Environmental Engineering Reference
In-Depth Information
Undrained vs. Drained Strength
Undrained conditions exist when a fully saturated slope is sheared to failure so rapidly
that no drainage can occur, as when an embankment is placed rapidly over soft soils. Such
conditions are rare except in relatively impervious soils such as clays. Soil behavior may
then be regarded as purely cohesive (see
Figure 3.29)
and
0. Results are interpreted in
terms of total stresses, and
s
u
, the undrained strength, applies. The case of sudden draw-
down of an adjacent water body is an undrained condition, but analysis is based on the
consolidated undrained (CU) strength of the soil before the drawdown. This strength is
usually expressed in terms of the CU friction angle.
Drained or long-term conditions exist in most natural slopes, or some time after a cut is
made and drainage permitted. Analysis is based on effective stresses, and the parameters
φ
φ
' and
c
' will be applicable.
Peak and Residual Strength
The foregoing discussion, in general, pertains to peak strengths. When materials continue
to strain beyond their peak strengths, however, resistance decreases until a minimum
strength, referred to as the ultimate or residual strength, is attained. The residual strength,
or some value between residual and peak strengths, normally applies to a portion of the
failure surface for most soils; therefore, the peak strength is seldom developed over the
entire failure surface.
Progressive failure
, when anticipated, has been approximately evaluated by using the resid-
ual strength along the upper portion of the failure surface, and the peak strength at maximum
normal stress along the lower zone (Conlon as reported in Peck, 1967; Barton, 1972).
Stiff fissured clays
and
clay shales
seldom fail in natural slopes at peak strength, but rather
at some intermediate level between peak and residual. Strength is controlled by their sec-
ondary structure. The magnitude of peak strength varies with the magnitude of normal
stress, and the strain at which peak stress occurs also depends on the normal stress (Peck,
1967). Because the normal stress varies along a failure surface in the field, the peak
strength cannot be mobilized simultaneously everywhere along the failure surface.
Residual strength
applies in the field to the entire failure surface where movement has
occurred or is occurring. Deere and Patton (1971) suggest using
φ
r
(the residual friction
angle), where preexisting failure surfaces are present.
Other Strength Factors
Stress levels affect strength. Creep deformation occurs at stress levels somewhat lower
than those required to produce failure by sudden rupture. A steady, constant force may
cause plastic deformation of a stratum that can result in intense folding, as illustrated in
Figure 9.61.
Shear failure by rupture occurs at higher strain rates and stress levels and dis-
tinct failure surfaces are developed as shown in
Figure 9.62.
The materials are genetically
the same, i.e., varved clays from the same general area.
The strength of partially saturated materials (see
Section 3.4.2)
cannot be directly evalu-
ated by effective stress analysis since both pore-air and pore-water pressures prevail.
Residual soils, for example, are often partially saturated when sampled. In Brazil, effective
stress analysis has sometimes been based on parameters measured from direct shear tests
performed on saturated specimens to approximate the most unfavorable field conditions
(Vargas and Pichler, 1957). Depending upon the degree of field saturation, the saturated
strengths may be as little as 50% of the strength at field moisture. Apparent cohesion
results from capillary forces in partially saturated fine-grained soils such as fine sands and
silts; it constitutes a temporary strength that is lost upon saturation and, in many
instances, on drying.
