Geology Reference
In-Depth Information
Mine in Japan, on the basis of descriptive parameters such as rough-
ness, aperture and orientation, as observed in the drift (at the exposed
traces of discontinuities in the mine). They used a neural network
approach to train the analysis, but with limited success. Indeed, it
seems clear that connectivity is the most important factor that controls
flow through rock, rather than locally measurable characteristics such
as fracture intensity, aperture or spacing. The fact that the most
transmissive and well-connected features might not be adequately
sampled during investigation, is a major problem for all geotechnical
projects, not least for potential nuclear waste repositories, where it is
recognised that large-scale conductive features need to be identi
ed
and dealt with in a deterministic manner (Black et al., 2007; Nirex,
2007).
In weathered rock pro
flow is sometimes concentrated in
fractured rock, underlying weathered saprolite or colluvium, and this
can result in transmission of water (and high water pressures) from one
part of a hillside to another, or even between catchments, where the
water then feeds into the overlying weathered mantle and may trigger
landslides. During the Mid Levels study in Hong Kong, it was recog-
nised that con
les, water
ned conditions could occur where material of lower
conductivity overlies rock of higher conductivity. Strong upward
hydraulic gradients from bedrock to the decomposed rock aquifer
were identi
ed in some areas (GCO, 1984b), and these observations
were used in setting up a numerical model of the hydrogeology (Leach
& Herbert, 1982). Jiao &Malone (2000) and Jiao et al. (2005, 2006)
have extended this concept of a highly transmissive zone at depth to
explain several deep-seated landslides and evidence of artesian pres-
sure in Hong Kong. Montgomery et al. (2002) report an intensely
instrumented site (more than 100 shallow piezometers) in Oregon,
USA, and similarly noted artesian
flow from the underlying bedrock,
which they found surprising for such a steep hillside. They also com-
mented that, whilst the seepage from bedrock might effectively deter-
mine the speci
flows might initiate, the
distribution and connectivity of the near-surface bedrock fracture
system are almost impossible to predict. Similar upward
c locations where debris
flows from
bedrock into the overlying soil mantle are reported for steep granitic
terrain by Katsura et al. (2008).
3.7 Geological hazards
Numerous hazards can be regarded as essentially geological, including
the potential for subsidence, swelling, clay shrinkage, natural noxious
gases and mining, as generally addressed in
Chapter 4,
when consider-
ing elements to be targeted during site investigation. The most
