Geotechnics, 28(4), 235253

13.Liang F.Y., Chen L.Z. and Han J. (2009) Integral Equation Method for Analysis of PiledRafts with Dissimilar Piles under Vertical Loading, Computers and Geotechnics, 36 (3),

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14.East China Electric Power Design Institute. (1997) Shangai Waigaoqiao Power PlantStatic Load Test Report,

15.Shanghai Building Design Institute. (1989) Shanghai Code for Design of BuildingFoundationDBJ08-11-89, Shanghai construction committee.

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Prediction and Measurement of Settlement of Piled Raft Foundation Over Thick Soft Ground