Environmental Engineering Reference
In-Depth Information
Translation
slide
Slow-fast
Low-
moderate
Plane,
wedge
slide
Block glide
Grain
slide
Rotation
slide
Slow
Low-
moderate
Circular
slide
Debris slump
Soil slump
Creep/heave Very slow
Low
Rock creep
Talus creep
Solifluction
Cambering
Grain creep and rain
splash
Compound translation
failure
Combination of two or more, changing along a continuum
of modes and rates during movement
a primary fracture surface (
D
1
) dips into the rock wall and appears stable but is
intersected by more widely spaced fractures (
D
2
). Columnar blocks, defined by height (
h
)
and breadth (
b
), topple when their centre of gravity overhangs a pivot. Fracture geometry,
failure modes and rock wall height relationships are shown in Figure 13.14.
Circular sliding is uncommon in hard rock, where the 'massive' nature of orthogonal
fracture sets provides sufficient release surfaces, but may be seen in densely fractured
material, soft sediments (Plate 13.9) or when hard rock overlies less resistant strata. In the
former, initially steep slides are released along less steep fractures as they approach the
rock wall foot, leaving circular scars. In the latter, the
incompetent
stratum (e.g. clays,
shales) slumps, and may subsequently flow, carrying away
competent
overburden in
celebrated cases such as the Black Ven cliffs in Dorset.
Cambering
occurs as the rigid
overburden creeps forward before shearing over a steepening slope. Debris may distort
underlying strata during rock creep (Plate 13.10).
Slides release small rock volumes to spectacular failures. 36,000 m
3
of rock failed in a
single wedge slide during glaciation at the site in Plate 13.8b but quantities two to three
orders of magnitude larger were moved in catastrophic events such as the Franks Slide,
Alberta, in 1903, and Blackhawk Slide, Southern California (Plate 13.11). Free
rockfalls
from an overhang or prised off by frost weathering involve smaller volumes. All rock
failures contribute blocky debris either to downslope scree or avalanches, which continue
as collision-buoyed flows. The special case of rock glaciers is covered in Chapter 15.
FAILURE MODES ON DEBRIS SLOPES
Various terms are used for the forms of denudation 'debris' or 'detritus'. Several stages
in the progressive breakdown of rock are recognized.
Rock debris
or regolith is blocky
material derived from rock walls and retains its angular character with little or no further
breakdown. Debris forms
scree
or
talus
deposits on slopes at
