Which one of the following situations is NOT at all suitable for tunneling ?
- AIf the beds are parallel to horizontal or have approximately zero degree inclination
- BIf the beds are vertically dipping and the axis of the tunnel is perpendicular to the strike
- CIf the strike of involved beds is parallel to the axis of the tunnelCorrect
- DIf the dip of involved beds is parallel to the axis of the tunnel
Solution
Concept
Orientation of rock beds relative to the tunnel axis
Step-by-step solution
1. Beds that are horizontal, or vertical with the tunnel at right angles to the strike, give favourable conditions. 2. When the strike is parallel to the tunnel axis, the roof rests on a bedding plane and the sides on the dipping planes, which gives unstable conditions. 3. When the dip is parallel to the axis (strike perpendicular to the axis), the conditions are comparatively good.
Explanation
Tunnelling is most difficult where the strike runs along the tunnel, because bedding planes parallel to the axis allow slabs to fall from the roof and the sides.
Why the other options are wrong
(a), (b) and (d) are acceptable orientations.
Shortcut
Tunnel axis ⟂ strike = good; ∥ strike = bad.
Common mistake
Choosing the dip-parallel case.
Correct answer: C) If the strike of involved beds is parallel to the axis of the tunnel