Civil Engineering Reference
In-Depth Information
The Geologic Map Database from the United States Geologic Survey (USGS) provides
access to geologic maps of all parts in the world, where available. On a national scale,
geologic maps can be obtained from the British Geological Survey (United Kingdom)
and the Bureau des Recherches Géologique et Minières (France). In Germany, geo-
logic maps are available from the Staatliche Geologische Dienste Deutschlands (Na-
tional Geological Services of Germany). Several German Federal States and the BGR
in Hannover (Federal Institute for Geosciences and Natural Resources) offer geologic
maps via the internet by means of the so-called “Web Map Server”. In Austria and
Switzerland, geologic maps can be obtained from the Geologische Bundesanstalt (Fed-
eral Institute for Geology) and the Bundesamt für Landestopografie (Swisstopo), re-
spectively.
Valuable information on the project area can also be provided by aerial photographs,
satellite images, survey drawings and experience gained during construction of neigh-
boring structures.
During the evaluation of available documents, a close cooperation between engineers
and geologists is important. Knowledge on regional and historical geology may con-
tribute valuable information with regard to the assessment of rock mechanical condi-
tions in the project area and the planning of an exploration program.
13.3
Rock Exposures
The mapping of rock exposures and outcrops at the ground surface such as natural
slopes, excavated cuts and quarries enables the gathering of information on the bedrock
without great cost. Figure 13.2 shows an outcrop at a natural slope near Nuttlar. How-
ever, it should be kept in mind that the rock mass at such exposures is frequently altered
by weathering and may not represent the undisturbed state. Mapping of rock surfaces
and its evaluation is described in Section 13.9.
13.4
Test Pits
Test pits allow direct access to the bedrock if the overburden of the rock mass with soils
is not too large and seepage water can be kept under control. Test pits and test trenches
are frequently carried out in the foundation areas of dams.
Test pits are excavated mechanically or, where necessary, by blasting. They must be
carried out deep enough to encounter the unweathered, undisturbed rock, and wide
enough to enable access for mapping. Thus, at least one wall should be stepped or in-
clined (Fig. 13.3). Mapping (Section 13.9.1) and taking of rock samples should be car-
ried out as soon as the test pit is excavated. Otherwise, the rock surface must be protect-
ed, from weathering.
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