Geology Reference
In-Depth Information
highly fractured shear zone, rather than a discrete plane with gouge,
and such zones are often highly permeable.
Faults are a particular concern in geotechnical engineering in that
they can be associated in sudden and often rather unexpected changes
in rock quality. They may act as barriers to
flow (termed fault seals in
oil reservoirs) or, conversely, they can be highly permeable zones, full
of water, and lead to a sudden inrush of water into tunnels. Faults are
also, of course, the main source of earthquakes. By de
nition, faults
disrupt the rock mass and may throw rocks of very different engineer-
ing characteristics together. As a result, a tunnel may pass from hard
and good rock to extremely poor rock conditions over a very short
distance and without warning. Such situations can be very dif
cult to
deal with, necessitating a change in excavation methods, support
requirements and sometimes a complete rethink of a project
(e.g. Ping Lin Tunnel
a case study in
Chapter 7).
For foundations,
there may be a sudden change over a few metres from simple pad
foundations resting on rock to the need for deep piles to carry the load
of a structure.
Not all faults cause problems for projects, so there is a danger of
being over-cautious, leading to over-expensive investigation and unne-
cessary allowance in design for potential poor ground. Furthermore,
faults shown on geological maps are sometimes conjectural, inferred
by the mapping geologist on some topographic feature such as a valley
or other lineament. However, lineaments and river systems can re
-
ect
geological features other than faults and the drainage system may owe
its geometry to ancient geological history.
Another type of fault that can cause considerable problems because of
low shear strength is that formed close to the Earth
is surface due to
periglacial processes. Such shear surfaces can be formed by a number of
different mechanisms and can be extensive laterally (Spink, 1991).
Numerous failures of slopes and embankments have been attributed to
their presence (Early & Skempton, 1972), including Carsington Dam,
during construction as described in
Chapter 7.
Skempton et al. (1991)
note that soli
'
nd, evenwhen you are
fairly certain that they are there. Patient and detailed logging will
be required, possibly with trial pits left open for several weeks to allow
the shear surfaces to become apparent as the ground dries out and stress
relief occurs.
uction surfaces are often dif
cult to
3.4.6 Joints
Joints are fractures in rock that, by de
nition, show no discernible
displacement relating to their time of origin, which distinguishes them
