giant landslide in Tibet 2000

Shear strength

5.1 General

Shear strength

5.1 General

granary foundation failure on clay

Shear strength

5.1 General

Shear strength

5.1 General

Foundation soil liquefaction caused by earthquake

Shear strength5.1 General

5.1 General

Embankment

Foundation

Retaining wall

Key problem: shear strength

Shear strength

5.1 General

Shear strength

Translation slip

Collapse

Rotational slip flow slide

5.1 General

Shear strength

5.2 Coulomb law

τf = c + σ’ tan φτf = shear strengthc = cohesionφ = angle of internal friction

σ1

σ1 major principle stress

σ3 σ3Minor principle stressConfining stress

σ n

τ f

f =c+

tan

粘土c

f

Shear strength

Consider the following situation:A normal stress is applied vertically and held constantA shear stress is then applied until failure

33

1

1

3

1

dl

dlcos

dlsi

n

Shear strength

O 13 1/2(1 +3 )

2

A(, )

2

312

2

31 2

1

2

1

• For any given normal stress, there will be one value of shear stress• If the normal stress is increased, the shear stress will typically increase in sands

and stay the same in clays

Shear strength

5.3 Mohr–Coulomb failure criterion

3 1

c f2 f

A

cctg1/2(1 +3 )

31

31

2

1cot

2

1

sin

c

245tan2

245tan2

31

oo c

245tan2

245tan2

13

oo c

For sandy soil: c=0

245tan2

31

o

245tan2

13

o

Shear strength

f2 f

3 1

c

A

cctg1/2(1 +3 )

2

45902

1 f

45max

max

In the case represented by the figures in this chapter, in which it is assumed that the vertical direction is the direction of the major principal stress, the planes on which the stresses are most critical make an angle π/4 − /2 with the vertical direction. Thus it can be expected that sliding failure will occur in planes that are somewhat steeper than 45. If for instance = 30, which is a normal value for sands, failure will occur by sliding along planes that make an angle of 30 with the vertical direction.

Shear strength

Soil

Normal stress σn

Shear stress σ3

(1) Direct shear test

5.4 Shear testShear strength

Direct shear test is Quick and InexpensiveShortcoming is that it fails the soil on a designated plane which may not be the weakest one

Shear strength

Shear strength

Shear strength

Shearstress

Shear displacement

Peak Strength

Residual Strength

• The discussion thus far have referenced failure of the soil.• Failure is indicated by excessive strain with little to no increase (even decr

ease) in stress.• After failure, the soil strength does not go to 0• The soil retains residual strength

Shear strength

φ

Shearstress

normal stress

Typical plot for clays - drained condition

OverconsolidatedOCR >1

normallyconsolidatedOCR=1

c

Shear strength

(2) Triaxial shear test

3 3

3

3

3

3

△

△

•The test is designed to as closely as possible mimic actual field or “in situ” conditions of the soil.

Shear strength

Shear strength

Triaxial tests are run by:saturating the soilapplying the confining stress (called σ3)Then applying the vertical stress (sometimes called the deviator stress) until failure

3 main types of triaxial tests:Consolidated – DrainedConsolidated – UndrainedUnconsolidated - Undrained

Shear strength

The specimen is saturatedConfining stress (σ3) is applied

This squeezes the sample causing volume decreaseDrain lines kept open and must wait for full consolidation (u = 0) to continue with test

Once full consolidation is achieved, normal stress applied to failure with drain lines still open

Normal stress applied very slowly allowing full drainage and full consolidation of sample during test (u = 0)

=’

f=f

Shear strength

(3) Unconfined compression test

qu

qu

• The specimen is not placed in the cell

• Specimen is open to air with a σ3 of

0 • Test is similar to concrete compress

ion test, except with soil (cohesive – why?)

• Applicable in most practical situations – foundations for example.

• Drawing Mohrs circle with σ3 at 0 a

nd the failure (normal) stress σ3 defi

ning the 2nd point of the circle – often called qu in this special case

• c becomes ½ of the failure stress

Shear strength

Shear strength

Shear strength

加压框架

量表 量力环

升降螺杆

无侧限压缩仪qu

qu

Shear strength

unconfined compression apparatus

Shear strength

unconfined compression apparatus

Shear strength

unconfined compression apparatus

qu

cu

u=0

2u

uf

qc

Shear strength

unconfined compression apparatus

sensitivity

High sensitivity St>4

Middle sensitivity 2< St≤4

Low sensitivity 1<St≤2

'u

ut q

qS

the effects of disturbance of soil constitutive property on soil strength

Shear strength

(4) Vane shear test

This test is used for the in-situ determination of the undrained strength of intact, fully saturated clays; the test is not suitable for other types of soil. In particular, this test is very suitable for soft clays, the shear strength of which may be significantly altered by the sampling process and subsequent handling. Generally, this test is only used in clayshaving undrained strengths less than 100 kN/m2. This test may not give reliable results if the clay contains sand or silt laminations.

3

2

2

max

DHD

Mf

Shear strength

ExampleShear strength

Shear strength

Shear strength

• Triaxial tests rarely run• The unconfined test is very common• In most cases, clays considered φ = 0 and c is used as the strengt

h• Sands are considered c = 0 and φ is the strength parameter• Direct shear test gives us good enough data for sand / clay mixes

(soils with both c and φ)• Tables showing N value vs strength very commonly used (page

567 for clays for example).

Remark

Shear strength