The method of placing tubes or tracks is called cut and cover method and not tunnelling in which, top cover remains undisturbed and intact during excavation.

Horizontal strata are rare in occurrence for longer tunnels. If y are countered for small tunnels or short lengths of long tunnels, horizontally layered rocks might be considered quite favourable.

During twentieth century most of tunnelling has been in connection with hydropower generation. Such tunnels are aptly called ?hydropower? tunnels.

Information regarding mineralogical composition, textures and structures of rocks through which proposed tunnel is to pass is of great importance in deciding: method of tunnelling, strength and extent of lining and, thus cost of project.

Choice of right method will, refore, be possible only when nature of rocks and ground all along alignment is fully known. This is one of most important aim and objects of geological investigations.

Folded rocks are often best storehouses for artesian water and also ideal as aquifers. When encountered during tunnelling unexpectedly, se could create uncontrollable situations.

The or type of hydropower tunnels are those which feed water under great pressure to turbines and are distinguished as pressure tunnels.

Public utility tunnels include a variety of underground excavations made for specific purposes such as for disposal of urban waste (sewage tunnels), for carrying pipes, cables and supplies of oil, water etc.

When tunnel axis may be running parallel to dip direction, layers offer uniformly distributed load on excavation. The arch action is said to happen, where rocks at roof act as natural arches transferring load onto sides.

The traffic tunnels include all tunnels which are excavated to divert traffic load of whatsoever type from surface to subsurface routes for a short length with a view of facilitating flow of traffic at a desired speed, maximum convenience and at minimum cost.