Environmental Engineering Reference
In-Depth Information
As discussed in Chapter 2, Section 2.10, it is very important that evidence of any cur-
rently active or past landsliding in the project area is recognised early. Fortunately the
presence of active or ancient landslides is usually indicated by:
- unvegetated scarps, cracks and areas of exposed soil or rock;
- characteristic topographic forms, e.g. scarps, spoon-shaped troughs, hummocky or
steep ground and areas of internal drainage;
-areas of anomalous vegetation, e.g. where trees are dead, or younger than elsewhere,
or where the vegetation is more dense due to an area of deeper or wet soil;
- evidence of past or current restriction or damming of streams.
Rib and Liang (1978) provide guidance on the recognition of landslides on aerial pho-
tographs in a range of geological environments.
In densely vegetated areas, evidence of active or very recent landsliding may not be
readily visible on photographs. For example, in some tropical areas shallow landsliding
produces subdued topographic forms and revegetation is rapid. As shown in Figure 3.34
large landslides show up well even in tropical rainforest. Fookes et al. (1991) describe how
examination of several sets of aerial photographs assisted the analysis of slope movements
at Ok Ma damsite, Papua New Guinea.
Another use of photo-interpretation is in terrain analysis where the features observed,
including topography, geology, soils, drainage and vegetation are used to divide the area
into land units with similar characteristics (Grant 1973, 1974). This type of study is more
suited to regional assessment of catchment or reservoir areas or to the location of con-
struction materials, than to localised assessments, e.g. dam sites.
Figure 5.3
is an example
of a terrain map prepared to assist in the location of construction materials. Such maps
form a logical basis for the planning of ground surveys followed by exploration of possi-
ble borrow areas. These maps should not be relied on for selection of sites without a field
check of site conditions.
5.2.3
Photographs taken during construction
Any review of an existing dam should include a thorough search for photographs taken
during construction, and systematic review of the photographs. They are particularly
valuable as a record of foundation preparation and clean-up, and for assessing matters
such as segregation of filter and transition zones.
5.3
GEOMORPHOLOGICAL MAPPING
Before embarking on a subsurface investigation program, which may be (a) expensive and
(b) provide limited information of dubious relevance, it is recommended that the surface
evidence should be systematically recorded.
The ground surface reflects both the underlying geology and the geomorphological
development of the area. Geomorphological mapping of surface features can provide an
indication of the distribution of subsurface materials, their structure and areas of possible
mass movement, e.g. landslides. Geomorphological mapping should be carried out at two
levels - an overall appraisal of the study area and a detailed recording of site data using a
topographic map as the base. Hutchinson (2001), Varnes (1978) and Brunsden et al.
(1975) describe mapping methods.
The overall appraisal involves an examination of the topographic features of the area,
surface shape, drainage, local depressions and areas of distinctive vegetation.
An hour spent on a convenient hilltop reviewing the geomorphology of the whole area
before commencing detailed mapping may provide an understanding of the problem
