Faults Folds and Joints Questions and Answers

Answer: A

Correct answer: 55 per cent

Only pieces of 100 mm or more count, so the 80 mm piece is excluded:

Including the 80 mm piece would give 60 per cent, which is the trap. Note also that total core recovery here is 900 of 1500 mm, a separate figure from the RQD.

Answer: B

Correct answer: very poor

The standard classification runs: 0 to 25 per cent very poor, 25 to 50 poor, 50 to 75 fair, 75 to 90 good, and 90 to 100 excellent. Twenty per cent therefore falls in the worst class, indicating a heavily fractured rock mass that will need substantial support in a tunnel and cannot be relied on to stand in a steep face.

Answer: C

Correct answer: 0.027 m\( ^3 \)

Three sets at similar spacing intersecting one another cut the mass into roughly cubical blocks of side equal to the spacing:

which is about 70 kilogrammes of rock - small enough to be handled by bolting and mesh, but large enough to kill if it falls from a tunnel roof. The 0.9 answer multiplies rather than cubes, and 0.3 uses the spacing itself.

Answer: D

Correct answer: a plane on which a block may slide

Planar sliding requires the discontinuity to dip out of the face - to daylight - and to dip less steeply than the face but more steeply than its friction angle. Here the bedding dips at 25 degrees out of a face cut at 50, so it daylights and the condition is met. Toppling would require steep discontinuities dipping into the face, and circular failure occurs in soil or heavily broken rock with no dominant plane.

Answer: A

Correct answer: bed B

Bed B has been cut out where the hole crosses the fault, so the displacement must be at least enough to carry the whole of B past the borehole - that is, at least its thickness. It is a minimum rather than an exact value, since a larger throw would omit part of A or C as well, and the hole gives no way of telling how much of the omission is accounted for. Omission itself indicates a normal fault.

Answer: B

Correct answer: 1-Q, 2-P, 3-R

Each fault type records a different stress regime. A normal fault drops the hanging wall and lengthens the section, so it records tension. A thrust pushes the hanging wall up and over, shortening the section, so it records compression. A strike-slip fault moves the blocks past each other horizontally without shortening or lengthening, so it records shear. Reading the stress history from the structures is the whole point of the classification.

Answer: C

Correct answer: 1-Q, 2-P, 3-R

The three features record three quite different processes. Slickensides are polished striations produced by blocks grinding past each other, so they mean fault movement. Columnar jointing is a primary igneous feature produced by contraction as a lava or sill cools. Sheet jointing forms as erosion removes the overburden and the rock expands upward, so it records unloading rather than any tectonic event.

Answer: D

Correct answer: 2, 4, 3, 1

The sequence traces the rotation of the axial plane as compression increases. A symmetrical fold has a vertical axial plane and equal limbs. Continued compression tilts it, giving an asymmetrical fold with unequal limb dips. Further compression carries one limb past the vertical so that it is inverted - an overturned fold. Finally the axial plane becomes horizontal and the fold is recumbent, with one whole limb upside down.

Answer: A

Correct answer: 1 and 3 only are correct

Statements 1 and 3 are correct: a joint is by definition a fracture without appreciable displacement, and repetition of a bed in a vertical hole indicates that the section has been stacked by a reverse or thrust fault. Statement 2 reverses the convention - hade is measured from the vertical, and dip from the horizontal, the two being complementary.

Answer: B

Correct answer: columnar jointing in the rock mass

Columnar jointing is produced by contraction as an igneous body cools, and involves no displacement at all, so it is evidence of a cooling history rather than of faulting. The other three are all diagnostic of a fault: repetition of a stratum shows the section has been stacked, slickensides record blocks grinding past each other, and breccia and gouge are the crushed products of that movement.