Shear Strength and Stiffness: Clays v Sands

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Shear Strength and Stiffness Clays v Sands
D. A. Cameron 2006
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(CLEAN) SANDS

• Drained triaxial test, or
• Direct shear

?wet ? ?dry i.e. ?? ? ?
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Influence of Density of Sand
(?1 - ?3)
?, shear stress
dense
(??p)
(??cv)
loose
??n, normal stress
?a, axial strain
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Influence of Density of Sand
?v (volume strain)
(?1 - ?3)
dense
loose
- ?v
?a, axial strain
?a, axial strain
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SUMMARY

• Dense Sand
• Has higher ??p
• Is strain softening
• Shows dilatancy
• Has same ??cv value

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Typical values of the strength
parameter for sands

• ??p ? 35 to 50?
• Strength fn(density, psd and grain shape)
• ??cv ? 27 to 35?

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CLAYS Undrained Strength

• A Unconsolidated, Undrained (UU)
• The sample is not conditioned back to field
  state
• stress relief
• greater void space
• de-saturation ?

When tested undrained ?V 0, ??3 generates ?u
??3 if saturated SO, ??3? 0
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Undrained Shear Strength, cu

• ?peak c? (??N)tan??
• BUT not much change in effective
  stress in the short term
• SO revert to total stress?
• ?peak c (?N)tan?
• NOW, for saturated NC clays ? ? 0
• ?peak cu

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cu undrained cohesion
?, shear stress
What is the unconfined compression strength ?
? ? 0
?3 50
100
200
2cu
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B Consolidated, Undrained Tests
(CIU)Consolidated Isotropically, Undrained

• A better test for the conditioning of
• the sample
• Shear strength, cu fn(consolidation pressure
  or, void ratio
• after the consolidation phase)

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C Consolidated, Undrained Tests with pwp
measurement

• Effective stresses measured
• effective strength parameters, c? and ??
• CLAYS ?? ? 20 to 35?
• c? 0 kPa for NC clays
• BUT c? ? 30 kPa for OC clays

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The influence of OC on saturated clay
(?1 - ?3) or u
(?1 - ?3) or u
CIU
CIU
OC, over-consolidated
NC, normally consolidated
?a
?a, Axial Strain
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The influence of OC on saturated clay
(?1 - ?3) or ?V
(?1 - ?3) or ?V
CID
CID
OC, over-consolidated
NC, normally consolidated
?a
?a, Axial Strain
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SOIL CONSISTENCY - descriptors

• CLEAN SANDS
• Based on density index, ID ()
• e.g. loose 15 35 dense 65 85
• Peg test?

• CLAYS
• Based on undrained shear strength ?
  cu (kPa)
• e.g. firm 25 50 stiff 50 100
• Thumbnail test?

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Field Strength?

• Saturated Clays or Silts
• Shear vane pushed into soil, then rotated

Width D, height H
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Resistance to turning shear strength x area x
moment arm
Torque (?DH)cuD/2 ..
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Shear Vane

• Is useful for residual strengths of soft to firm
  clays
• Can measure SENSITIVITY of clays, silts
• SENSITIVITY cu undisturbed / cu remoulded
• QUICK clays , sensitivity 16 to 100!

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Shear Vane
Since rate of shear is fairly fast, reduction
factor needed (after Bjerrum) for more plastic
clays (heavier)
e.g. PI 20 no reduction PI 40, 83 cu
vane PI 80, 65 cu vane
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GROUND COVERED

• Shear model
• Coulomb shear strength
• cohesion, friction
• Total v effective stress strength parameters
• Clean Sand v Clay
• Clay
• UNDRAINED loading
• NC clay - no frictional response
• same soil tested slowly, has c? ??
• Dilatancy - when does it occur?

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SUMMARY, continued

• Strength tests
• Direct shear
• Triaxial
• Vane shear
• Mohrs circles
• failure planes
• Pole
• Stress paths
• t v s