Giant landslide in Tibet 2000 Shear strength 5.1 General Shear strength 5.1 General.
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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
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