Geology Reference
In-Depth Information
example, as part of the drainage system behind concrete retaining
walls. Plastic sheets (geomembranes) are used as barriers to water
ll sites. Great care must be taken to ensure
that sheets are welded one to the other and that those welds are tested.
Membranes must be resistant to and protected from puncturing. Any
leakage may be extremely dif
flow, especially for land
cult and expensive to rectify at a later
stage. In
Chapter 7,
an example is given where a combination of
permeable geotextiles and impermeable geomembranes were used to
reduce leachate loss from a quarry used for land
ll.
6.8.8 Grouting
Grouting is generally used to increase strength of a rock or soil mass
and to reduce permeability (Warner, 2004). It is routinely used below
dams to provide a cut-off curtain to restrict seepage through the
foundations. A main consideration is the type of grout
usually
cement, but sometimes chemical grouts or resin must be used to
penetrate low-permeability ground. The pattern of holes to be used,
phases of grouting necessary, and pressures to be adopted are also
matters for specialist design. Grouting might jack open existing joints
in rock or form new fractures in soil and weak rock (claquage).
Grouting is sometimes used to correct settlement or other deforma-
tions caused by engineering works such as tunnelling (e.g. Harris
et al
.,
1994), but care must be taken that the grouting does not make matters
worse, as per the Heathrow Express Tunnel collapse described in
-
Cavities that engineering geologists and geotechnical engineers
need to contend with include natural cavities such as those
often found in limestone areas, more rarely in other rock types,
including unlikely candidates such as weathered granite (Hencher
et al
., 2008). The other main problem is mining. Ground inves-
tigation for such voids is a matter of careful desk study (includ-
ing the mining method that might have been used if that is the
hazard of concern), focused investigation, possibly using geophy-
sics such as micro-gravity and resistivity and probing, perhaps
using percussive drilling to keep the costs down. If and when
voids are found, these can be explored and characterised using
cameras, echo sounders and radar. In the case of old mine work-
ings, inspection may be required by suitably equipped and experi-
enced persons following proper safety procedures. Depending on
their extent, voids may be back
lled, grouted or structurally
reinforced, as appropriate. When extensive mine workings were
encountered unexpectedly during tunnelling for the high-speed
