Geology is the most important factor that determines the nature, form and cost of a tunnel. For example, the route, design and construction of a tunnel are largely dependent on geological considerations. Estimating the cost of tunnel construction, particularly in areas of geological complexity, is uncertain.

Prior to tunnel construction, the subsurface geology is explored by means of pits, adits (drifts), drilling and pilot tunnels. Exploration adits driven before tunnelling proper commences are not usually resorted to unless a particular section appears to be especially dangerous or a great deal of uncertainty exists. Core drilling aids the interpretation of geological features already identified at the surface.

A pilot tunnel is probably the best method of exploring tunnel locations and should be used if a major-sized tunnel is to be constructed in ground that is known to have critical geological conditions. It also drains the rock ahead of the main excavation. If the inflow of water is excessive, the rock can be grouted from the pilot tunnel before the main excavation reaches the water-bearing zone.

Reliable information relating to the ground conditions ahead of the advancing face obviously is desirable during tunnel construction. This can be achieved with a varying degree of success by drilling long horizontal holes between shafts, or by direct drilling from the tunnel face at regular intervals. In extremely poor ground conditions, tunnelling progresses behind an array of probe holes that fan outwards some 10 30 m ahead of the tunnel face.