geotehcnical analysis

CIVL 4720 Geotechnical Analysis and Design

CIVL 4720 – Geotechnical Analysis and Design (2014/2015)

Lecture Schedule : Monday (09:00 - 10:20) LTE

Wednesday (09:00 - 10:20) LTE

Tutorial 1 Monday 18:30 – 19:20 Room 2302

Tutorial 2 Monday 14:00 – 14:50 Room 3006

Instructor : Professor Jidong Zhao

Website: To be confirmed by Professor Jidong Zhao

Applications of fundamental principles of soil mechanics to

geotechnical analyses and designs.

This course covers lateral earth theories, design of earth

retaining structures, slopes, shallow and deep foundations

and reinforced earth structures, and geotechnical centrifuge

modelling.

Course Descriptions

1. Lateral earth pressure

1.1 Lateral earth pressure at rest (Ko)

1.2 Rankine’s theory

1.3 Coulomb’s theory

1.4 Assignment and worked examples

2. Design of retaining walls

2.1 General design philosophy

2.2 Gravity & cantilever concrete walls

2.3 Cantilever sheet pile walls

2.4 Anchored sheet pile walls

2.5 Soil arching

2.6 Pore pressure distributions behind retaining wall


Course Contents

Weeks 1-2

Weeks 3-4

3. Braced cuts or multi-propped excavations

3.1 General design philosophy

3.2 Short-term lateral wall stability & strut loads

3.3 Base heave in clays

3.4 Piping in sands

3.5 Ground settlement & swelling

3.6 Case history 1 - Lion Yard at Cambridge

3.7 Case history 2 - Dragon Centre in Kowloon


4. Shallow Foundations

4.1 General behaviour and design principles

4.2 Terzaghi’s general ultimate bearing capacity theory

4.3 Other bearing capacity theories

4.4 Vertical stress distributions below shallow foundations

4.5 Settlements of shallow foundations


Course Contents

Weeks 5-6

Weeks 7-8

5. Deep Foundations

5.1 Types and uses of pile foundations

5.2 Design principles of vertically loaded single piles

5.3 Design of barrettes, rock socketed piles and resisting NSF piles

5.4 Pile tests


6. Reinforced Earth Structures

6.1 Types and considerations of soil reinforcement

6.2 Failure mechanisms


Course Contents

Weeks 9-10

Week 11

7. Geotechnical Centrifuge Modelling

7.1 Introduction & history of centrifuge modelling

7.2 Principle of centrifuge modelling and its applications

7.3 Scaling laws

7.4 Examples

7.5 Limitations and Future development

8. Slope Stability

8.1 Slope characterisation

8.2 Methods of slope stability analysis

8.3 Practical consideration of methods of analysis

8.4 Choice between total and effective stress analysis

8.5 Examples

8.6 Assignments

Course Contents

Week 11

Weeks 12-13

Reference Books

1. Craig. R.F. (2012) Soil Mechanics. 8th edition, E & FN SPON.

2. Budhu. M. (2011). Soil Mechanics and Foundations John Wiley, 3rd edition.

3. Das. B. M. (2011). Principles of Foundation Engineering. 7th edition, 2011.

ISBN: 0495082473. . 416p.

4. Das. B. M. (2012). Fundamentals of Geotechnical Engineering. 4th edition.

ISBN: 0534492940

5. Ng, C.W.W., Simons, N. & Menzies, B. (2008). Soil-structure Engineering

of Deep Foundations, Excavations and Tunnels. Publisher:

Thomas Telford, UK. ISBN:0727732633. 3rd Reprint. 416p.

6. Powrie. W. (2004). Soil Mechanics - Concept and Applications, 2nd edition,

E & FN SPON.

Ng, C.W.W., Simons, N. & Menzies, B. (2008). Soil-structure Engineering of Deep Foundations, Excavations and Tunnels.

Publisher: Thomas Telford, UK. ISBN: 0-7277-3263-3. 424p. 3rd re-print

Mark Allocations

Tutorial Assignments 15%

Mid-term Examination

Final Examination

25%

60%

**Chapters 1-3 in November

TAs Name: Feng Song (Mainland China)

Ph.D Topic: Theoretic study of coupled water-gas-

heat flow in unsaturated soils considering the

effect of vegetation and methane oxidation

Date of Registration: Fall 2012

Name: Ni Junjun (Mainland China)

Ph.D Topic: Bioengineering of landfill cover


Name: Sk Belal Hossen (Bangladesh)

M.Phil Topic: Hydro-mechanical properties of

unsaturated loess


TAs

Name: Chen Zhong Kui, Bruce (Mainland China)

Ph.D Topic: Experimental study of water-gas-heat

coupled flow mechanisms in landfill covers


Name: Song Dongri (Mainland China)

Ph.D Topic: Debris impact on flexible barrier


Name: Su Yuchen, Andy (Mainland China)

M.Phil Topic: Debris impact on rigid retaining

wall protected by energy absorption materials


CIVL 4720 - Geotechnical Analysis and Design

Chapter 1 -

Lateral Earth Pressure

Course Contents

1.0 - Lateral earth pressure

1.1 - Lateral earth pressure at rest

1.2 - Rankine’s earth pressure theory

1.3 - Coulomb’s earth pressure theory

1.4 - Assignment

What is Geotechnical Engineering Design ?

1. Soil Mechanics - theory and science

2. Foundation Engineering - art, experience,

judgement, and applications of the principles of soil

mechanics and some structural theories to the

analysis and design of foundations

What are the Design Limit States ?

1. Serviceability limit state - serviceable

2. Ultimate limit state - prevent catastrophic collapse

1. 1 - Lateral earth pressure at rest

Earth pressure at rest

• If the lateral strain in the soil is zero, the

corresponding lateral pressure is called the earth

pressure at rest (Ko) and is usually expressed in

terms of effective stress by the following equation:

Earth pressure at rest

• Since the at-rest condition does not involve failure of the soil,

the Mohr circle representing the vertical and horizontal stresses

does not touch the failure envelope and the horizontal stress

cannot be evaluated using the Monhr circle. Therefore, the value

of Ko is mainly determined by experimental means such as using

a triaxial apparatus or by field measurements.

One-dimensional loading and unloading loop (Ng, 1995)

• AB=deposition or

lowering of water table

• BC=erosion or rising

water table

• CB=re-deposition or

refilling

Some common expressions of K0

•By considering conditions at the centre of the base of a heap of granular

material, Jaky (1944) related Konc (for normally consolidated soils) with the

maximum angle of friction and derived the following equation:

• For engineering purposes, the above expression has been simplified as follows:

• The expression has subsequently become one of the most widely known

geotechnical engineering formulae.

Empirical approach – first approximation

Figure 1-3

Figure 1-4

Empirical approach – first approximation

Ng, C.W.W. (1995). Numerical analysis of geological effects on Ko.

Proc. 10th Asian Regional Conf. on Soil Mech. & Fdn. Engng, Beijing , Vol. 1, 55-58

P

Q O

Path-dependent behaviour (i.e., different Ko at A, B and C)

S

Schmidt (1966) studied laboratory results from five clays with different

mineralogy and stress history, and proposed the following empirical

mathematical expression for clays subjected to loading and unloading:

where σ‘1max is the maximum principle effect stress. He commented that the

formula did not fit data from tests on sand.

How can we account for some aspects of the observed behaviour of soils

subjected to unloading-reloading cycles ?

Along PQ

To account for some aspects of the observed behaviour of soils subjected to

unloading-reloading cycles, Schmidt (1983) proposed the following equation

to describe soil behaviour at a state between

OCR= OCRmax and OCR=1,

where OCR and OCRmax are the current and the maximum over consolidation

ratios respectively.

Along QS



OCRmax

OCRA=OCRB=OCRC

How to estimate Ko at A, B and C ??????

Mayne & Kulhawy (1982) reviewed laboratory data from over 170 soils and presented an empirical relationship between Ko and OCR for primary loading-unloading-reloading conditions as follows:

However, it should be noted that for the case of reloading, the expression was merely based on limited available data at low OCR. More importantly, the last two equations do not seem to have based on experimental data with more than one loading-unloading-reloading cycle.

Some other common expressions of K0



E

F G

1.2 - Rankine’s earth pressure theory

Rankine’s Theory of Earth Pressure (1857)

• This lower bound plastic solution with the following assumptions:

• Plane strain

• Perfectly plastic, homogenous and isotropic

material

• Frictionless wall with horizontal ground surface

Rankine’s Theory of Earth Pressure • For a Mohr-Coulomb material in a state of plastic equilibrium, we have:

where P is the pole point of the Mohr stress circle

Figure 1-5

Craig (1997)

Why did he has to assume frictionless wall ?

What is active earth pressure? Consider a normally consolidated soil element behind a smooth wall, its stress state can be

represented by the Mohr stress cycle (M1) shown in the diagram. If the soil is assumed to be

homogenous and isotropic, the soil element at depth z is subjected to a vertical stress σz and a

horizontal stress σx and, since there can be no lateral tranfer of weight if the surface is

horizontal, no shear stresses exist on horizontal and vertical planes. The vertical and horizontal

stresses, therefore, are principle stresses. Why the stress component, σz, is the major principle

stress in this situation? Will there be a difference if the soil element, locates at 5m below ground

(say), has been subjected to an overburden pressure of 1000 kPa (say), which was then

subsequently removed and the wall was rigid throughout?

Figure 1-6

Craig (1997)

Correction - No dash

Active earth pressure If there is a movement of the wall away from the soil, the value of

horizontal stress σx decreases as the soil dilates or expands outwards. In

terms of Mohr stress circle representation, this means that there is an

increase in size of the circle from M1 to M2 as shown in Figure 1-6. The

decrease in σx is a non-linear function of the lateral strain and soil stiffness.

If the expansion is large enough, the value of σx will decrease to a minimum

value such that a state of plastic equilibrium develops. Since this state is

developed by a decrease in the horizontal stress σx, this must be the minor

principle stress (σ3). The vertical stress σz is the major principle stress (σ1).

The stress σ1= σ2 is the overburden pressure at depth z and is a fixed value

for any depth. The value of σ3=σx is determined when a Mohr circle through

the point representing σ1 touches the failure envelop for the soil. The

relationship between σ1 and σ3 when the soil reaches a state of plastic

equilibrium can be derived from this Mohr-circle.

Craig (1997)

•From the previous figure,

•Rearrange,

•At stated, σ1 is the overburden pressure at depth z

Active earth pressure Craig (1997)

Correction - No dash

•The horizontal stress for the condition is defined as the ACTIVE

PRESSURE (Pa), being due directly to the self-weight of the soil. If

is defined as the active pressure coefficient then


• When the horizontal stress becomes equal to the active pressure, the soil is

said to be the ACTIVE RANKINE state, there being two sets of failure planes

each inclined at to the horizontal (the direction of the major principle

plane).

Figure 1-7


Passive earth pressure • In the above derivation a movement of the wall away from the soil was

considered. If, on the other hand, the wall is moved against the soil mass,

there will be lateral compression of the soil and the value of σx will increase

until a state of plastic equilibrium is reached, i.e. the Mohr circle will shrink

initially from M0 to M1 and then gradually expands to M3 at which the cycle

touches the failure envelope. For this condition, σx becomes a maximum value

and is the major principal stress σ1. The stress σz, equal to the overburden

pressure, is then the minor principle stress i.e,

Figure 1-8

Craig (1997)

• In this case, the horizontal stress is defined as the PASSIVE PRESSURE (Pp)

Representing the maximum inherent resistance of the soil to lateral

compression. By considering the Mohr stress circle, it can be easily shown that

define as the passive pressure coefficient,

then

•When the horizontal stress becomes equal to the passive pressure, the soil is

said to be in the PASSIVE RANKINE state, there being two sets of failure

planes each inclined at to the vertical as shown previously.

Passive earth pressure Craig (1997)

• Inspection of the active and passive pressure equation it is obvious that the

pressures increase linearly with depth as shown below.

•When C=0, triangular distributions are obtained in each case. When C>0, the

value of Pa is zero at a particular depth z0. It can be shown that

•This means that in the active case, there is a tension crack formed between the

surface and depth z0 as soils cannot take tension generally.

Figure 1-9

Active and passive pressure distributions

Active and passive pressures Craig (1997)

Notes:

1.The depth of the tension zone Z0

may be calculated. Negative earth

pressures within this zone should be

ignored. Water in the tension crack is

assumed to exert a hydrostatic

pressure.

• is the soil unit weight above the

water table, and are are the

saturated unit weights of the soil

below the water table.

2. Water pressure is assumed to be

hydrostatic, which implies that the

retaining wall and the lower boundary

are impermeable. For non-hydrostatic

conditions and the presence of

perched water tables, the effect of

pore water pressures should be

properly assessed.

Figure 1-11 Calculation of Active Earth Pressure for a Vertical Retaining Wall Using the Rankine-Bell Equation

Geoguide 1 (1993)

Canadian Foundation Engineering Manual, CFEM (1993)

Effect of wall movement on earth pressure in sand (CFEM, 1993)

Geoguide 1 (1993)

1.3 - Coulomb’s earth pressure theory

Coulomb’s theory (1776) of earth pressure

1. Consideration of the stability of soil as a whole

2. Defining a trial failure plane and calculating limiting equilibrium

of the soil mass

3. Consideration of wall friction

4. Upper bound plastic solution (choose a failure plane) -

underestimate active & overestimate passive thrust

5. When , the Coulomb theory is identical to

Rankine theory

Craig (1997)

(lower bound plastic solution)

Assumptions

1. The point of application of the total active thrust is not

given by the Coulomb theory but is assumed to at a

distance of H/3 above the base of the wall

2. The acceptance of plane failure surfaces

Craig (1997)

Figure 1-12

General range of wall friction

angles for masonry concrete

walls

Backfill material Range of δ (deg)

Gravel 27-30

Coarse sand 20-28

Fine sand 15-25

Stiff clay 15-20

Silty clay 12-16

Craig (1997)

Actual failure curves

Coulomb theory: active case with c = 0

Craig (1997)

Direction of block movement (ABC) = downward (relative to the wall)

• The maximum force P between the soil and the wall is given

by

where


Craig (1997)

Graphical Method

Figure 1-14

Alternative Expression:

Ref: Principle of Foundation Engineering by DAS


Figure 1-15 Coulomb theory: active case with c > 0

Coulomb theory: active case with c > 0 and a

tension crack

Craig (1997)

Coulomb theory: passive case with c = 0

Craig (1997)

Direction of block movement (ABC) = upward (relative to the wall)

A

B

C

• The minimum force P between the soil and the wall is given

by

where

Coulomb theory: passive case with c=0

Craig (1997)

Ref: Principle of Foundation Engineering by DAS

Graphical Method

Figure 1-16

Coulomb’s passive pressure

Alternative expression:

Figure 1-17 Nature of failure surface in soil with wall friction for (a) active pressure

case and (b) passive pressure case (Das, 2002)

Active pressure case

Passive pressure case

Active and passive pressure cases

Note: Figure based on NAVFAC

(1982b) and Caquot & Kerisel

(1948)

Figure 18- Active Earth Pressure Coefficients

for Different Wall Configurations and

Retained Slope Angles

Note: Figure based on NAVFAC

(1982b) and Caquot & Kerisel

(1948)

Figure 19- Passive Earth Pressure Coefficients

for a Vertical Wall Retaining Sloping Ground

Effective Stress Analysis - Drained or Undrained (Analysis)

where

where or

Total Stress Analysis Undrained (Analysis)

where

or

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Chapter 1 - Assignments 1-4

geotehcnical analysis

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