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Period #24:
Shear Strength of Soils (IV)
A. Review and Motivation
•
The MohrCoulomb shear strength model for soils is of the form:

τ
f
 = c +
σ′
n
tan(
φ
D
)
where:

τ
f

is the absolute value of shear stress that causes failure on a given plane;
c
≥
0
is the cohesion of the soil;
σ′
n
is the normal
effective
stress on a plane; and
φ
D
≥
0
is the socalled
drained
angle of friction for a soil.
Typically, one does not expect to see a value of
φ
D
greater than about 45
o
.
•
Note that in the above shear strength criterion, the normal stresses are
always effective stresses.
•
For saturated, finegrained soils subjected to loads, the effective stresses
change over long periods of time as excess pressures dissipate.
•
Therefore, the shear strength of a saturated, finegrained soil will generally
change (increase) with time as excess pore pressures dissipate.
•
This period, we discuss different strength models for finegrained soils
that are based on combinations of total stresses and effective stress.
•
Typically, models based on effective stresses, apply to the long term
drained behavior of finegrained soils.
•
Models based on total stresses typically apply to shortterm behaviors
such as in the first few weeks/months after a structure is built upon
a finegrained soil deposit.
1
53:030 Class Notes; C.C. Swan, University of Iowa
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View Full Document B.
Drained Shear Strength Behavior of Clays
•
For normally consolidated clays, c
≈
0, and
the friction angle is denoted
φ
D
.
•
For overconsolidated clays, c > 0, and the friction angle is denoted by
φ
D1
.
•
When the soil is overconsolidated: (or when
σ′
<
σ′
c
)

τ
f
 = c
1
+
σ′
n
tan(
φ
D1
)
•
When the soil is normally consolidated: (or when
σ′
=
σ′
c
)

τ
f

=
σ′
n
tan(
φ
D
)
•
Typical values of
φ
D
for silty/clayey soils:
12
o
≤
φ
D
≤
30
o
,
•
φ
D
≈
30
o
for low plasticity soils (PI = 510);
•
φ
D
≈
12
o
for high plasticity soils (PI = 50100);
•
12
o
≤φ
D
≤
30
o
for moderate plasticity soils (10
≤
PI
≤
50)
2
53:030 Class Notes; C.C. Swan, University of Iowa
τ
c
1
φ
D
φ
D1
σ′
c
σ′
53:030 Class Notes; C.C. Swan, University of Iowa
Example 24.1:
3
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View Full Document C.
Drained and Undrained Strength Behaviors of FineGrained Soils
For simplicity, we will first discuss this in the context of triaxial compression
tests, and then discuss the drained/undrained strength behaviors in terms of
geotechnical applications.
1.
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This note was uploaded on 11/10/2010 for the course CIVIL 53:30 taught by Professor Swan during the Fall '09 term at University of Iowa.
 Fall '09
 swan
 Shear Stress

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