Geology Reference
In-Depth Information
damage to adjacent structures; piles can become overloaded by negative
skin friction (
Chapter 6).
Similarly, rising water levels can cause diffi-
-
culties due to buoyancy, which could require holding down anchors or
piles. Rising water might also weaken the ground supporting a
structure, sometimes due to dissolution of cementing agents.
For tunnels and other underground structures, in
ows can be very
dif
cult to predict accurately because it depends so much on the
geological situation, which is rarely well understood before the
works commence. Given knowledge of water pressure and hydraulic
conductivity, then in
ows can be predicted following standard equa-
tions or by numerical simulation, but the controlling parameters are
dif
cult to measure or estimate and predictions can be wildly out, as
found for trials for nuclear waste investigations (e.g. Olsson & Gale,
1995). This is especially so for tunnels passing through variable geol-
ogy, as discussed by Masset & Loew (2010). The pragmatic solution is
to probe ahead of the tunnel face periodically, and if in
ows from the
probe holes are high, then to improve the ground in front of the tunnel,
usually by injecting cement or silica grout to reduce the hydraulic
conductivity. Alternatives are dewatering the ground or freezing the
ground temporarily. If predictions of groundwater conditions are
badly incorrect, this can have major consequences for the suitability
of tunnelling method or machinery (e.g. the degree of waterproo
ng of
equipment). Where there is a risk of high water in
ow, it is normal
practice to drive the tunnel uphill to reduce the risk of inundation,
danger to workers and damage to machines. Where tunnelling under
the sea, lakes or rivers, there may be a risk of disastrous in
ows, as
occurred during the construction of the Seikan Tunnel in Japan
(Matsuo, 1986; Tsuji et al., 1996). Other examples where the severity
of groundwater conditions was underestimated with severe conse-
quences include the SSDS tunnels in Hong Kong and Ping Lin Tunnel
in Taiwan, which are discussed in
Chapter 7.
Most landslides are caused by rainfall and in Hong Kong, for example,
rises in water level during a storm of more than 10m have been recorded
(Sweeney&Robertson, 1979). Therefore, there is great interest in trying
to predict changes that might occur, as these will greatly affect any
numerical calculations of slope stability, as well as other engineering
projects. Geological pro
les are generally depicted for groundwater
modelling, as made up of discrete, homogeneous and often isotropic
units of given hydraulic conductivity (Todd, 1980), and most commer-
cially available, hydrogeological software only deal with homoge-
neous units. To be more realistic, models may need to incorporate
local barriers such as fault seals, fracture
ow or more variable geological
conditions such as local lithofacies (e.g. Fogg et al., 1998).
