Environmental Engineering Reference
In-Depth Information
FIGURE 9.62
Section of 3-in.-diameter undisturbed
specimen of varved clay taken from a
depth of 11m in the failure zone showing
the rupture surfaces after collapse of an
excavation in Haverstraw, New York.
Failure Surface Modes and Stability Relationships
General: Two Broad Modes of Failure
Infinite slope
mode involves translation on a planar surface whose length is large compared
with its depth. This mode is generally applicable to cohesionless sands, some colluvial and
residual soil slopes underlain by a shallow rock surface, and some cases of clay shale slopes.
Finite or limited slope
mode involves movement along a surface limited in extent. The
movement can be along a straight line, a circular arc, a log-spiral arc, or combinations of
these. There are two general forms of finite slope failures: wedges and circular failures.
Wedge analysis forms are generally applicable to jointed or layered rock, intact clays on
steep slopes, stratified soil deposits containing interbedded strong and weak layers, and
clay shale slopes. Cylindrical failure surfaces are typical of normally consolidated to
slightly overconsolidated clays and common to other cohesive materials including resid-
ual, colluvial, and glacial soils where the deposit is homogeneous.
Infinite-Slope Analysis
The infinite slope and forces acting on an element in the slope are illustrated in
Figure 9.63.
In the infinite-slope problem, neither the slope height nor the length of the failure surface
is considered when the material is cohesionless.
