Denudation, weathering and mass wasting - Fundamentals of the Physical Environment - page 304

120

PLANE FAILURE
Slope angle, S
>
Discontinuity angle, D
>
Friction angle

Circular
failure

Toppling
failure

S

D

100

Dry slopes

I

WEDGE FAILURE
Discontinuity intersect, I
>
Slope angle, S
>
Friction angle

80

S

60

b

Wedge
failure

TOPPLING FAILURE
Tangent of slope angle, S
>
breadth/height of blocks,
b
h

h

40

Saturated slopes

20

Plane failure

CIRCULAR FAILURE
Restricted to non-
cohesive material and
intensely fractured
rock mass

0

0 0 0 0 0
Discontinuity angle, °degrees

50 60 70 80 90

Figure 13.18 Critical height of a vertical rock slope
determined by fracture geometry and condition. Note the
major reduction caused by saturated intermediate to high-
angled discontinuities.
Source: After Hoek and Bray (1977)

Figure 13.17 Failure modes in rock slopes, related to
discontinuity, geometry and friction strength.

Failure modes on rock slopes

Sliding and toppling failures dominate rock slopes, taking
one or more of four forms determined by relations
between discontinuity geometry and slope ( Figure 13.17 ).
Failure rarely needs to shear intact rock and might find
this more difficult ( Plate 13.17 ). The sole requirement for
sliding in discontinuous rock mass is one fracture surface
( D ) steeper than the friction angle ( F ) but at a lower angle
than the rock wall slope ( S ), which allows it to appear,
or daylight , in the rock wall. Single fractures provide
translation or release surfaces in planar slides (see Plate
13.6 ) but most rock walls fail by wedge slides ( Plate 13.6 ),
where the release surface is the intersecting plane ( I )
between two fractures. Toppling failure occurs where a
primary fracture surface dips into the rock wall and
appears stable but is intersected by more widely spaced
fractures. 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 13.18 .
Circular sliding is uncommon in hard rock, where
the 'massive' nature of orthogonal fracture sets provides
sufficient release surfaces, but may occur in densely frac-
tured material, soft sediments or when hard rock overlies

Plate 13.17 Quarry surfaces during blasting, showing
structural control on the release of blast energy and the
surviving rock wall. The tectonic fracture geometry deter-
mines the alignment and dip of every rock face, major or minor.
Photo: Ken Addison

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