Geological Investigations for Projects Questions and Answers

Answer: C

They map a physical property continuously, but the property must be calibrated against something actually seen in a hole. Boreholes give certainty at points and geophysics gives continuity between them; neither alone suffices.

Answer: D

It exposes the fabric, the fissures and the lateral variation that a borehole core cannot show, and block samples can be cut from it. Its depth is limited by stability and by the water table, so a few metres is the practical limit.

Answer: A

Correct answer: the rock mass must be examined and tested in place

A borehole samples a line a few centimetres wide; an adit exposes the rock mass itself - its joints, shear zones and their infillings - and allows in situ tests at the level of the works. The cost is only justified where those questions govern the design, which in practice means large caverns, dam abutments and major tunnels.

Answer: B

Correct answer: recovers an intact core for examination

Percussion boring destroys the structure and returns disturbed chippings, from which nothing can be said about jointing or weathering. Rotary coring brings up a cylinder in which the discontinuities, their spacing, orientation and infilling are all visible and can be logged, and from which specimens for testing may be cut.

Answer: C

It states how much of the drilled interval was actually brought to the surface, so poor recovery marks weak or broken ground. Rock quality designation goes further by counting only the sound pieces of at least 100 millimetres.

Answer: D

Fractured or soft material washes away with the flush or falls from the barrel, so the very ground that matters most is the ground least likely to be recovered. Triple-tube barrels and careful drilling improve matters.

Answer: A

Correct answer: establish the true dip and strike of the discontinuities

A core recovered from a vertical hole records the angle a joint makes with the core axis but not the direction it faces, since the core is free to rotate as it comes up. Orienting it - by a scribe, an impression packer or an oriented core barrel - restores the missing information, which is what stability analysis requires.

Answer: B

Correct answer: driving a split spoon sampler a stated distance

The test drives a standard split spoon into the base of a borehole with a 63.5 kg hammer falling 760 mm, and the blow count for the second and third 150 mm increments is the N value. It is crude, and correlations from it are empirical, but it is cheap, universally understood and returns a disturbed sample at the same time.

Answer: C

Loading a rigid plate in stages and recording the settlement gives a direct measure of the ground's stiffness. Its weakness is that it stresses only a shallow zone, which may not represent the depth a real foundation influences.

Answer: D

Correct answer: expanding a probe against the walls of a borehole

A pressuremeter expands a cylindrical membrane against the borehole wall and records pressure against volume change, from which the in situ stiffness and a limit pressure are derived. Because it loads the ground where it lies, it avoids the disturbance that sampling causes - the reason it is preferred in ground that cannot be sampled undisturbed.