Shear strength - University of Waterloo...1 CIVE 554/650 Shear Strength of Soil Direct shear test in...

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CIVE 554/650Shear Strength of Soil

Direct shear test in sand: (a) schematic diagram of test equipment; (b) plot of test results to obtain the friction angle φ’

© 2004 Brooks/Cole Publishing / Thomson Learning™

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Range of relative density and corresponding range of effective stress angle of friction for coarse-grained soil(modified from U.S. Dept. of the Navy, 1971)


Variation of friction angle φ’ with void ratio for Chattachoochee River sand (after Vesic, 1963)

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σ’1

σ’1

σ’3 = K σ’1σ’3

Soil Sampling

Triaxial Test Equipment

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Sequences of stress application in triaxial test

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Plot of deviator stress vs. axial strain-drained triaxial test


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Triaxial TestingApplication of cell pressure (sigma 3):

UNCONSOLIDATED (U) –No drainage allowed e constant (fixed) if Sr =1

- CONSOLIDATED (C) –Drainage allowed when applying cell pressuree decreases due to sample consolidation.

Triaxial TestingApplication of deviatoric stress (σ1):Deviator stress = σ1 – σ3

Undrained (U) No drainage allowed e constant (fixed) if Sr =1

- Drained (D) Drainage allowed when applying cell deviator stresse decreases due to consolidation. Stress applied so slow no excess pwp

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Triaxial TestsUU – Unconsolidated UndrainedUD – Unconsolidated DrainedCU - Consolidated UndrainedCD - Consolidated Drained

Undrained Soil Strength Parameters (Su, φ = 0)

Note: Always test soil at void ratio on the field

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Skempton’s Porewater Coefficients

Developed “B” for a measure of degree of Water saturation

3σμ

ΔΔ

=B

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Peak- and residual-strength envelopes for clay


Skempton’s Porewater CoefficientsDeveloped “A” parameter for indication of sample OCR

interest ofpoint at stress deviatoric)(interest ofpoint at pressure pore

pressure pore initail)(

:)(

31

31

=−==

−=Δ

−Δ

=

i

i

o

oii

i

i

where

A

σσμμ

μμμ

σσμ

What about negative values of Af

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Soil StrengthM-C Failure Envelope

Sample will fail at intersection with envelope

τ’

σ3(1) σ1(1)

σ1(2)

σ1(3) σ1(Failure)

σ’n

σ3(2)

c’

σ1(1) σ1(2) σ1(3) σ1(Failure)

Test 1 Failure Circle

τfailure (1)

τfailure (2)

Test 2 Failure Circle

NOTE: all stresses are effective

Each test is performed at a set void ratio

Drained Soil Strength Parameters (c’, φ’)

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Soil Strength Envelope

Soil Strength

Stress Path of Sample

1

2

3

α’

t = d' + S tan α'

s = (σ1' + σ3')/2Centre of Circle

d = c' cos φ'tan ψ = sin φ'

S

(σ1 - σ3)/2 = tRadius of Circle

d

s

t

sinφ’ = tanα’

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Unconfined compression test in progress (courtesy of Soiltest, Inc., Lake Bluff, IL)

Unconfined compression test: (a) soil specimen; (b) Mohr’s circle for the test; (c) variation of qu with the degree of saturation

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Variation of friction angle φ’ with plasticity index for several clays (after Bjerrum and Simons, 1960)


Variation of φ’r with CF (Note: pa = atmospheric pressure)

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Variation of φ’r with liquid limit for some clays (after Stark, 1995)

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Figure 1.40 Clay deposit


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Variation of cu/σ’0 with liquidity index (based on Bjerrum and Simons 1960)


Su vs Effective Overburden StressFor normally Consolidated soft clays (OCR <1.2)

22.0'

≈v

uSσ

Shear strength - University of Waterloo...1 CIVE 554/650 Shear Strength of Soil Direct shear test in...

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