Environmental Engineering Reference
In-Depth Information
30
20
FIGURE 3.32
Approximate relationship between the
drained angle of residual shearing
resistance and plasticity index for rock
gouge material. (From Patton, F. D. and
Hendron Jr., A. J.,
Proceedings of the 2nd
International Congress,
International
Association Engineering Geology, Sao
Paulo, 1974, p. V-GR. 1. Kanji, M. A.,
M.S. Thesis, Department of Geology,
University of Illinois, Urbana, 1970. With
permission.)
10
0
20
40
60
80
Plasticity index (%)
Sensitivity S
t
: The ratio of the natural peak strength to the ultimate undrained strength
when a sample is completely remolded at unaltered water content is referred to as the soil
sensitivity, expressed as
S
t
s
u
/
s
r
(3.34)
A clay classification based on
S
t
is given in Table 3.18. Soils termed as “quick” have high
sensitivities and are extremely sensitive to vibrations and other forms of disturbance. They
can quickly lose their strength, change to a fluid, and flow, even on very flat slopes (
see
Glacial-Marine Clays
Section 7.6.5
).
Thixotropy
refers to the regain in strength occurring in remolded soils because of a “reha-
bilitation of the molecular structure of the adsorbed layers” (Lambe and Whitman, 1969).
Dynamic Shear
Concepts
Under dynamic or cyclic loading a soil specimen subject to shear initially undergoes defor-
mations that are partially irreversible, irrespective of the strain amplitude; hence,
stress-strain curves in loading and unloading do not coincide. If strain amplitude is small,
the difference between successive reloading curves tends to disappear after a few loading
TABLE 3.18
Clay Classification by Sensitivity
S
t
a
Sensitivity
s
u
/
s
r
b
Classification
2
Insensitive
2-4
Moderately sensitive
4-8
Sensitive
8-16
Very sensitive
16-32
Slightly quick
32-64
Medium quick
64
Quick
a
From Skempton, A.W.,
Proceedings of the 3rd International Conference on Soil Mechanics and Foundation
Engineering,
Switzerland, Vol. I, 1953, pp. 57-61.
b
s
u
peak undrained strength,
s
t
remolded strength.
