Geology Reference
In-Depth Information
generalised parameters such as hydraulic diffusivity, which are often
dif
ne and only apply to simple geological conditions. More
recent modelling methods are described by Blake et al. (2003).
It is often argued that it is more realistic to instrument a site and then
to extrapolate rainfall response to a design rainstorm (GCO, 1984a).
However, instrumenting slopes tomeasure critical water pressures is not
easy because instruments must be installed (and monitored remotely) at
the precise locations where water pressures will develop. Quite often
rock and soil pro
cult to de
flow carried in
narrow channels, pipes or through particularly permeable zones, as
discussed below, so the success of any instrumentation programme
will depend very much on the geological and hydrogeological insights
and skills of the investigating team, as well as a degree of good fortune.
les are compartmentalised and water
3.6.3 Preferential
flow paths through soil
Soil formed by sedimentation might be expected to suit isotropic mod-
elling but intrinsic and systematic variability is to be anticipated.
Sediment piles of even relatively uniform sand can be expected to con-
tain intercalations of
ow
characteristics. Conductivity will generally be higher in the horizontal
direction than vertical. For soils derived from the weathering of rock, the
mass can be incredibly complex and likewise the hydraulic conductivity.
Degree of weathering varies from place to place and the soil mass will
contain remnant fabric and relict joints from the parent rock that will
probably control water through
ner material that will result in heterogeneous
ow. In practice and empirically, the
mass might be represented by a simple set of parameters characterised
for a representative elemental volume (REV), but those parameters
probably do not actually represent the physical and mechanical
processes taking place in anything other than very simple situations.
Richards &Reddy (2007) provide a comprehensive review of piping,
particularly as related to earth dam construction, which is where piping
was
ne several
types of piping, which are also relevant to natural ground, namely:
first recognised as an important phenomenon. They de
1. Suffosion (or eluviation): the washing out or dissolution of mate-
rial en-masse, leaving a loose framework of granular material,
which is prone to collapse.
2. Dispersion of clay soils by rainwater in the vadose zone.
3. Backwards erosion from a spring. The pipe forms (for some
reason) and then material is gradually lost from that opening.
4. Erosion along some pre-existing opening such as a master joint.
The majority of pipes investigated by geomorphologists are con
ned
to the upper few metres in the ground pro
le (e.g. Jones, 1971; Uchida
et al., 2001). They are particularly common in forested areas within
shallow soil pro
les and are associated with shallow landslides
