Environmental Engineering Reference
In-Depth Information
Plate 1
(a) Karren etched into the surface of a clint on limestone
pavement (hammer for scale). (b) Dolines on limestone covered
with glacial till and slope debris below Pen-y-Ghent, Yorkshire
Dales.
Photos: Ken Addison.
wasting also occurs in more formally recognized environments, and every landform is a
composite of slope forms. It is not surprising that producing schemes of slope
classification and evolution is a busy industry!
SLOPE STABILITY
In its simplest form, slope stability depends on the ratio of stabilizing forces resisting
movement to destabilizing forces encouraging it and is reflected in the engineering term
factor of safety
,
F
s
. A state of
limiting equilibrium
or incipient failure exists when
F
s
=
1, as we saw in the Mohr-Coulomb equation earlier. Movement can be triggered by any
further deterioration. The natural angle of rest in granular slope materials approximates
F
s
= 1, which is too close for comfort in the engineering world. A compromise is struck
between increased stability and cost. Hence
F
s
> 1 is measured in expensive stabilization
schemes or loss of space/resources in road cuttings or quarries by reducing slope angles.
F
s
= 1 is measured by the costs of failure during the design life of a structure in human
lives, resource value, etc. We simplify the balance of forces on natural slopes to the
relationship between shear
strength
, shear
stress
and slope angle - and revisit Mohr-
Coulomb (see box, p. 269).
In granular materials, moderate pore water levels generate cohesion or negative pore
water pressure. Positive pore water pressures develop as water content rises, transferring
