Environmental Engineering Reference
In-Depth Information
-
the location of tracks, roads, test pits and trenches, with summary logs of the soils and
rocks exposed;
-
the position of drillholes and geophysical traverse lines;
-
the proposed works in outline including the full supply level of the proposed storage.
Figure 5.6
is an example of the first stage of compilation of a geotechnical plan at the
site for a concrete faced rockfill dam about 35 m high. At this stage the plan shows a factual
record of surface geological and geomorphological features. Three relatively important
features are inferred - the landslide upstream from the site and the two faults. Other
important features might also be suspected, for example bedding-surface faults at the
boundaries for the sandstone beds, because no actual shale-sandstone boundaries are
exposed in outcrop, even at river level.
It should be noted that such a plan would normally be prepared on 1 : 500 or even
1:200 scale on an A2 or A1 size sheet. For the sake of legibility for publication at its pres-
ent size the plan has been simplified greatly, i.e. it contains much less geological detail and
wider contour spacing than would usually be present.
Position identification is important during the mapping. In sparsely vegetated areas the
combination of enlarged air photos and contour plans at the same scale may enable posi-
tioning to an acceptable level of accuracy. Where required, more accurate control can be
achieved by ground survey methods or by use of GPS.
5.5
GEOPHYSICAL METHODS, SURFACE AND DOWNHOLE
Geophysical methods have been extensively used in dam investigations, both on dam con-
struction projects and in the assessment of the condition of existing dam structures. There
are many different methods; the selection of the method(s) relevant to a particular problem
will usually depend on the regional and local site conditions, the nature of the problem,
timing and cost.
The
advantages
of the use of geophysical methods include:
- They are non-invasive and can be carried out from the surface or from existing boreholes;
- They can provide information on site conditions between data points e.g. boreholes;
- They may be able to identify local areas of concern which have no surface expression e.g.
cavities;
-The surveys can usually be performed quickly and cover a relatively large area;
-
Recent development of computer analysis and presentation of results (tomography) has
assisted interpretation.
The
disadvantages
of the use of geophysical methods include:
- Each method measures a particular physical subsurface property which may or may
not be relevant to the problem under examination;
- Some borehole surveys are affected by the presence of steel casing;
- Each method requires expensive equipment and skilled operators;
- The results involve the recording and analysis of a great amount of numerical data;
- The mathematical analysis is based on an assumed subsurface model which may be dif-
ferent from the actual geological situation;
- Much of the data involves the averaging of information;
- The accuracy of the results may be lower than required;
-The interpreted subsurface profile usually needs to be confirmed by drilling;
-
Establishment of the equipment and operators is expensive and also stand-by time
waiting for other operations, e.g. drilling.
