Tunnels is a nearly horizontal underground passage which is open at both ends to the ground surface. Tunnelsare constructed below cities, rivers and through mountains for carrying railways, rocks, canals water supply and sewerage.
In majority of cases, the location and alignment of tunnels and size of the bore are established prior to the geological survey. Before starting the geological investigation, photo-geologic interpretation should be done along the tunnels alignments. This will provide information in the topography, nature of surface material, broad geologic structure, water condition and vegetation.
The geological investigation is done to disclose the nature and structure of the surface material and sub surface rocks. The geological defects such as faults, shear zones, joints and water bearing horizons, it present along the proposed tunnels lies are carefully outlined. In the job the help of drilling and geophysical survey are also taken. This makes it possible to demarcate and precisely the zones before hand is of great importance, as it helps in making preparation at a right time to avoid the hazards. This not only increases the safety of workers but also makes the construction economical.
A study of the hydrological condition is another important aspect of the geological survey. This involves in finding the depth of water table, direction and velocity of movements of groundwater, and the seasonal changes of a water table. A tunnels which is located above the water table will be safe from groundwater invasion and seasonal flooding.
The outline of the geological survey for construction a tunnels are as follows.
A detailed geological map is prepared showing various rock types present in the area. Theirlithological characters, texture and mechanical properties are determined. The geological structure such as dip, folds, faults, joints, shear zones, etc are studied and marked in the geological map.
The surface water seepage  if any and depth of water table at various places along the tunnelsalignment are also shown on the map.
The results of the geological survey may be confirmed by drilling and geophysical survey. In geophysical survey, mainly the electrical resistively survey is carried out to provide information on the extent of faulting and fracturing, depth of bed rock under soil cover and location of a particular rock formation.
The above said geological survey provides for tunnels are enough data to prepare geological section along the tunnels alignment. Such section show clearly the various lithological units, their structure, depth of buried rock surfaces at various places, position and depth of water bearing, horizons, faults and fracture zones. This helps in choosing the proper excavation methods and in forecasting the troubles in tunnels. Thus the geological survey is of great help to the design and construction engineer.