Topic 2.3 ive geo

Geotechnical and Foundation Engineering

SCE5331

Geotechnical and Foundation Engineering

Dr. Hong Chengyu, Joey

Office: 301, Tel: 2176-1545

Email: [email protected]

1

TOPICS & SYLLABUS:

Topic 1: Review of Soil Mechanics

Topic 2: Shallow Foundations

Topic 3: Lateral Earth Pressure and Retaining Walls

Topic 4: Pile Foundations

Topic 5: Subsoil Exploration

Topic 6: Slope Stability

Textbook: Braja M. Das. (2007). Principles of Foundation

Engineering, 6th Edition, ISBN 0-495-08246-5.

Reference book:

Foundation Design and Construction (2006), GEO

Publication No. 1/2006, 376 p.

2

2.1 Bearing Capacity of Shallow Foundation

General Concept

Terzaghis Bearing Capacity Theory

Factor of Safety

Modification of Bearing Capacity Equations for Water Table

The General Bearing Capacity Equation

Eccentrically Loaded Foundations

2.2 Settlement of Shallow Foundation

Stresses From Elastic Theory

Types of Foundation Settlement

Elastic Settlement Based on the Theory of Elasticity

Elastic Settlement of Foundations on Saturated Clay

Range of Material Parameters for Computing Elastic Settlement

2.3 Primary Consolidation Settlement and Creep Settlement

Primary Consolidation Settlement Relationships

Three-Dimensional Effect on Primary Consolidation Settlement

Vertical Stress Increase in a Soil Mass Caused by Foundation Load

Allowable Bearing Pressure in Sand Based on Settlement Consideration

Field Load Test

Tolerable Settlement of Buildings

3





Field Load Test


2.3 Primary Consolidation Settlement and Creep Settlement

4

3.14 Primary Consolidation Settlement Relationships (One-Dimensional Straining Vertical Compression Only)

einitialeratiovoide

e

estrainvertical

HdzS

o

o

z

cz

ifH

zpc

zc

;

1

constant

0

)(

conditionoedometerinsettlementSoedc

'''

''''

'''

)(2

1

)4(6

1

),,,,()(

mbt

bmtav

scoczz CCfb

')( zvz ma

5

cavc

czpc

cav

He

CHS

with

iiAreaclayedconsolidatnormallyFor

'

0

''

0

0

)(

'''

0

log1

:

)(

e'''

zoz

'

c'

o'

z

cC

es CorC

'

z)(i

)(ii

cavs

pc

cav

He

CS

with

iAreaclayedconsolidatoverFor

'

0

''

0

0

)(

'''

0

log1

:

)(

stresseffectiveverticialinitial

pressuredationpreconsoli

indexelasticswellingCC

indexncompressioCwhere

He

CH

e

CS

with

iiiAreasclayedconsolidatoverFor

o

c

es

c

c

avcc

cspc

avco

'

'

'

''

0

0

'

0

'

0

)(

''

0

''

/

log1

log1

:

)()(

conditionoedometerinsettlementSoedc

6

3.15 Three-Dimensional Effect on Primary

Consolidation Settlement

][ )3()1()3( Au7

22.3FigurefromratiosettlementK

conditionoedometerinsettlementS

KSS

oedc

oedcc

B: diameter of a circular

foundation or width of a

continuous foundation.

8




(Self review)


Field Load Test


Primary Consolidation Settlement and Creep Settlement

14

Stress due to a Concentrated Load

Boussinesg (1885) equation is

22

2/52

2 12

3

:

yxrwhere

z

rz

P

increasestressVertical

3.16 Vertical Stress Increase in a Soil Mass Caused by

Foundation Load (for Consolidation Settlement Calculation)

Self review

15

2/3

20

21

11

:

z

B

q

centrebelowincreasestressVertical

Stress due to a Circularly Loaded Area

16

Stress below a Rectangular Area

8.3

,

n)f(m,factorinfluence

)(2

)(3

:

00 0

2/5222

3

0

TableUse

z

Ln

z

Bm

I

Iqzyx

zdxdyq

cornerthebelow

increasestressverticalThe

L

y

B

x

17

Table 3.8 Variation of Influence Value I

18

Table 3.8 Variation of Influence Value I

19

Below any point

say O

20

Calculate stress increase

below the centre of a

rectangular area

8.3

28.3.

TableasSame

FigfromI

Iq

Iq

o

o

22

))(( zLzB

LBqo

23

)4(6

1 ''''bmtav

Calculate the average stress

increase of a soil layer

)84.5(

Example:

A flexible rectangular area measures 1.5m3m in plan. It supports a load of 100kN/m2. Determine the vertical stress increase due to the

load at a depth of 3.75 m below the center of the rectangular area.

Solution 1: using Table 3.8

Solution 2: using 2:1 method

kPa12.13

kPa6.12

24

cCoe

3.30

16.5

For a OC soil, c=60kPa, what is Sc ?

What is immediate

settlement for the

clay layer, Se ?

25

26

)84.5(

cavc

czpc

cav

He

CHS

with

iiAreaclayedconsolidatnormallyFor

'

0

''

0

0

)(

'''

0

log1

:

)(

27

22.3FigurefromratiosettlementK

conditionoedometerinsettlementS

KSS

oedc

oedcc

29

For each layer Hj, if mv and are constant with depth z, then:

jvjvcj HmHS'

In case of normally consolidated clay, using Cc:

jc

jvcj He

CHS

'

0

'

1

0

log1

For multi-layer Hj (j=1,2,3, n),

summation of settlements in all layers :

called

nj

j

cjc ss1 30

vU

ionconsolidatof

degreeaverageU

SUS

v

cvt

31

Settlement due to Secondary (Creep) Consolidation

Ht

tC

Ht

t

e

CHS

p

ezsc

1

2

1

2)(

log

log1

)/log(loglog 1212 tt

e

tt

eCC e

Ce

C

p

e 1

32

For Hong Kong Marine

Clays:

C = (0.3% to 1%) w (in %)

33

Why a clayey soil creeps?

Creep is due to

viscous adsorbed water (double layers) on clay particles

viscous re-arrangement/sliding/deformation of clay

particles/plates

viscous deformation of clay plates

Adsorbed water is NOT free water

Adsorbed water is NOT free to flow under gravity.

34

Under

effective

stress

Creep movement !

36

Creep always exists under the action of effective stresses

(loading), independent of the excess pore water (or pore

pressure).

Therefore, creep has nothing to do with the primary

consolidation.

Creep exists during and after primary consolidation.

Creep rate depends on stress/strain state:

Creep rate is large in a normally consolidated state.

Creep rate is small in a over-consolidated state. 37

Example 3.11:

a. Determine the primary consolidation settlement of a foundation with 1.5m 3m in plan.

b. Assume the pore water pressure parameter A for the clay is 0.6, estimate the

consolidation settlement considering the 3D effect.

c. Assume that the primary consolidation settlement is completed in 3 years. Also let

C=0.006. Estimate the secondary consolidation settlement at the end of 10 years.

39

3.17 Allowable Bearing Pressure in Sand Based on

Settlement Consideration

Meyerhof (1956) proposed a correlation for the net allowable bearing

pressure for foundations with SPT (N1)60.

Original Meyerhof method, for 25mm estimated maximum settlement:

)22.1)((28.3

128.399.7)/(

)22.1)((98.11)/(

2

60

2

)(

60

2

)(

)(

mmBforB

BNmkNq

mmeterinBforNmkNq

Dqq

allnet

allnet

fallallnet

Researchers observed Meyerhofs results are rather conservative. 40

Example 3.12:

A shallow foundation measuring 1.75m 1.75m is to be constructed over a layer of sand. Given Df = 1m; N60 is generally increasing with depth, the average value of N60 is 10. The

estimated elastic settlement of the foundation is 14.7mm. Use Meyerhofs method

(modified form by Bowles) to calculate the allowable bearing pressure of the sand.

qnet(all) = 115.6 kPa 42

Standard Penetration Test (SPT)

SPT N-Value:

Standard hammer weight is

622.72 N(62.3 kg or 140 lb)

Hammer drop height is

762 mm (or 30 in)

Number of blows for spoon

penetration of three 152.4mm

(6 in) is recorded

The blow number of last 2

penetrations (2 x 152.4=304.8mm)

is the SPT N-Value: (1) 152.4mm 4 blows

(2) 152.4mm 5 blows

(3) 152.4mm 7 blows

SPT N-Value=5+7=12 43

)(8.0 MPainNEs

44

Example 3.13:

A shallow square foundation for a column is to be constructed on sand. The foundation

must carry a net vertical mass of 102,000 kg. The standard penetration numbers (N60)

obtained from exploration are given in the figure. Assume that the depth of the foundation

will be 1.5m and the tolerable settlement is 25mm. Determine the size of the foundation.

Depth (m) N60

2 3

4 7

6 12

8 12

10 16

12 13

14 12

16 14

18 18

45

)(5

)()(

smallerwhicheverBorHz

averagez

zEE

is

s

Consider the non-homogeneous

nature of soil deposits:

3.18 Field Load Test

46

Plate load test simulates field loading conditions and predicts settlement on proposed

foundation. Bearing capacity and modulus of subgrade reaction.

48

where

qq

clayintestsFor

PuFu )()(

:

platetestofwidthB

foundationproposedofwidthB

where

B

Bqq

soilssandyintestsFor

P

F

P

FPuFu

)()(

:

(Independent of the size of the plate)

platetestofcapacitybearingultimateq

foundationproposedofcapacitybearingultimateq

Pu

Fu

)(

)(

49

3.20 Tolerable Settlement of Buildings

ratiodeflectionL

EAlinereferencefrom

deflectionrelative

lij

)(

)jandipoints

betweendistanceis(

l

Sdistortionangular

pointssuccessivetwo

betweengradient

''

ij

T(ij)

50

In Hong Kong:

(a)25mm for important structures; (b) 50mm less important

(c) 100 mm for walk road, and (d) 200mm for gardens etc.

Prof. A.W. Skempton

51

Topic 2.3 ive geo

Documents

Transcript of Topic 2.3 ive geo