Geology and ground models - Practical Engineering Geology - page 87

environments, sometimes associated with expansive formation of salt
in pores. The depth of weathering in cold climates is far less than in
tropical environments.
In temperate climates such as the UK, weathering is rarely very
signi cant and certainly far less so than in countries such as Brazil,

Malaysia and Singapore, where weathering has extremely important
consequences for investigation, design and construction (e.g. Shirlaw
et al., 2000). In the UK and much of northern Europe, most weathered
rocks were stripped from the landscape by recent glaciations.

3.5.2 Weathering pro les

Weathering reduces the strength of rock material, as illustrated in
Figure 1.5 and discussed in Chapter 5 in terms of geotechnical para-
meters. Weathering processes generally operate at upper levels in the
saturated zone and in the vadose zone above the water table, although
it should be noted that water tables change periodically and have done
so over the millions of years that it will have taken for the development
of some thick weathered pro

les. Therefore, current water levels may
not be related to depth of weathering at a site.
As a general rule, weathering works in from free surfaces where
chemicals in water (including the water itself) can attack the parent
rock ( Figure 3.51). Eventually, it may leave a framework of corestones
of less weathered rock separated by severely weathered zones marking
out the loci of the original joints ( Figure 3.52). The process is illu-
strated in Figure 3.53. The wide varieties of conditions that can be
encountered in weathered terrain are discussed by Ruxton & Berry
(1957), and Figure 3.54 is based on their interpretation of one type of
weathered pro le from Hong Kong. Despite the conceptual usefulness

of Ruxton & Berry les, exposures are often encountered that do

not conform and such exposures provide challenges to mass weath-
ering classi

' s pro

cations such as the current European standard, as dis-
cussed in Chapter 4 and Appendix C. Furthermore, other rock types
often weather without the development of corestone-type pro les.

Mudstone sequences tend to develop a gradational weathering pro le,

as characterised for the Keuper Marl by Chandler (1969). Limestone is
often karstic, with large caves and open joints, as illustrated in
Figure 3.55 and classi ed by Fookes & Hawkins (1988).

At any particular location, the weathering pro le is a function of

parent geology, groundwater conditions and the geological and geo-
morphologic history of the site. The pro les may be ancient and bear

little relationship to current geomorphologic setting. Given these and
other factors, weathering pro les can be rather unpredictable from

examination of the current topography. Valleys might be associated
with deep weathering along faults (e.g. Shaw & Owen 2000), but not
always.