Geotechnical and Foundation Engineering

SCE5331

Geotechnical and Foundation Engineering

Dr. Hong Chengyu, Joey

Office: 301, Tel: 2176-1545

Email: cyhong@vtc.edu.hk

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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.

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COURSE DESCRIPTION:

The course introduces civil engineering students the fundamental

concepts of foundation analysis and design. Upon completion of this

course, students should be able to interpret field and laboratory data

to get design properties and able to design and analyze shallow

foundations, retaining walls and pile foundations.

PREREQUISITE:

SCE4231 - Engineering Geology and Soil Mechanics

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ASSESSMENT METHOD:

Final Marks = 50% (Assignments + Quiz + Reports) +

50% (Final exam)

QUIZ and EXAMS:

Quiz and exams will consist of a mixture between discussion and

technical questions to evaluate your comprehension of the material.

The final exam will be closed book. However, formulas, design

charts, and similar materials will be given when needed. In addition,

you should bring a straight edge and calculator to the exams.

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ASSIGNMENTS:

Prepare your homework in a professional manner and show discussions and all steps of calculations in your assignments. Any submission which is illegible or difficult to understand will receive a reduced grade.

Students may consult with each other about homework assignments. However, each student is responsible for preparing their own homework and displaying their understanding of the principles behind the homework solution.

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REPORTS:

Prepare your Reports in a professional manner and show discussions and all steps of your experimental processes (using photos or ?). Prepare the lab report in a logical and clear manner. All figures, tables and data should be clearly presented and analyzed in your report.

Participation in the work of a course is a precondition for a students achievement of credits in that course.

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ATTENDANCE:

The foundation of a structure is in direct contact with the ground and transmits the load of the structure to the ground. When designing foundations, two principal criteria must be satisfied:

Bearing Capacity There must be an adequate factor of safety against collapse (plastic yielding in the soil and catastrophic settlement or rotation of the structure).

Settlement Settlements at working loads must not cause damage, nor adversely affect the serviceability of the structure

Other considerations that may be relevant to specific soils, foundation types and surface conditions.

FOUNDATION DESIGN

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Soil mechanics

Engineering geology

Proper judgment from past experience

Foundation Engineering is an art!

The most basic aspect of foundation engineering deals with the selection of the type of foundation. Foundations are commonly divided into two categories:

shallow and deep foundations.

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Foundation Types

Mat/Raft foundation

Spread footings Concrete footing

Wall footings Battered Piles Caissons

Shallow Foundations: D 3~4 B 9

The problems for

Regina homeowners

(Regina is the capital of Saskatchewan, Canada.)

UNEVEN SETTLEMENT!!!

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Regina

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Source of the problem:

glacial-lake clays

Many communities in

southern Saskatchewan

experience foundation

problems. All these

communities share one

thing in common - they are

built on clay sediments

deposited in ancient glacial

lakes.

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Looking for solutions?

Swelling and shrinking are limited to the uppermost part of the

ground, which gains and loses moisture through the year due to

changes in precipitation and vegetation growth.

Below this 'active zone', the ground is stable.

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One engineering solution

is to build foundations on piles

that extend through the active zone

to stable ground below.

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Any other suggestions?

Some Historical Cases for Foundation Engineering

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Leaning Tower of Pisa (unsuccessful)

Shallow foundation on deep soft deposits.

The axis of the tower is not straight.

Maximum inclination reached 5.5 in 1990s.

Using soil extraction to stabilize the tower in 1999.

Further info: http://en.wikipedia.org/wiki/Leaning_Tower_of_Pisa

Eiffel Tower (successful)

Adjacent to the Seine River underlain by deep and soft alluvium.

Two legs closest to the river were founded on 12m below the ground surface.

Two legs furthest from the river were on shallow but firm soils.

The tower has not experience excessive differential settlement for over 100 years.

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Soft Soil Engineering Problems

Transcona ,191331m,23m,8.8m,1.5m,27 degree38850T,4m

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Tallest buildings in the world

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Ultima Tower, is it possible?

3200 m high, more than 1 million residents, 500 floors

To reach a far-distance goal by starting here

All total buildings are founded on the ground

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Geotechnical Properties of Soil Fo

un

dat

ion

De

sign

The Load that will be transmitted by the superstructure to the foundation system

The requirements of the local building code

The behavior and stress-related deformability of soils

The geological conditions of the soil

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Grain Size Distribution

Weight-Volume Relationships

Relative Density

Atterberg Limits Standard

Compaction Test

Hydraulic Conductivity of

Soil

Steady State Seepage

Effective Stress Consolidation

Consolidation Settlement

Shear Strength Unconfined

Compression Test

Review of Soil Mechanics

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Weight-Volume Relationships

23

24

Atterberg Limits

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The optimum water content Wopt,

which results in the maximum dry

density rdmax for a given soil under the same compaction energy

Too wet Too dry

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ds

s

wssw V

M

M

MM

VM

MM

VM

wr

r

/

/1

/

1

wV

M sd

1

rr

Standard Compaction Test

Proof:

The optimum water content Wopt, which is just right (water being

lubricate and soil having enough air voids) to achieve the maximum

dry density rdmax 27

Effective Stress

Without seepage With seepage Quick condition (failure by heave)

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u

A

uA

A

N

A

P

uANP

'

'

'

Note: Particle contact areas are zero.

Thus, force due to u is uA

Consider the fully saturated soil (water and solids only, no air)

Consider vertical force equilibrium:

Hydraulic Conductivity of Soil

o Laboratory tests

o Typical values

o Empirical equations

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Steady State Seepage

2-dimension

3-dimension

FLOW NET METHOD

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Hydraulic Conductivity of Soil

The quick condition

icr is the critical hydraulic gradient

Effective stress at A:

)( satwwvA zh

)]([

)(

)(

wAw

AAwA

w

hzh

zhu

zhu

Effective stress =0 as critical

case or quicksand condition

wh

Ah

zA

Az

crAA

w

wsat

Awwsat

wAwsatwwv

vv

iz

h

z

h

hz

hzhzh

u

0)(

)()('

'

e

Gi s

ww

wsatcr

1

1'

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Bedrock or soil

Marine Deposits

Water Table

Pre-loading fill

Sand fill

Confined or 1-D Straining Consolidation

(or Oedometer) Condition:

Soil layers are horizontal and uniform

Loading is uniform

Deformation & water flow are in vertical only

Consolidation

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Consolidation

34 Void ratio-effective stress relationship

From the e-log s curve, what parameters can be determined?

Preconsolidation pressure

35 e-log relationship

Compression: for each layer Hj (thickness), if mv and are constant with depth z, then (loading):

jvjvcj HmHs'

Compression: for normally consolidated clay, use Cc for loading:

''

1

'

2

'

1

'

2

0

log1

j

cjvcj H

e

CHs

Recompression/Heave/Swelling: for normally consolidated clay, use

Cr or Ce for un-loading:

je

jvcj He

CHs

'

1

'

2

0

log1

Consolidation Settlement

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)1(,1

,1

111

0

00

0

00

0000

0

0

10

0

10

0

10

0

eH

He

H

e

H

e

H

H

e

e

H

H

AH

HA

V

V

V

V

VV

VV

e

ee

e

ezv

V

sv

vv

V

V

V

V

V

Vs

s

v

s

v

s

v

)/log( '' 1

1

ii

iic

eeC

37

22

z

uc

t

u ev

e

wv

vm

kc

The initial value of excess pore water pressure (initial condition):

The boundary conditions of excess pore water pressure:

Free-draining, double drainage

1-D Terzaghi Consolidation Thoery

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848.0%90

196.0%50

v

v

TU

TU

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Determination of coefficient of consolidation

wv

vm

kc

t

dTc

d

tcT vv

vv

2

2;

(1)The log time method

(due to Casagrande)

(2) The root time method (due to Taylor)

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2196.0

t

dcv

90

2848.0

t

dcv

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Correction for construction period

Previous solution/chart is for suddenly applied load

Common real load is ramp loading (construction loading)

linear increase and then constant

How to find a solution to this or make a correction?

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Shear Strength

Direct shear test

Triaxial tests

CD Tests CU Tests UU Tests

Attention to:

Effective stress parameters

Total stress parameters

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Unconfined Compression Test

Sometimes conducted on

unsaturated soils

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