Burland Bridge

7/25/2019 Burland Bridge

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2ndInternational Conference on New Developments in Soil Mechanics and Geotechnical Engineering,

28-30 May 2009, Near East University, Nicosia, North Cyprus

Observed and predicted settlement of shallow foundation

Rasin Dzceer

KasktaA..-stanbul, Turkey

KEYWORDS: Settlement, Shallow foundations, Load tests, CPT, SPT, Finite Element Method.

ABSTRACT: The objective of this paper is to compare the predictive capabilities of different

methods of estimating settlements of shallow foundations on sands. For this purpose 2.10 x.2.10 msquare concrete footing was statically load tested. Prior to load test, standard penetration test

(SPT), cone penetration test (CPT) and laboratory tests were performed to determine the

engineering properties of soil layers. Predictions of footing settlement were performed by

conventional (semi-empirical) and finite element method (FEM). The results of static load test

revealed that the settlements were over predicted by Finite element method. Finite element analysis

using either SPT or CPT derived input parameters provided conservative settlement estimates.

However, most of the empirical methods employed in this study provide reasonable estimates

using CPT derived parameters as input.

1 INTRODUCTION

The design of shallow foundations on cohesionless soils is often controlled by settlement, rather

than bearing capacity limitations. Several methods have been proposed for predicting settlement of

shallow foundations on cohesionless soils. Settlement prediction methods can be divided into two

categories, conventional or semi-empirical methods and the finite element based methods.

Semi-empirical methods are the predominant techniques used to estimate settlements of shallow

foundations on cohesionless soils. These methods have been correlated to large databases of tests

such as the SPT and CPT (Kimmerling 2002).

In this paper a 2.1x.2.1 m precast concrete footing was statically load tested to 1.50 times the

proposed design load of 200 kPa to examine the settlement behaviour of the footing. Prior to load

test, SPT, CPT and laboratory tests were performed to determine the engineering properties of soil

layers. Settlement of the footing resting on cohesionless soil was estimated by several methods

based on semi-empirical correlation and FEM. Measured settlement of the footing was comparedwith the settlements estimated by conventional methods and FEM.

2 REVIEW OF THE SETTLEMENT PREDICTION METHODS

Allowable bearing pressure for footings on sand is generally limited by the consideration of

settlement rather than safety against bearing capacity failure. Due to the difficulties of obtaining

relatively undisturbed samples of cohesionless soils, semi-empirical approaches rely on

correlations between the observed foundation settlements and some parameters from in situ tests.

Many methods have been proposed to predict the settlement of foundations on cohesionless soils

based on SPT N values and CPT point resistance, qc. Some of the methods used to estimate

settlement are summarized in Table 1. There are several other methods used to estimate settlement

of foundations based on dilatometer (DMT) and pressuremeter (PMT) derived parameters.

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591

In addition to the conventional approaches in estimating settlement of the shallow foundations,

new methods and techniques are becoming available as more sophisticated electronic and

computational tools are being developed. These include centrifuge modeling (Sargand et al. 1997),

nondestructive test methods such as the wave-activated stiffness (WAK) test (Briaud and Gibbens

1997), and neural networks (Shahin, et al. 2002).

Several studies were performed to compare the predictive capabilities of different methods of

estimating settlements of shallow foundations on sands.

Gifford et al. (1987) concluded that the methods proposed by DAppolonia et al. (1967) and by

Burland-Burbidge (1984) were more accurate than the other methods. The Peck-Bazaraa method

had a tendency to underpredict the field settlement, while the methods by Hough and Schmertmann

(1970) often overpredicted the field settlement.

Briaud and Gibbens (1997) conducted a survey among bridge and foundation engineers for

research work for FHWA. Briaud and Gibbens concluded that the best predictions resulted from

the methods by Briaud (1992), Burland-Burbidge (1984), Peck-Bazaraa and Schmertmann (1986).

Briaud (1992) and Burland-Burbidge (1984) were somewhat conservative for their methods, while

the other two were slightly unconservative.Berardi and Lancellotta (Lancellotta 1995) have compared the reliability and accuracy of

different methods. They concluded that the most accurate empirical methods appear to be methods

suggested by Burland and Burbridge and DAppolania et al.

3 SOIL CONDITIONS

Standard penetration test (SPT), cone penetration test (CPT) and laboratory tests were performed to

determine the engineering properties of soil layers. The soil investigation has revealed that very

loose to loose silty sand up to 4 m depth, followed by dense to very dense silty sand down to 12 m

exist at the site. The ground water table was encountered at a depth of 2 m below ground level. The

results of SPT and CPT are presented in Figure 1.

0 2 4 6 80

1

2

3

4

5

6

7

8

f (%)Friction Ratio.

Depth(m)

0 10 20 30 40 50

Cone Tip Resistance

0

1

2

3

4

5

6

7

8

Depth(m)

qc (MPa)

0

f (kPa)

0

1

2

3

4

5

6

7

8

Depth(m)

100 400

Sleeve Friction

0 10 20 30 40 50

SPT N

0

1

2

3

4

5

6

Depth(m)

Blows / 30cm DESCRIPTION

Light brown to dark

gray, very loose to

medium dense, fine to

medium SAND with

traces of silt

(SP - SM)

200 300

Rs( SPT )

Figure 1: SPT and CPT Results


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Observed and Predicted Settlement of Shallow FoundationDzceer, R.

Table 1 Summary of Settlement Prediction Methods

METHODEXPRESSION FOR

SETTLEMENTDEFINATIONS EXPLANATIONS

D'Appolonia(1967) 0 1

qBS

S = settlement (inches);0 = embedment influence

factor; 1= compressible strata influence factor; q= applied pressure (tsf); M = modulus of

compressibility (tsf).B= footing width (ft)

Department of the

Navy (1982)

2

1

4

1v

q BS

K B

KV1= modulus of subgrade reaction (tons/ft3); Valid for 20B feet

Peck and Bazaraa

(Anderson et al.

2007)

22 2

1D W

q BS C C

N B

CD= embedment correction factor;

Cw= water table correction factor;

N= corrected SPT-N value;

1 0.4

'

f

D

vw

v

CC

q

C

Peck-Hanson-

Thornburn

(D'Appolonia

1967)

10.11 w

qS

C N

Cw = water table correction factor;

N1= average corrected SPT-N value within depth of

1Bbelow the base of footing.1

1

1

0.5 0.5

200.77 log 0 ' 0.25

'

2 ' 0

0.4 0 ' 0.25

w

w

f

v

v

v

v

DC

D B

N N for ts

N N for

N N for t sf

Anagnostropoulos

(1991)

0.87 0.7

1.2

2.37q BS

N

S= settlement (mm); q= applied bearing pressure

(kPa)B= footing width (m);

N= average uncorrected SPT-Nvalue

Bowles (1996)2

(1 ) 's f

s

qBS I

E

I

= Poissons ratio; q= applied bearing pressure(ksf);B=B/2for footing center and =Bfor footingcorner (ft);Es= modulus of elasticity of bearing soil

(ksf);IsandIf influence factor.

Burland and

Burbidge (1984)

2

1 2 3

1.25( / )'

0.25 ( / )

L BS

L B

Bq

S= settlement (ft); 1= a constant (0.14 for normallyconsolidated sands; 0.047 for overconsolidated sands);

2= compressibility index; and 3= correction for thedepth of influence; q'= applied stress at the level of

foundation (tsf);

Meyerhof

(Anderson et al.

2007) 2

8

'12

' 1

qS

Nq B

SN B

q= applied bearing pressure (ksf);B= footing width(ft); 'N Corrected SPT- N value

4

4

B feet

B feet

' 15 0.5 15N N

Schmertmann

(1978)1 2

0

zZ

z

s

IS C C q z

E

S= (in); C1= foundation depth correction factor; C2=

soil creep factor; q= applied pressure;Iz= strain

influence factor; andEs= modulus of elasticity.

1

2

1 0.5

1 0.2 log0.1

fDC

q

tC

Schultze and

Sherif (Anderson

et al. 2007) 0 87 0 41f.

fq BS

. DN

B

f= influence factor; q= applied bearing pressure (tsf);

B= footing width (ft);N= average SPT-N value

within 2Bfrom the base of footing; andDf= footing

embedment depth (ft).

Terzaghi and Peck

(Anderson et al.

2007)

23 2

1

D w

q BS C C

N B

CD= embedment correction factor; Cw= water table

correction factor;N= average uncorrected SPT-N

value for depthBbelow the base of footing; q=

applied pressure (tsf); andB= footing width (ft).

CW=1.0 forDw 2B;

CW= 2.0 forDw B

CD=1.0-4DB 4

DDB

Buisman- De Beer

(1965)

0

0

'

'log

HS

C

H=thickness of layer; C=Compressibility of sand

'o =Effective stress; v =Change in Effective stress

due to applied load; qc=Cone resistance

c

0 '

q=1.5C

Hough

(Kimmerling 2002) 1

'1log

' '

no v

c

i o

S H

C

c

H =thickness of layer ; C=Bearing Capacity Index

'o =Effective stress; v =Change in Effective stress

due to applied load

Janbu (CFEM

1992)

1

1

''1

' '

j j H

'n

o

i r r

S H

mj

=thickness of layer =Initial Effective stress;

1 = final effective stress; =Reference stress 100

kPa; m=modulus number; j=Stress exponent=0.50 for

sandy and silty soils

'o

'r

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4 LOAD TEST RESULTS

Full scale footing load test was conducted at site. A 2.1 x 2.1 m square concrete footing was used

for the test. The test was conducted at the proposed foundation embedment depth. Load test set up

is presented in Figure 2.

Figure 2 Load Test Set up

The full scale load test was performed on a precast footing up to 1.50 times the proposed safe

bearing capacity of footing. The settlement of the footing and the applied loads recorded during the

test are presented in Table 2.

Table 2 Measured Settlements vs. applied test loads

Loading Sequence (kPa)

0 40 80 120 160 180 200 220 240 260 280 300Measured

Settlement(mm)

0 0.6 4.3 7.4 8.9 10.2 12.5 13.8 15.2 17 18.8 22.4

5 INPUT PARAMETERS FOR EMPRICAL AND FINITE ELEMENT METHODS

Numerical calculations were performed with finite element method. Plaxis 9.0 (Brinkgreve and

Broere 2008) was used for this purpose. The square footing is represented by an equivalent area

circular footing using axisymetric conditions. The Mohr-Coulomb model was used to conduct the

analysis. The geometry and the generated mesh are given in Figure 3.

The angles of shearing resistance were correlated from SPT N values from the followingequation:

=53.88-27.6x10(-0.0147 N )

(1)

The angles of shearing resistance were correlated from CPT using the correlation based onthe effective overburden pressure v, qcvalues (Robertson and Campanella 1988)

The following correlations were used to obtain the Modulus of Elasticity with SPT N values and

CPT qcvalues for normally consolidated sands (Bowles 1996):

E (kPa)=500(N+15) (2)

E (kPa)=2-4 qc (3)

The following semi emprical relationship between the soil modulus and the adjusted cone tip

resistance, qc was used to obtain modulus number, m in Janbu method (Canadian foundation

Engineering Manual 1992), as proposed by Massarsch (1994):

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cM

r

qm a

(4)

m = Modulus number ; =Emprical modulus modifier, which depends on soil typeacM

q =Stress adjusted cone stress;r

=Reference stress=100 kPa

'

r

cM c

m

q q

(5)

cq = Unadjusted cone resistance; '

m =Mean effective stress

'm

= 01 2'3

v

K

(6)

0K =Coefficient of horizontal earth pressure

Input parameters used in finite element analysis and empirical methods are given in Table 3.

Table 3 Soil properties from in situ tests

SPT CPT

Description of SoilLayers (CPT)

Depth(m)

N( bl/30

cm)

(kN/m3)

(Deg)

E(MPa)

qc(MPa)

(Deg)

E(MPa)

SAND-GRAVELLY SAND -1.50 to -2.00 12 19 35 13.50 18 48 48

SAND-SILTY SAND -2.00 to -3.50 7 18 32 11 7 41 21

SANDY SILT- CLAYEY SILT -3.50 to -4.00 3 18.5 29 9 1.5 37 4.5

SAND -4.00 to -6.00 23 18 41 19 15 43 45

Figure 3 Finite element mesh for analysis

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6 COMPARISON OF SETTLEMENT PREDICTION METHODS

In this study, ten conventional methods among the available methods have been selected to be

incorporated in settlement predictions.

The comparison of settlement predicted with conventional methods and FEM are summarized

in Table 4 and Table 5 respectively. Comparison of predicted versus measured settlement ispresented in Figure 4.

Table 4 Measured versus predicted settlements by conventional methods

Settlements (mm) Loading Sequence (kPa)

60 120 180 240 300

Measured 2.20 7.34 10.2 15.2 22.4

Buisman- De Beer (3) 6.5 11.3 15.2 18.5 21.4

Burland and Burbidge (1) 6.2 12.3 18.5 24.6 30.8Elastic Theory (2) 3.1 6.1 8. 9 12 14.9

Anagnostropoulos (1) 7.1 13.1 18.5 23.8 28.9D'Appolonia (1) 2.1 4.3 6.4 8.5 10.7Hough (1) 13.8 23.3 30.7 36.9 42.1

Schmertmann (3) 6.3 12.6 18.9 25.2 31.4

Schultz & Sherif (1) 3.2 6.4 9.7 12.9 16.1Department of the Navy (NAVFAC) (1), ( 3) 6.9 13.8 20.6 27.5 34.4

Janbu Tangent Modulus (1), (2) 4.2 10.5 15.8 20.4 24.7

(1) SPT based Method; (2) Elastic Theory base Method; (3) CPT based Method

Table 5 Measured Settlements versus finite element analysis with Plaxis using insitu data

Settlements (mm) Loading Sequence (kPa)

60 120 180 240 300

Measured 2.20 7.34 10.2 15.2 22.4

SPT 3.5 13.2 25.3 39.1 54.1

CPT 1.7 7.4 14.3 21.5 28.9

7 DISCUSSION OF RESULTS

Examination of Table 4 indicates that the six of the conventional methods investigated in this

study, overpredict the settlement of the footing. The most accurate settlement was estimated withthe Buisman - De Beer method, 21.4 mm. The next most accurate method is the Janbu method,

with the settlement of 24.7 mm at 300 kPa.

On the other hand, the Hough method is the least accurate method with 42 mm of total

settlement. This method is considered to be the most conservative among conventional methods in

predicting settlement in sands.

However, the previous studies have shown that, Hough method overpredict settlement by a

factor of 1.8 -2.0 (Gifford et al 1987). It is interesting to note that, an overprediction factor of 1.88

was obtained in this study for Hough method. Following the Hough method Navfac method is the

second conservative method with a total settlement of 34 mm. On the other hand, DAppolania

method, underpredicted the settlement by a factor of 0.48, which is very close to the factor of 0.50

determined in previous studies (Duncan and Tan 1991). Elastic and Shultz - Sherif methods also

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underpredict the settlement. The other methods; the Schmertmann, Burland - Burbridge and

Anagnostrospoulos methods are situated in the middle.

As for the finite element analysis, better accuracy of the estimation is obtained using the input

data from CPT testing. The results of settlement estimate corroborate the conclusion from the

Anderson et al (2007) studies. The settlement predicted from the CPT derived input parameters

was smaller than SPT as the CPT estimated modulus of elasticity and angle of shearing resistances

are higher. The predicted settlements from the SPT and CPT input parameters are 54.1 mm and

28.9 mm respectively. The settlement predicted from the SPT input parameters is less accurate then

CPT.

0

5

10

15

20

25

30

35

40

45

50

55

60

0 50 100 150 200 250 300 350

Applied Pressure (kpa)

Settlement(mm)

Measured

Navfac

D'Appolania

Shultz-Sherif

Burland-Burbridge

Anagnostropoulos

Elastic

Buisman De Beer

HoughPlaxis (CPT)

Plaxis (SPT)

Janbu

Schmertman-1978

Figure 4: Comparison of the predicted and measured settlements.

8 CONCLUSIONS

1. A static load test was conducted to study the settlement behaviour of the footing. The CPT andSPT data were used to estimate the settlement of shallow foundation on sand.

2. Among the CPT based conventional methods, Buismann-De Beer, provide more accurate

estimations of settlement.

3. Janbu method using CPT derived modulus number m, provide good settlement estimate. Thecorrelations proposed by Massarsch provide accurate estimates of modulus number.

4. The correlated input parameters from the CPT data are more consistent than the SPT blowcount in both conventional methods and finite element method.

5. Finite element analysis using CPT derived input parameters provided reasonable settlementestimates whereas the SPT derived input parameters provided poor settlement estimates.

6. The settlement estimations using FEM with CPT and SPT derived parameters corroborate theresults obtained from the previous studies.

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Anderson B.J., Townsend F. C. and Rahelison L. (2007). Load testing and settlement prediction of shallowfoundation. Journal of Geotechnical and Geoenvironmental Engineering ASCE. Vol 133, No 12:1494-1502

Briaud, J-L (1992) The pressuremeter. Balkema,Brookfield,Vt.Briaud, J-L., and Gibbens, R. (1997). Large Scale Tests and Database of Spread Footings on Sand. Report

No. FHWA-RD-97-0680, Federal Highway Administration, McLean, VA,Brinkgreve, R. B. J., and Broere, W. (2008). Finite element code for soil and rock analysis. Plaxis Version

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Sargand S.M., Masada T. and Abdalla B. (2003). Evaluation of cone penetration test based settlementprediction methods for shallow foundations on cohesionless soils at highway bridge construction sites.Journal of Geotechnical and Geoenvironmental Engineering ASCE., Vol 129, No 10 : 900-908

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