Environmental Engineering Reference
In-Depth Information
FIGURE 9.61
Creep deformation in varved clays (Roseton, New York).
Spontaneous liquefaction occurs and the mass becomes fluid in fine-grained, essentially
cohesionless soils when the pore pressure is sufficiently high to cause a minimum of grain-
to-grain contact (see
Section 8.3.2).
After failure, as the mass drains and pore pressures dis-
sipate, the mass can achieve a strength higher than before failure. High pressures can
develop in pore air or pore water.
Changes with time occur from chemical weathering, lateral strains, solution of cement-
ing agents, or leaching of salts (see
Section 9.1.4)
.
In Situ Rock Strength
Effective stress analysis normally is applicable because the permeability of the rock mass
is usually high. In clay shales and slopes with preexisting failure surfaces, the residual fric-
tion angle
φ
r
is often applicable, with pore pressures corresponding to groundwater con-
ditions.
Two aspects that require consideration regarding strength are that strength is either gov-
erned by (1) planes of weakness that divide the mass into blocks (see
Section 6.4.4,
dis-
cussion of block behavior) or (2) the degree of weathering controls, and soil strength
parameters apply.
Seepage or cleft-water pressures affect the frictional resistance of the rock mass in the
same manner that pore pressures affect the strength of a soil mass.
