Environmental Engineering Reference
In-Depth Information
5.4
GEOTECHNICAL MAPPING
Geotechnical mapping is essentially geological mapping aimed at answering engineering
questions. At any particular site it involves the location and plotting on suitable scales of
all data which assists in understanding the geotechnical conditions at that site.
5.4.1
Use of existing maps and reports
Some useful data can often be obtained from existing maps and reports prepared for other
purposes.
Maps showing the regional geology on scales ranging from 1:100,000 to 1:1,000,000
are usually available from government agencies. Some mining areas and areas of existing
or proposed urban development may have been mapped at larger scales. The regional
maps are often accompanied by explanatory notes which are useful.
The regional maps show the inferred distribution of the main rock types and the
inferred geological structure. Areas where rock is overlain by unconsolidated sediments,
surface soils or talus are not differentiated unless these deposits are known to be wide-
spread and of significant depth. The data plotted on the maps is obtained from examina-
tion of surface outcrops, information from stratigraphic and exploration drilling, and air
photo interpretation.
The main value of these maps is providing an understanding of the stratigraphy, struc-
ture and geological history of the region, i.e. the broad geological environment in which
the project is to be located. As set out in Chapter 3, this understanding can provide a use-
ful insight into the range of geotechnical conditions which might be expected at the proj-
ect sites. The maps are also useful during studies associated with access routes, material
sources and reservoir and catchment areas.
Local deposits of alluvial, colluvial or residual weathered materials are usually ignored
or briefly mentioned on regional geological maps. Because of this the maps generally do
not answer detailed questions related to dam projects.
Geological reports prepared for different purposes, e.g. for mineral exploration, can
provide some useful information.
Regional maps showing distribution of soil groups, classified for agricultural purposes,
may also be available. These maps may be a useful supplement to the regional geology
map, particularly in the search for construction materials.
Land capability maps, based on the regional soil surveys with particular attention to
potential landsliding and erosion, may be available for limited areas.
5.4.2
Geotechnical mapping for the project
5.4.2.1
Regional mapping
When published regional geological maps are available they are usually able to provide
the regional geological understanding required for a dam project. For major dam projects,
and for all projects where these maps are to be used in access, material or reservoir stud-
ies, they should be checked on the ground and on air photos. Where necessary they should
be updated by the addition of data of engineering importance. Such data will include lin-
eaments, landslides, scarps, swamps, springs and areas of problem soils.
If no satisfactory regional geological map is available, and the proposed dam is of
major size and high hazard, then it would be prudent to prepare a regional map specifi-
cally for the project.
However produced, the regional geological plan as used and included in the project
reports should show the location of the proposed works and the outline of the proposed
storage.
