Environmental Engineering Reference
In-Depth Information
major reduction caused by saturated intermediate to high-
angled discontinuities.
Source: After Hoek and Bray (1977).
equilibrium angles in the range 35°-45°, depending on lithology and block shape.
Colluvium
is a general term for reworked rock debris derived from slope or other
(especially glacial) sediments. It has undergone further breakdown and chemical
weathering, with a wider range of particle sizes, and acquired a less blocky, more
grain/matrix form. Colluvium is unlikely to be well sorted or stratified, reflecting the
episodic downslope movement of separate pulses of debris. Pedogenesis will begin but
subsequent burial testifies to the influx of new material. Equilibrium slope angles range
from 32° to 37°, rising above 40° towards an upslope boundary with scree.
Soil
refers to
material showing substantial pedogenesis and pedological character, normally supporting
vegetation.
Debris is generally characterized by high void ratios, weak and irregular potential
shear surfaces and zero cohesion, except at low water content. Dry movements occur
when tangential forces exceed shear resistance, assisted by
ground heave
through
hydration or ice formation, or rain splash. At very low velocities and in the absence of
clearly defined failure surfaces, this amounts to the relentless
creep
of particles
downslope either as single blocks or grains or
en masse
by
solifluction
as lobes or sheets.
The latter term is used both generally and also in the restricted sense of the permafrost
environment. Debris/soil slumps develop on well defined failure surfaces by mass
rotation. Cohesive debris, with suction forces or cementation, may behave initially like
rock mass and slide, fall or topple (Plate 13.12) before disintegrating. Wet movements are
promoted by mass fluidization, and the resultant slurry velocity is usually moderate to
fast as
mud flows
or very fast as debris flows, where grain collisions provide buoyancy
(see box, p. 276).
