Geological Investigations for Projects Questions and Answers

Answer: C

Correct answer: 15 to 20 m

The stress beneath a foundation spreads with depth and has fallen to a small fraction of the bearing pressure by about one and a half to two times the width, so a 10 m foundation must be investigated to fifteen or twenty metres. Boring to only a few metres because that is where the foundation sits is the classic way to miss a compressible layer beneath.

Answer: D

Correct answer: 12

A regular grid at 30 m centres over 90 m by 60 m has four positions along the longer side and three along the shorter, since the first is at zero: \[ \left(\frac{90}{30}+1\right) \times \left(\frac{60}{30}+1\right) = 4 \times 3 = 12. \] Forgetting to add one to each direction gives six, which is the trap in this arithmetic.

Answer: A

Correct answer: three times as permeable

At the same injection pressure the take is proportional to the permeability, so \[ \frac{12}{4} = 3. \] The comparison is valid only because the pressure is the same in both sections; at different pressures the flows must be normalised before any such statement can be made.

Answer: B

Correct answer: very loose

On the standard classification an N value of 0 to 4 in sand is very loose, 4 to 10 loose, 10 to 30 medium dense, 30 to 50 dense and above 50 very dense. A value of 4 therefore indicates a deposit liable to large settlement and, below the water table, potentially to liquefaction.

Answer: C

Correct answer: 10

\[ \frac{15}{1.5} = 10. \] The factor of ten between wet clay and dry sand shows how completely the ground governs radar performance. It is why the method is excellent for locating services beneath a dry made-up road and nearly useless on a saturated clay site.

Answer: D

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

Each method senses a different physical property. Seismic refraction times the arrival of waves refracted along an interface, so it gives depth to layers of contrasting velocity. Resistivity responds to pore water and clay, so it maps saturation and clay content. Gravity responds to mass, so it finds density contrasts such as cavities.

Answer: A

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

Three tests on three quantities. The packer test seals a length of borehole and measures the water it will take, giving permeability. The plate bearing test loads a plate and records the settlement, giving stiffness and bearing behaviour. And the standard penetration test counts the blows to drive a sampler, giving a resistance correlated with density.

Answer: B

Correct answer: 2, 4, 1, 3

The sequence moves from cheapest and broadest to most expensive and most specific, and does not end at construction. The desk study comes first because it costs least and directs everything after it; the walkover checks it against the site; detailed exploration answers the questions that remain; and review during construction tests the whole model against the ground as it is actually exposed.

Answer: C

Correct answer: 1 and 2 only are correct

Statements 1 and 2 are correct. Refraction depends on velocity increasing downwards, so a soft layer beneath a stiff one is a hidden layer the method cannot see. And geophysics measures a physical property rather than identifying material, so it interpolates between boreholes rather than replacing them. Statement 3 is the error of the core stone: the first hard layer may be a boulder, and the hole must prove it is not.

Answer: D

Correct answer: a standard penetration test in a borehole

The desk study is by definition work done before going on site, using material that already exists - maps, memoirs, aerial photographs, records of neighbouring works. A standard penetration test requires a rig, a borehole and a crew, which places it firmly in the exploration stage.