Tunnels

TUNNELS

Definition

Ø Tunnels may be defined as underground routes or passages driven through the ground without disturbing the overlying soil or rock cover.

Ø     Tunnels are driven for a variety of purposes and are classified accordingly.

Ø         Chief  classes  of  tunnels  are:

Traffic Tunnels hydro-power tunnels and public utility tunnels.

Geological Investigations

These determine to a large extent solutions to following engineering problems connected with tunnelling:

Selection of Tunnel Route (Alignment).

There might be available many alternate alignments that could connect two points through a tunnel.

the final choice would be greatly dependent on the geological constitution along and around different alternatives

Selection of Excavation Method.

Tunnelling is a complicated process in any situation and involves huge costs which would multiply manifolds if proper planning is not exercised before starting the actual excavation.

And the excavation methods are intimately linked with the type of rocks to be excavated.

Choice of the right method will, therefore, be possible only when the nature of the rocks and the ground all along the alignment is fully known.

This is one of the most important aim and object of geological investigations.

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. Whether the tunnel is to be circular, D-Shaped, horse-shoe shaped or rectangular or combination of one or more of these outlines, is more often dictated by the geology of the alignment than by any other single factor.

Thus, in self-supporting and strong rocks, either, D-shape or horse-shoe shape may be conveniently adopted but these shapes would be practically unsuitable in soft ground or even in weak rocks with unequal lateral pressure.

Assessment of Cost and Stability.

These aspects of the tunnelling projects are also closely interlinked with the first three considerations.

Since geological investigations will determine the line of actual excavation, the method of excavation and the dimensions of excavation as also the supporting system (lining) of the excavation, all 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.