Liquefaction of Soils Questions and Answers

Answer: C

A large earthquake shakes for longer and delivers more cycles, so the resistance available per cycle must be adjusted from the reference magnitude of about seven and a half. The factor exceeds unity for smaller events.

Answer: D

Weighting each layer by how deficient its factor of safety is and by how thick and how shallow it is produces a single number for the site. That allows sites to be ranked and mapped, which a layer-by-layer factor of safety does not.

Answer: A

Both tests apply reversing shear to a saturated specimen and record the pore pressure and strain cycle by cycle. Sampling loose sand without disturbing it is the practical difficulty, which is why field methods dominate routine work.

Answer: B

Driving or pushing a tube into loose sand densifies it, and withdrawing the sample releases the confining stress, so the specimen tested is no longer the soil in the ground. Ground freezing is used for research work but is far too costly for routine investigation.

Answer: C

The total stress is \( 20 \times 5 = 100 \) kN/m² and the pore pressure \( 9.81 \times 5 = 49 \) kN/m², so the effective stress is 51 kN/m². Subtracting rather than adding the water pressure is the point of Terzaghi's relation.

Answer: D

Dividing 100 by 51 gives 1.96, and that ratio appears directly in the cyclic stress ratio expression. A high water table raises the ratio and therefore the demand, which is one reason a shallow water table is so unfavourable.

Answer: A

Correct answer: 0.30

\[ CSR = 0.65 \times \frac{a_{max}}{g} \times \frac{\sigma_v}{\sigma_v'} \times r_d = 0.65 \times 0.24 \times 1.96 \times 0.98 = 0.30. \] The 0.65 converts the peak stress into an equivalent uniform cyclic stress, on the reasoning that an earthquake delivers a number of cycles of varying size rather than repeated peaks.

Answer: B

Correct answer: 1.20

\[ FS = \frac{CRR}{CSR} = \frac{0.36}{0.30} = 1.20. \] Above one, so liquefaction is not predicted - though a margin of twenty per cent is modest given the scatter in the empirical curves, and many codes require 1.2 to 1.3 before a layer is treated as safe.

Answer: C

Correct answer: is expected to liquefy in the design earthquake

A factor of safety below one means the demand exceeds the resistance, so liquefaction is predicted and the consequences - settlement, loss of bearing, lateral spreading - must be assessed. The assessment does not need a laboratory test to reach that conclusion, and the water table is already in the calculation.

Answer: D

Removing 3 m of water pressure adds \( 9.81 \times 3 = 29 \) kN/m² to the effective stress at any depth below the new level. Higher effective stress means higher resistance, which is why dewatering is one of the mitigation measures.