Geology Reference
In-Depth Information
Figure 6.27
Rock-
fall nets and
avalanche shelter,
near Cape Town,
South Africa.
holes can be very effective, but all drains will seldom yield water
ows,
and the effectiveness of individual drains will probably change with
time as sub-surface
flow paths migrate. Typically, drains comprise
plastic tubes with slotted crests and solid inverts, inserted into pre-
drilled holes of tens or even hundreds of metres. Inner geotextile liners
might be used that can be withdrawn and replaced if they get clogged
up. Drains might need to be
flushed out periodically. Attention should
be made to detailing the drain outlets properly otherwise the slope face
may backsap. If not maintained, vegetation can block outlets reducing
their effectiveness.
In rock slopes, there is a need to target sub-surface
flow channels,
many of which will be shallow and ephemeral. The paths may be
tortuous and hard to identify and drainage measures can therefore be
rather hit or miss. If the exposed joint is badly weathered, the weak
material may backsap and possibly pipe, leading to destabilisation,
partially caused by lack of free drainage, and careful detailing will be
required to prevent deterioration. No-
nes concrete, whilst appearing
to be suitable to protect weathered zones, often ends up with lower
permeability than designed and should not be relied upon without
some additional drainage measures.
