Geology of Tunnels Questions and Answers

Answer: A

Crossing the strike means each troublesome bed is met over a short length rather than followed for kilometres, and the roof is not held up by a single bedding plane. Driving along the strike keeps the tunnel in one formation, for good or ill.

Answer: B

Beds dipping into one wall support it while those dipping out of the other let slabs slide into the excavation, so the two sides behave quite differently. Support must then be designed asymmetrically for the same tunnel.

Answer: C

Correct answer: slabs delaminating and dropping from the crown

A horizontally bedded roof behaves as a stack of beams spanning the opening. Each bed sags under its own weight, separates from the one above, and eventually drops as a slab. The failure is progressive rather than sudden, and it is why bedded roofs are bolted early - the bolts tie the beds together so they act as one thick beam instead of several thin ones.

Answer: D

Three intersecting sets define blocks whose dimensions follow from their spacings, and it is those blocks rather than the intact rock that fall. Support is chosen for the block size the joint pattern actually produces.

Answer: A

Correct answer: excavation beyond the intended profile

Overbreak is rock removed outside the designed line, whether by blast damage or by blocks falling from the crown. It costs money twice over: the material must be mucked out and the resulting void filled with lining concrete. Controlled perimeter blasting exists chiefly to limit it.

Answer: B

Weak rock at depth cannot sustain the stress concentration and creeps into the opening over hours or days. Yielding support that allows controlled deformation is preferred to rigid support that would simply be crushed.

Answer: C

Correct answer: sudden and violent rather than slow

Both are stress problems but they differ in the manner of failure. Squeezing is a slow plastic inward movement of weak rock, developing over hours or days and giving warning through convergence measurements. A rock burst is the sudden brittle failure of strong rock, throwing fragments violently with no warning, and it is far more dangerous for that reason.

Answer: D

Correct answer: strong, brittle and under high stress

A burst requires rock able to store elastic strain energy and to release it suddenly, which means strong and brittle material carrying a stress close to its strength. Weak or heavily jointed rock yields gradually instead and cannot store the energy. Bursts are therefore a deep hard-rock problem, not a weak-ground one.

Answer: A

Correct answer: montmorillonite or anhydrite

Both take up water and expand. Montmorillonite is the swelling clay mineral of smectite clays and shales, and anhydrite hydrates to gypsum with a large volume increase. Either can generate pressures capable of destroying a rigid lining, so swelling ground is dealt with either by a yielding support or by keeping water away from it.

Answer: B

Vertical stress grows roughly with the weight of the cover, so at some depth it exceeds what the rock can carry around an opening. The ratio of rock strength to in situ stress is the quantity that predicts the behaviour.