Civil Engineering Reference
In-Depth Information
(a)
(b)
(c)
(d)
Tension cracks
2050
Circular sliding
surfaces
2000
Toppling at
pit crest
1950
1900
1850
Sandstone
Talus
1800
Fault
1750
Siltstone
Conglomerate
Figure 9.4
Secondary toppling modes: (a) toppling at head of slide; (b) toppling at toe of slide with shear
movement of upper slope (Goodman and Bray, 1976); (c) toppling of columns in strong upper material due to
weathering of underlying weak material; (d) toppling at pit crest resulting in circular failure of upper slope
(Wyllie and Munn, 1978).
sandstone and columns of sandstone, with their
dimensions defined by the spacing of the vertical
joints, topple from the face. At some locations
the overhangs can be as wide as 5 m, and failures
of substantial volumes of rock occur with little
warning.
The example of the slide base toppling mode
shown in Figure 9.4(d) is the failure of a pit slope
in a coal mine where the beds at the crest of the
pit dipped at 70
◦
into the face, and their strike
was parallel to the face. Mining of the pit slope at
an angle of 50
◦
initiated a toppling failure at the
crest of the pit, which in turn resulted in a circular
failure that extended to a height of 230 m above
the base of the topple. Detailed monitoring of
the slope showed that a total movement of about
30 m occurred on the slope above the pit, result-
ing in cracks opening in the crest of the mountain
that were several meters wide and up to 9 m deep.
Continuous movement monitoring was used to
allow mining to proceed under the moving slope,
and finally the slope was stabilized by back-filling
the pit (Wyllie and Munn, 1979).
A further example of the toppling mechan-
ism is illustrated in Figure 9.5 (Sjöberg, 2000).
In open pit mines where the depth of the slope
progressively increases, minor toppling move-
ment may eventually develop into a substantial
failure. Careful monitoring of the movement, and
recognition of the toppling mechanism, can be
used to anticipate when hazardous conditions are
developing.
