Geology Reference
In-Depth Information
As an alternative to deep drains drilled into the slope from the
surface, drainage adits and tunnels are sometimes used to lower
the water table, generally with drainage holes drilled radially into
the rock mass from the tunnel walls. Other solutions include deep
caissons constructed at the rear of the slope to intercept through-
flow, with inclined drains leading away from the slope at their
base (McNicholl
et al
., 1986). Pumped wells are also occasionally
used, pumps being activated when water levels reach critical
heights within the slope.
Stability can be improved by a variety of reinforcement options. For
rough matching joints, provided there has not been previous move-
ment, the interlocking nature provides considerable shear strength. If
the joint can be prevented from movement by reinforcing at strategic
locations, then full advantage can be taken of the natural shear
strength. Depending on con
guration, rock may be stabilised by pas-
sive dowels, tensioned bolts or cable anchors. Passive dowels allow
both mobilisation of a normal force (due to the resistance provided by
the fully grouted dowel) plus active shear restraint provided by the
steel of the dowels resisting block slide mobilisation (Spang & Egger,
1990).
The Geotechnical Engineering Of
ce in Hong Kong has pub-
lished some guidelines on prescriptive measures for rock slopes
and in particular gives guidance on rock dowelling for rock blocks
with volume less than 5m
3
(Yu
et al
., 2005). In essence, it is
advised to use pattern dowels with one dowel per m
3
of rock to
be supported, with minimum and maximum lengths of 3 and 6m
respectively, and where the potential sliding plane dips at less than
60 degrees. The dowels are to be installed at right angles to the
potential sliding plane, with the key intention to allow the dowels
to act in shear, whilst also enhancing the normal restraint due to
asperity ride during sliding. In practice, dowels frequently need to
be used in more variable orientations. Designs must be checked in
the
field during installation, to check that the perceived ground
model is correct. If not, then the design must be revised.
Sub-horizontal cable anchors can be used if capacities larger than
about 20 tonnes per reinforcement member are required. Great care
needs to be taken to ensure that such tensioned anchors are adequately
protected against corrosion, and regular checking and maintenance
will be required. Several cases of anchors that have failed due to
corrosion are discussed in
Chapter 7.
For weaker rock and soil, pattern
soil nailing is now commonly used. The nails, which typically comprise
50mm or so diameter steel bars connected, as necessary, by couplers
every 6m, are usually installed in pre-drilled holes, held centrally by
