Geological Investigations for Projects Questions and Answers

Answer: C

The method assumes velocity increasing with depth, so a soft layer under a stiff crust is a hidden layer that the travel times cannot reveal. A weathered zone beneath a hard cap is the classic case and a dangerous one.

Answer: D

Correct answer: the water table and variations in saturation

Resistivity depends strongly on the amount and salinity of the pore water, so a saturated horizon contrasts sharply with a dry one above it. That makes the method well suited to locating the water table, tracing saline intrusion and mapping clay content. It says nothing about strength or structure.

Answer: A

Correct answer: rapid attenuation in clay and saturated ground

Radar depends on the ground being electrically resistive, and clay and saline water are conductive: they absorb the signal within a metre or two. In dry sand or clean gravel the same equipment reaches ten metres or more. Penetration is therefore governed by the material rather than by the instrument, which is why a trial line is worth running before the survey is planned.

Answer: B

Correct answer: density

A gravity survey measures very small variations in the earth’s gravitational field caused by density contrasts at depth. A cavity or a buried valley filled with soft deposits shows as a negative anomaly, a dense intrusion as a positive one. It is the method of choice for detecting voids in limestone, where nothing else works over large areas.

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.