Environmental Engineering Reference
In-Depth Information
The field survey team should liaise with the site investigation group, and provide a series of
clearly labelled ground control marks throughout the project area to assist in the location
of features identified during the geotechnical mapping. All boreholes, pits and trenches
should be located, levelled and clearly labelled.
5.2
INTERPRETATION OF SATELLITE IMAGES AERIAL PHOTOGRAPHS AND
PHOTOGRAPHS TAKEN DURING CONSTRUCTION
Satellite image maps and aerial photographs are available from government agencies in most
countries. For investigations of existing dams, photographs taken during construction are
an invaluable aid to assessing the geology and construction of the dam.
5.2.1
Interpretation of satellite images
Standard LANDSAT images are at 1:100 000 scale but enlargements at 1:500 000 and
1:250 000 are also available.
These small scale images or image maps provide a broad view of the region in which the
project is to be located. This broad view can indicate correlations between geological fea-
tures or the position of geological boundaries or faults, when these features are of such
great extent that they are not recognisable on larger scale photographs covering smaller
areas.
The broad view also provides an indication of relationships between the regional geol-
ogy and landforms, drainage, soils, vegetation and land-use, which may be useful in:
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planning of access routes to and within the project area;
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location of potential sources of construction materials, and
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assessing reservoir siltation rates.
5.2.2
Interpretation of aerial photographs
5.2.2.1
Coverage
If the existing aerial photographs in the project area are inadequate in quality, coverage or
scale, it is best to take new photographs with the following advantages:
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the photographs will show present conditions;
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the required scales and coverage can be specified, and
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the photographs can be used to prepare topographic maps.
Aerial photographs taken at different dates can indicate changes in site conditions.
5.2.2.2
Interpretation
Photogeological interpretation using a stereoscope forms a major part of the initial
appraisal of regional and local site conditions during the pre-feasibility and feasibility stages
(see Section 4.4.1.2 and Table 4.3). It usually gives a good indication of likely geotechni-
cal constraints on the project and the extent of investigations required.
Much has been written about the techniques of photogeological interpretation and
their application in engineering (Rib and Liang, 1978; Rengers and Soeters, 1980; Bell,
1983b; Lillesand, 1987; and Hunt, 1984). Comments will therefore be limited to aspects
of special significance in dam engineering.
The stereoscope provides a three-dimensional image with a vertical scale which is
exaggerated by a factor which depends on the distance between the photo-centres. This
