Geology of Tunnels Questions and Answers

Answer: C

Correct answer: a fault zone or a karst conduit

Both are concentrated pathways for groundwater: a fault zone is crushed and permeable along its length, and a karst conduit is an open dissolution channel that may carry the drainage of a wide area. Unfractured massive rock holds little water and yields it slowly, so the danger is the discontinuity rather than the rock.

Answer: D

Correct answer: pre-grouting ahead of the face

Grout injected through holes drilled ahead of the face seals the fissures before they are opened, so the water is excluded rather than pumped. This is the standard treatment where inflow is expected, and it is far safer than driving into the zone and dealing with what comes. Advancing faster into a water-bearing zone invites exactly the inrush it is meant to avoid.

Answer: A

Correct answer: act as a drain and lower the water table

A tunnel below the water table in permeable rock is an enormous drain at atmospheric pressure, so water flows into it and the table falls around it, sometimes over a wide area. The consequences reach beyond the works: springs dry up, wells fail, and settlement may follow where the drawdown consolidates compressible ground.

Answer: B

Correct answer: crushed, weak and often water-bearing material

A fault zone combines the two worst conditions at once. The rock has been crushed and sheared, so it is weak and has little stand-up time, and that same crushing makes it permeable, so it often carries water under pressure. Meeting one unexpectedly gives no opportunity to pre-grout or to prepare heavy support.

Answer: C

Correct answer: support the ground ahead of the face before excavating it

Forepoles are bars or tubes driven forward over the crown at a shallow angle, forming an umbrella that carries the ground ahead while the next round is excavated beneath them. They are used where the stand-up time is too short to open the face safely, and they are the standard treatment for running ground.

Answer: D

Correct answer: both hard rock and soft ground together

A mixed face means the excavation is partly in rock and partly in soil, which is the worst condition for any machine: the cutting tools suited to one are wrong for the other, the face is unstable where it is soft while the rock resists, and steering is difficult because the machine tends to deviate towards the softer side.

Answer: A

Correct answer: stall or damage the excavation machinery

A soft-ground machine has cutting tools designed for soil, and a boulder neither cuts nor passes through the openings. It jams the cutterhead, damages the tools, and often has to be broken up by hand from within the chamber - a slow and hazardous operation. The boulders neither weaken the ground nor make it more permeable.

Answer: B

Correct answer: the degree of jointing of the rock mass

Rock quality designation is computed from a core run as the percentage made up of intact pieces longer than 100 mm, so it measures how closely the mass is broken by discontinuities rather than how strong the rock itself is. That is why it appears in classification systems alongside a separate term for intact strength.

Answer: C

Strength, jointing, joint condition, water and stress are scored and combined into a single index, which is then read against empirical support recommendations. Both rest on records of what has worked in comparable ground.

Answer: D

Correct answer: remain stable after being opened

Stand-up time is the interval between exposing the ground and its beginning to fail, and it determines how far the excavation may advance before support must be installed. It falls sharply as the rock mass quality falls and as the span increases, and in very poor ground it may be effectively zero, which is when forepoling is needed.