DHARM726 GEOTECHNICAL ENGINEERING
engineering problems may arise where structures settle because of mine workings in their
proximity.
Seismic potential or potential seismic activity is a major factor in structural design in
many regions of the world, especially in the construction of major structures such as dams and
nuclear power plants. Maps are now available showing the earthquake zones of different de-
grees of vulnerability. A lot of work has been done in this regard by the ‘‘Centre for Research
and Training in Earthquake Engineering’’ located at Roorkee in India.
18.2.3Aerial Photography
Aerial photography is now a fairly well-developed method by which site investigation may be
conducted for any major project. Air photo interpretation is the estimation of underground
conditions by relating landform development and plant growth to geology as reflected in aerial
photographs.
Photographs are obtained in sequence by flying in more or less straight lines across a
site with a two-thirds overlap in the direction of flight and one-quarter overlap between suc-
cessive flight lines. For general mapping, a scale of 1 : 20,000 may be adequate but for more
detailed work larger scales obtained by low altitude photography are necessary.
Analysis consists of an identification of all the natural and man-made features and
their grouping by geological association. Finally the probable geological formation of soil or
rock is determined from the total pattern of associations.
The features include topography, stream patterns, erosion details, colour and tone, veg-
etation, man-made features, natural and man-made foundations and micro details in topogra-
phy such as sink holes, rock outcrops and accumulations of boulders. Each of these features is
used to associate with a particular type of rock or soil stratum. For example, vegetation differ-
ences frequently reflect both drainage and soil character.
Land form represents the total effect of environment and geological history on the un-
derlying rock and soil foundations. The study of evolutionary processes that produce a given
land form is termed ‘geomorphology’. Once the land form is identified the geological associa-
tions are defined.
Air photo interpretation requires a thorough grounding in geology, geomorphology, ag-
riculture and hydrology. The technique, though highly specialised, is a valuable preview and
supplement to site reconnaissance.
18 .3 Soil Exploration
The subsoil exploration should enable the engineer to draw the soil profile indicating the se-
quence of the strata and the properties of the soils involved.
In general, the methods available for soil exploration may be classified as follows:- Direct methods ... Test pits, trial pits or trenches
- Semi-direct methods ... Borings
- Indirect methods ... Soundings or penetration tests and geophysical methods
In an exploratory programme, one or more of these methods may be used to yield the
desired information.