The preliminary surveys include: Knowing general topography, lithology, hydrological conditions in area and structural condition of rocks. Driving pilot tunnels, bore-hole drilling are considered under detailed surveys.

In self-supporting and strong rocks, eir, D-shape or horse-shoe shape may be conveniently adopted but se shapes would be practically unsuitable in soft ground or even in weak rocks with unequal lateral pressure. In those cases, circular outline may be first choice.

In most cases hydropower tunnels are driven through rocks for purpose of conveying water under gravity from one point to anor. In such cases, y are called discharge tunnels.

The layers that dipping at angles up to 45° may be said as moderately inclined strata. The tunnel axis may be running parallel to dip direction, at right angles to dip direction or inclined to both dip and strike direction.

Subways and tube railways also fall in category of excavations but y are, in most cases, not tunnels in strict sense because y are excavations made in ground and n covered from top.

Tunnels may be defined as underground routes or passages driven through ground without disturbing overlying soil or rock cover. Tunnels are driven for a variety of purposes.

Rocks considered as hard and crystalline are granites, syenites, gabbros, basalts and all related igneous rocks, sandstones, limestones, dolomites, quartzites etc. However soft rocks include shales, friable and poorly compacted sandstones, chalk and porous varieties of limestones and dolomites, slates and phyllites with high degree of cleavage.

In many cases, problems created by jointing in rocks due to jointing can be grouting. In or cases, lining of tunnel in fractured zones might have to be applied.

Traffic tunnels may vary in length from a few meters to many kilometres and have been excavated in almost all major countries of world.

In most cases, soft rocks are not self-supporting. Hence, temporary and permanent lining becomes necessary that would involve extra cost and additional time.