elastic settlement a critical review

8/6/2019 Elastic Settlement a Critical Review

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EELASTICLASTIC SSETTLEMENTETTLEMENT OFOF

SSHALLOWHALLOW FFOUNDATIONSOUNDATIONS ONON

GGRANULARRANULAR SSOILOIL—— BBRAJARAJA M. DM. DASAS

Settlement, S

• , e

• Primary, S p

S = Se + S + Ss

landmark

entitled

The Science

Foundations, Karl Terzaghi wrote:

“Foundation problems, throughout, are of such character that

impossible. The only way to handle them efficiently consists

of finding out, first, what has happened on preceding jobs of a

similar character; next, the kind of soil on which the

operations were performed; and, finally, why the operations

have led to certain results. B s stematicall accumulatin

such knowledge, the empirical data being well defined by the

results of adequate soil investigations, foundation engineering

”‐ . . . .“The bulk of the work—the systematic accumulation of

em irical data—remains to be done.”

To evaluate the current state of the art for

settlement redictions of shallow foundations in

sand, in an attempt to promote the use of shallow

foundations.

A FHWA initiative

. e era g way m n s ra on

2. Texas A & M University

.4. American Society of Civil Engineers (ASCE)

exas n vers yNational Geotech Experiment Site

Approximately

5 Square Footings: 1m 1m.5m .5 m

2.5m 2.5m

3m 3m (South)

Predict the load at 25 mm settlement

In Situ Test Summary

Bore o e s ear test 3

Dilatometer test 4Pressuremeter test 4

Step blade test 1Standard penetration test 6

Number of participants: 31

15 consultants

16 academics

Israel – 1 Brazil – 1

Canada – 2 Italy – 1

– – USA – 21

Methods Used for Settlement Prediction

Schmertmann (1970, 1978), Burland and Burbidge (1985)

and FEM being popular

Alpan (3 times) Menard/Briaud (5)

Buisman, DeBeer (3)

Burland & Burbidge (9)

NAVFAC (4)

Oweis (4)Canada Found. Manual (1)

D’Appolonia (4)

DeBeer 1

Parry (1)

Peck (2)

Robertson & Cam anella 1

Decourt (1)

FEM (1)

Schmertmann (18)

Schulze & Sherif (3)

Leonard

Frost (4)

Vesic (6)

re cte vs. easure a ues o 25

Footing (m)

Item 11 1.51.5 2.52.5 33 33

Pre ct on

range (kN)591100 1162950 2954271 4075600 4156400

Existing methods for predicting settlement may be

grouped into three categories:

A — Methods in which observed settlement of

structures

linked

in situ test

results

s an ar pene ra on es , cone

penetration test, Pressuremeter tests)

— em ‐emp r ca me oC — Use of theory of elasticity and modulus of

e ast c ty, E s

CATEGORY A Terzaghi and Peck (1948, 1967)

Meyerhof (1956, 1965)

DeBeer and Martens (1957) Hough (1969)

Peck and Bazaraa 1969

Burland and Burbidge (1985)

Se(1) = sett ement o a test p ate measur ng B1B1

B is usuall of the order of 0.3m to 1m

Terzaghi and Peck (1948, 1967)

60 3.0

qC C S DW e

where q is in kN/m²; B is in m; S is in mm

C W = ground water table correction

below foundation

= 2 if de th of water table is less than or e ual

C = correction for de th of embedment

= 1 – (D f

va ugan, c ers ey an

analyzed 79 settlement records

of foundations provided by

Je alan and Boehm 1986

and Papadopoulos (1992).

)1( BSe

43.0 )1(SB e

4 )1(60 SN ee

75.0)1(Se

Me erhof

)(kN/m2 2q

)(kN/m3

195622

3.060 BNe

m)22.1(

)(kN m25.1(mm)

196560

m)22.1(

3.0)(kN/m2(mm)

Note: q increased by about 50%

m)22.1( 25.1

(mm) Bq

C C S DW e

andkN m2

22 Bm.

0.1W C

eyer o s na ys s

StructureB

(m)AverageN60

q(kN/m2) )observed(

)predicted(

T. Edison, Sao Paulo

Banco do Brasil, Sao Paulo

I aran a Sao Paulo

C.B.I. Esplanada, Sao Paulo

Riscala, Sao Paulo

Ministry, Dusseldorf

Chimney, Cologne

Average ≈ 1.50

DeBeer and Martens (1957)

‘o = effective overburden pressure at a depth

= increase in pressure due to foundation loadingH = thickness of layer considered

cqC 5.1

Field Test Results:o 9.1predicted eS

o servee

DeBeer (1965)

Hough (1969)

)( beaC oc

Value of

Type of soilconstant

(uniformity coefficient C u ≤ 2)Clean gravel 0.05 0.50

Coarse sand

Medium sand

Fine sand

0.50Inorganic silt 1.00 0.50

Well‐graded cohesionless soil

Silty sand and gravelClean, coarse to fine sand

Coarse to fine silt sand

0.090.12

0.200.35

Sandy silt (inorganic) 0.18 0.25

Peck and Bazaraa (1969)

)(kN/m2

(mm)22

BqC C S DW e

where B is in m

(N1)60 = corrected standard penetration number

C = ‘ at 0.5B below the bottom of foundation o = total overburden pressure

‘ = effective overburden ressure

C D = 1.0 – 0.4(D/q) 0.5

Peck and Bazaraa (1969)

kN m754 260

01.025.3 o

)kN/m75(

4)( 260

Peck and Bazaraa’s Method

a ter ppo on a et a .

GRANULAR SOILBurland and Burbidge (1985)

60)(60 25.1 ravel sand

or gravel For NN a

)15(6.015 water ground the

below sand silty

60)(60 NN a

where N60(a) = adjusted N60 value

N60(a) [or

N60(a)]

approximately

constant

increasing) with depth,

BR = reference width = 0.3m

B = width of the actual foundation m

Depth of Stress Influence, z'

If N60(a) [or N60(a)] is decreasing with depth, calculate

z‘ = 2B and z‘ = distance from the bottom of the foundation to the bottom of the soft soil layer (z“ ).

‘ = ‘ = “

Depth of Influence

12factor,Correction

H = thickness of compressible layer

S 71.1

aR NNB

)(6060 ]or[

L7.025.1

where L = length of the foundation

pa = atmospheric pressure (= 100 kN/m2)

Overconsolidated

Soilpressure)idationoverconsol ;( ccq

S 57.0

aR NNB 4.1)(6060 ]or[

BL25.0

For Overconsolidated Soil

:)( cq

eS 57.014.0

aR NNB ]or[

.)(6060

LB 67.0

25.1 7.0

Probability of Exceeding 25‐mm Settlement in the Field

Predicted methods

Predicted

settlement Terzaghi & Peck Schmertmann

Burland &

Burbidge

0.0310

0.260.31

0.270.32

0.420.49

CATEGORY B Schmertmann (1970),

Briaud (2007)

Terzaghi, Peck and Mesri (1996)

Akbas and Kulhawy (2009)

Schmertmann (1970)

])21[()1(

])21)[(1( B AE

I sssz

qC C Ss

q = net stress at the level of the foundation

1 = correc on ac or or e ep o e oun a on

= 1 – 0.5(qo /q)

foundation

= 1+0.2log(t /0.1)

E s = 2qc

The same 79 foundations records

given by Jeypalan and Boehm (1986)

and Papadopoulos (1992)

were anal zed b

Sivakugan et al. (1998).

Schmertmann et al (1978)

Schmertmann et al. (1978)

(peak) 1.05.0

Item L/B = 1 L/B 10

Iz at z = 0 0.1 0.2

z p /B 0.5 1.0

E s 2.5qc 3.5qc

Salgado (2008)

20111.01.0)0(at LI zz

41222.02 BBzo

Lee et al. (2008)

FEM Analysis

6at1of maximumawith

4315cos95.0 L

at4o max mumaw t B

Terzaghi et al.

(1996)

0z se E

4.1log4.01)1/(

5.3)1/( cBLs qE

dayslog1.0

cree oe

81 Foundation and 92 Plate Load Tests

L d S l C

Load‐Settlement Curve

Based on Pressuremeter TestBriaud (2007)

Pressuremeter Test

],,,[ (mean))/( p f f f f q pd eBL

functiongamma

SHAPE FACTOR

f BL 2.08.0)/(

ECCENTRICITY FACTOR

e f e Center33.01

eBe f e Edg1

LOAD INCLINATION FACTOR

Center90

(deg)1

Edge(deg)15.0

slope1:318.01.0

ong‐term sett ement, nc u ng creep =

1 t Se

t = design life (in hours)

L1 ‐ L2 Method

167 Axial compression field load tests

e(L1) = .

68.169.02

theorysVesic'of portionu NQ

theorysVesic'of portionqqu NQ

Note: Vesic’s theory includes compressibility factor. So

),,,,( BE f Q su

CATEGORY C

Use of Theory of Elasticity andModulus of Elasticit

soe II

E BqS )(

E s = average modulus of elasticity (z = 0 to z = 4B)

B‘ = B/2

s = Poisson’s ratio

Steinbrenner (1934)

Is = shape factor = f (m, n)

For Se at the center : = 4

Fox 1948

and,factordepth s f

center)(flexible,(rigid) 93.0 ee SS

Variation of I f

D f /BL /B

1.0 2.0 5.0

’ s = .

. . . .

Poisson’s ratio s = 0.40

0.20 0.932 0.955 0.970

0.600.80

0.7790.727

0.8360.788

0.8860.849

zE E i ss )( average,Weighted

s = 60

Mayne and Poulos (1999)

os zk E E

)1( 2s

E RGee

IIIBqS

HE ,ac orn uence

BBk ee

factorcorrectionrigidityfoundation

5.04 BL

6.14.022.1exp5.3

Berardi and Lancellotta (1991)

Is = influence factor for a rigid foundation (μs

= 0.15)

(Tsytovich, 1951)

H 1 /B

. . . .

1 0.35 0.56 0.63 0.69

Berardi and Lancellotta (1991) re‐analyzed field

erformance of 130 structures on redominantl

silica sand as reported by Burland and Burbidge

1963)(Janbu, 5.0

pa = a mosp er c pressure

at a depth B/2 below the foundation ando

npenetratiostandardcorrectedaverage601 Nzonen uencet ennum er

H15 = (1.2 to 2.8)B

25 = . o .

Influence zone for L B ≥ 10:

H15 (1.8 to 2.4)B

Skempton (1986)

602601

P d f C l l i SE ( d)

Procedure for Calculating SE (continued)

6. Determine

calculate Se.

. e . e 9. If so, the calculated Se in Step 7 is the answer.

. , e E .

11. Repeat Steps 5, 7 and 8 until the assumed and

Settlement Prediction in Granular Soils

ro a s c pproacSivakugan and Johnson (2004), Geotechnique, Vol. 54,

Predicted Settlement – 25 mm

Probability of

exceeding 25 mm

Method in the field

Terzaghi & Peck (1948) 0.26 (26%)

Schmertmann (1970)

Burland & Burbidge (1985)

0.27 (27%)

0.42 (42%)

erar ance otta .

1. Meyerhof’s relations (1965) simple to use. On the

average, will give Se(predicted)/Se(observed) 1.5 to 2.0.

2. Peck & Bazaraa method (1969) is not superior to

that of Meyerhof (1965).

3. Burland & Burbidge (1965) is an improved method

over that of Meyerhof (1965) and Peck & Bazaraa

(1969).

Difficult to estimate overconsolidation pressure

from field exploration.

S h t t l (1978) T hi t l (1996)

Schmertmann et al. (1978), Terzaghi et al. (1996),

Sal ado 2008 and Lee et al. 2008 will ive

reasonable results with proper values of E s .5. Su ested E relations:

BLs LE 4.1log4.01

qE 5.3)1/(

6. The E s (L/B = 1) relationship can be related to N60 via

7. Pressuremeter method of develo in load‐

settlement relationship is er effecti e b t ma

settlement relationship is very effective, but may

not be cost effective.

8. L1 – L2 (Akbas and Kulhawy) is a good method.

However proper assumption of E and needed to

estimate QL2.

9. Relationshi s

settlement

develo ed

theory of elasticity will give equally good results

provided a realistic E s is used.

Use of iteration method is suggested.

If not, used Terzaghi et al.’s relationship

(1996).

What we have seen is a systematic

accumu a on o now e ge over years.

parameters

comparing

settlement

reliability.

Reliability is the probability that the actual

settlement would be less than that computed

specific

method.

choosing

method

design

In choosing a method for design, it all comesdown to keeping a critical balance between

at times, knowing the non‐homogeneous

nature of soil in eneral. We cannot be over‐

conservative but, at the same time, not be

accurate.

We need to keep in mind what Karl Terzaghi

sa n e ames orres ec ure a e

Institute of Civil Engineers in London: “

‘an act of God’.”

elastic settlement a critical review

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