Exploratory Bore above the Tunnel Alignment in Rail Tunnel

Is it advisable to do the exploratory drill holes in the tunnel alignment as it would puncture the tunnel and then we have to fill the hole by grouting material that always create zone of weakness. One of consultant suggested to drill the borehole in the centre line of alignment in rail tunnel. As per understanding sideways drilling is ok but puncturing the tunnel by drilling is not so good. Kindly please put some light with reference literature.

Geology Design Tunneling & Underground Construction

Asked 05/07/2024 00:42, updated: today, 04:31
Rahul Sharma

2 Answers

Votes: 1

Stephen Buttling

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.

Votes: 1


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.