Question

When can the rocks act as a natural arch?

(a) Tunnel axis parallel to dip direction

(b) Tunnel axis normal to dip direction

(c) Tunnel axis inclined to dip direction

(d) Tunnel axis parallel to strike direction

The question was posed to me during a job interview.

This interesting question is from Geological Investigations in section Tunnels of Engineering Geology

Answer 1

Right option is (a) Tunnel axis parallel to dip direction

For explanation I would say: When the tunnel axis may be running parallel to the dip direction, the layers offer uniformly distributed load on the excavation. The arch action is said to happen, where the rocks at roof act as natural arches transferring the load onto sides.