A rail tunnel is probably about 6 m in diameter. A borehole is probably less than 100 mm in diameter. For a 1 m advance in the tunnel the borehole represents less than 0.03% of the volume. Having said that, it does depend to some extent on the nature of the material. Is it sound rock, sheared rock, weathered rock, hard soil, firm soil, soft soil? Also the holes should always be properly backfilled and grouted. This is often overlooked as no one is watching. Horizontal drilling gives a much higher proportion of useful information, but to cover a significant amount of advance requires a long hole which is difficult and expensive. Drilling along the alignment is easier, but gives relatively little information about the tunnel face, and for deep tunnels includes a lot of "dead" drilling.

It is always advisable to perform number of drill holes along the tunnel alignment (if it is feasible /accessible) prior to excavation. The location of proposed drill holes must be physically checked by geologists/engineers involved in the field. Preference should be given in stream / nala sections. The findings from the drill hole shall assist you better during actual excavation works w.r.t planning and precautionary measures, excavation methodology etc.

Thanks