Geology Reference
In-Depth Information
countries linked to seismic design codes. In some countries, there are
also local seismic micro-zoning maps showing locations of active faults
and hazards such as liquefaction susceptibility. Sources of information
for the UK are given in BS 5930 (BSI, 1999) and Clayton et al., 1995.
The Association of Geotechnical and Geoenvironmental Specialists
(AGS), whose contact details are given in Appendix A, also give
useful advice and sources of reference. Records of historical mining
activity and previous land use are especially important. In the UK, the
British Geological Survey (BGS) has made available a digital atlas of
hazards, including mining (but not coal), collapsible materials, swel-
ling and compressible soils, landslides and noxious gas. Landslide
hazard maps are published in the USA for southwest California and
in Hong Kong, as discussed below.
Air photographs can be extremely useful for examining sites. Pairs of
overlapping photographs can be examined in 3D using stereographic
viewers, and skilled operators can providemany insights into the geology
and geomorphological conditions (Allum, 1966; Dumbleton & West,
1970). Historical sets of photographs help to reveal the site develop-
ment and to assess the risk from natural hazards such as landslides. In
Hong Kong, it is normal practice to set out the site history for any new
project through air photo interpretation (API) of sets of photos dating
back to the 1920s. The role of API in helping to assess the ground
conditions at a site is illustrated in
Box 4-3.
Overlapping air photos allows a skilled earth scientist to examine the site topography in three dimensions.
According to Styles (personal communication), in order to do it well you must put yourself on the ground
mentally and walk across the terrain looking around in oblique perspective. Topographic expression and
other features such as the presence of boulders, hummocky ground, arcuate steps and vegetation, can be
interpreted in terms of terrain components and geomorphological development: landslide morphology,
degree of weathering, and distribution of super
cial deposits such as colluvium and alluvium. Broad
geological structure such as major joint systems, faults and folds, may be observed, interpreted and
measured in a way that would be more dif
cult working only by mapping exposures on the ground
(
Figure B4-3.2)
.
Where landslides are identi
ed on photographs, debris run-out can be measured, which may help in
assessing the degree of risk for existing and future developments. River channels can be traced and
catchments measured. Where a series of historical photographs is available, an inventory of landslide
events can be compiled and related to historical rainfall records. Anthropogenic development and use of
sites can be documented.
It is important that API is checked by examination in the field and this is known as ground truthing,
which is an integral part of site reconnaissance and
field mapping. Similarly, interpreted site history
should be checked and correlated against other documentary evidence such as old maps and photographs.
The preliminary ground model developed from API and
field studies can then be investigated further by
