Civil Engineering Reference
In-Depth Information
Overburden
4.5 m
Limestone
Intermediate benches
on weaker rock
6.0 m
Coal
Sandstone
15 m
Intermediate
bench on shale
6.0 m
Shale
Figure 12.20
Configuration of
benched cut in
horizontally bedded
shale and sandstone,
with weaker coal and
shale formations located
at toe of cut faces.
9 m max.
Sandstone
4.3 m
Roadside ditch
angles between about 40
◦
and 55
◦
, rocks will tend
to roll down the face and into the ditch.
To up-date the work carried out by Ritchie,
a comprehensive study of rock fall behavior and
the capacity of catchment areas has been carried
out by the Oregon Department of Transporta-
tion (2001). This study examined rock fall from
heights of 12, 18 and 24 m on slopes with five
increments of face angles ranging from vertical to
45
◦
(1V:1H). The catchment areas at the toe of
the slope were planar surfaces with inclinations
of 76
◦
(1/4H:1V), 80
◦
(1/6H:1V) and horizontal
to simulate unobstructed highway shoulders. The
tests observed both the impact distance from
the toe and the roll-out distance. The report
includes design charts that show, for all combina-
tions of slope geometry, the relationship between
the percent rock retained and the width of the
catchment area.
12.6.2 Ditches
Catch ditches at the toe of slopes are often a cost-
effective means of stopping rock falls, provided
there is adequate space at the toe of the slope
(Wyllie and Wood, 1981). The required dimen-
sions of the ditch, as defined by the depth and
width, are related to the height and face angle of
the slope; a ditch design chart developed from
field tests is shown in Figure 12.21 (Ritchie,
1963). The figure shows the effect of slope angle
on the path that rock falls tend to follow, and how
this influences ditch design. For slopes steeper
than about 75
◦
, the rocks tend to stay close to the
face and land near the toe of the slope. For slope
angles between about 55
◦
and 75
◦
, falling rocks
tend to bounce and spin with the result that they
can land a considerable distance from the base;
consequently, a wide ditch is required. For slope
