Faults Folds and Joints Questions and Answers

Answer: C

Correct answer: the size of the blocks that may fall

Joint spacing, together with the number and orientation of the sets, fixes the dimensions of the blocks into which the rock mass is divided, and it is those blocks that fall out of a tunnel roof or slide from a wall. Support is designed around them: bolt length and spacing must be matched to the block size, and closely jointed ground needs mesh or shotcrete as well.

Answer: D

Correct answer: extent along its own plane

Persistence is how far a joint continues in its own plane - a metre or the whole height of a slope. It matters because a discontinuous joint requires intact rock bridges to be broken before a block can move, and those bridges may carry the whole stability of the slope, whereas a fully persistent joint offers a continuous failure surface. Spacing, aperture and roughness are the other joint properties, each described separately.

Answer: A

A plane daylighting in the face with a dip shallower than the slope provides a ready surface for a block to slide on. The same plane dipping into the face is harmless, which is why orientation matters more than strength.

Answer: B

Correct answer: crushed and often water-bearing material

A fault zone is a band of broken rock, breccia and gouge, of low strength and often high permeability, so the tunnel meets ground that will not stand unsupported and may deliver a sudden inflow of water. Where gouge has sealed one side of the zone, high pressure water may be held behind it and released without warning when the face breaks through - which is why fault zones are located in advance by probe drilling.

Answer: C

Correct answer: provide a leakage path and a plane of weakness

A fault crossing a dam abutment threatens the works twice over. Its crushed and often permeable material gives water a route around or beneath the structure, and the reservoir head drives flow along it; and it is a surface of low shear strength on which the abutment or the foundation may slide. Both are addressed before construction, by excavating and backfilling the zone and by grouting, and both have caused dam failures.

Answer: D

Correct answer: reduce permeability along the joints

Grout injected under pressure fills the open joints and reduces the permeability of the rock mass, which is what a grout curtain beneath a dam is for. It also stiffens the mass somewhat by knitting the blocks together, but the primary purpose is watertightness. It does not alter the intact strength of the rock material, which is a property of the mineral fabric, and it does not remove the joints.

Answer: A

Existing fractures give the shock wave surfaces to reflect from and give the gases paths to open up, so less explosive is needed per cubic metre. The same joints make the excavation less stable once it has been formed.

Answer: B

Correct answer: 25 degrees

Hade is measured from the vertical and dip from the horizontal, so the two are complementary:

The steep fault has a small hade. Answering 65 degrees confuses the two conventions, which is exactly what the question tests.

Answer: C

Correct answer: 25 m

The throw is the vertical component of the slip:

A shallow-dipping fault therefore gives a small throw for a large slip, which is why a thrust may carry rocks many kilometres horizontally while displacing them only modestly in the vertical.

Answer: D

Correct answer: 43 m

The heave is the horizontal component of the same slip:

Throw and heave are the two components of the net slip, and \( 25^2 + 43^2 \approx 50^2 \) as they must be. At 30 degrees the horizontal displacement is the larger of the two.