SNU Geotechnical and Geoenvironmental Engineering Lab.
1.3 Shear Strength of Clays.
(1) Drained Strength.
NC Clay OC Clay
drained
UU behavior UU behavior
τ
, '
n n
σ σ
su su
'
pc
8 . 0
(max. past pressure)
0
φ = φ =0
φ'
τ
, '
n n
σ σ
SNU Geotechnical and Geoenvironmental Engineering Lab.
“Fundamental Sear Strength Parameters”
- Hvoslev Parameter.
ce, φe
- Work based on lab tests of saturated remolded clays.
Physico-chemical Forces (Intrinsic Forces)
Attractive Forces, A.
1) Electrostatic attraction.
2) Van der Waal’s Force.
SNU Geotechnical and Geoenvironmental Engineering Lab.
Repulsive Forces, R.
1) Electrostatic repulsion.
- Factors affecting intrinsic forces.
1. Type of clay.
2. Particle spacing.
3. Geometric arrangement of particles.
4. Specific surface.
SNU Geotechnical and Geoenvironmental Engineering Lab.
- Effective stress equations.
R A u + −
−
= σ σ '
S≡shear strength=Sgranular +Scohesive
e e
c c
g
c g
c g
c u
K R A K
K u
K R A u K
u S
S S
+
−
=
− + +
−
=
− +
− +
−
= +
=
φ σ
σ
σ σ
tan ) (
) (
) )(
(
) (
) (
Equivalent friction angle.
→material constant.
Equivalent cohesion.
SNU Geotechnical and Geoenvironmental Engineering Lab.
For NC Clay. (drained test)
f
f Mohr
Mohr Mohr
Mohr---Coulomb failure envelope-Coulomb failure envelopeCoulomb failure envelopeCoulomb failure envelope
e e e Hvorslev e
Hvorslev Hvorslev
Hvorslev failure envelopefailure envelopefailure envelopefailure envelope
SNU Geotechnical and Geoenvironmental Engineering Lab.
→ No longer 1 failure envelope, but a series of envelope for Hvorslev theory.
.
f
f Mohr
Mohr Mohr
Mohr----Coulomb failure envelopeCoulomb failure envelopeCoulomb failure envelopeCoulomb failure envelope
Hvorslev Hvorslev Hvorslev
Hvorslev failure envelopefailure envelopefailure envelopefailure envelope
SNU Geotechnical and Geoenvironmental Engineering Lab.
For O.C. Clay.
Ideal Soils → No ∆e during unloading.
In terms of Hvorslev parameters;
For any unloading (OCR > 1).
constant constant
=
=
e e
c φ
log P' log P' log P' log P'
e e e e
typical data typical datatypical data typical data
idealized data idealized data idealized data idealized data
σ’
SNU Geotechnical and Geoenvironmental Engineering Lab.
Practical decrease in strength due to increase in e during ‘actual’ unloading → So, Hvorslev ideal envelope (O.C. Clay) → upper bound strength.
Composite envelope (idealized)
ideal
practical τ
ce
σ'
φ'
φe
SNU Geotechnical and Geoenvironmental Engineering Lab.
Summary
An increase in effective stress has two effects on strength of clays.
1. Increase particle to particle contact forces.
→ increase in frictional resistance. → S =(σ −u) tanφe
increase 2. Decrease volume and void ratio, and increase (A-R) and ce.
