Geological investigation

Geological investigations are very essential in selection of tunnel site.Following are the classifications-

1. Selection of tunnel alignment-

• Although many alternate alignment might be chosen the final choice would be greatly dependent on the geological conditions along and around different alternative: the alignment having least geologically negative factors would be considered to be the various terms.

2.Selection of excavation Method

• Tunneling is a complicated process in any situation and involves a huge cost. Moreover, the excavation methods are intimately linked with the type of rocks to be excavated. Choice of the right method will be possible only when the nature of the rocks and ground all along the alignment is fully known. This is one of the most important aim and object geological investigations.

3.Selection of design for the tunnel

• The ultimate dimensions and design parameters of a proposed tunnel are controlled. Besides other factors, by geological constitution of the area along the alignment.

• The shape of the tunnel (D shape, Horse shoe shape, Rectangular or combined of one or more of these outlines.) is more often suggested by the geology along the alignment than by any other single factor.

• Thus in self supporting and strong rocks, either horse shoe or D-shaped may be continently adopted but these steps would be practically unsuitable in soft ground or even in weak rocks with unequal lateral pressure. In these cases, Circular outline may be the first choice.

4. Assessment of cost and stability-

• These facts are closely interlinked with the first three considerations, since geological investigations will determine the line of actual excavations, the method of excavation and the dimensions of excavation as also the supporting system i.e. lining of the excavation , all the estimates about the cost of the project would depend on the geological details.

• Similarly, tunnels passing through hard and massive rocks, even when left unsupported, may be regarded as stable. However, those passing through difficult grounds might still collapse or bulge at places or even completely fail, if geological situation is not perceived properly.

5.Assessment of Environmental hazards-

• The processes of tunneling, whether through rocks or soft ground involves disturbing the environment of an area more than one way. The Tunneling methods might involve vibrations induced through blasting or ground cutting and drilling, producing abnormal quantities of dust and interference with water supply system of the nearby areas.