ASSESSMENT OF ENVIRONMENTAL HAZARDS
The process of tunnelling, whether through rocks or through soft ground, and for whatsoever purpose, involves disturbing the environment of an area in more than one way.
The tunnelling methods might involve vibrations induced through blasting or ground cutting and drilling, producing abnormal quantities of dust and last but not the least, interference with water supply system of the nearby areas.
A correct appreciation of geological set up of the area, especially where tunnel alignment happens to be close to the populated zones, would enable the engineer for planning and implementing plans aimed at minimizing the environmental hazards in a successful manner.
These stages are:
A. Preliminary Surveys
Following geological characters are broadly established for the entire area in which the tunnel project is to be located as a result of preliminary surveys:
The general topography
The topography of the area marking the highest and the lowest points, occurrence of valleys, depressions, bare and covered slopes, slide areas, and in hilly regions and cold climates, the snow-line.
The litho logy
The litho logy of the area, meaning thereby, the composition, attitude and thickness of rock formations which constitute the area.
The hydrological conditions
The hydrological conditions in the area, such as depth of water table, possibility of occurrence of major and minor aquifers of simple type and of artesian type and the likely hydrostatic heads along different possible routes or alignments.
The structural condition
The structural condition of the rock, that is, extent and attitude of major structural features such as folding, faulting, unconformities, jointing and shearing planes, if developed.
Existence of buried valleys are also established during the preliminary surveys.
B. Detailed Surveys
bore-hole drilling along proposed alignments and up to desired depths; the number of bore-holes may run into dozens, scores or even hundreds, depending upon the length of the tunnel;
rock samples obtained from bore holes are analysed for their mechanical and geo-chemical properties in the laboratories;
Drilling shafts and adits, which allow direct approach to the desired tunnel for visual inspection in addition to the usual advantages of drilling;
Driving Pilot Tunnels
Driving pilot tunnels which are essentially exploratory in nature but could better be used as a main route if found suitable by subsequent enlargement.
The actual number of bore holes and shafts and adits and their depth and length are decided by the length and location of the proposed tunnel.
For tunnels with little overburden, these may be driven close to the proposed tunnel.