Recent alluvial deposits are more susceptible to liquefaction than older ones because they
Answer: C
Ageing, cementation at particle contacts and past seismic shaking all increase resistance, and a young Holocene deposit has had none of them. Geological age is one of the screening criteria used before any testing is done.
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Plastic fines in a sandy soil
Answer: D
Cohesion between plastic particles resists the collapse of the skeleton, so plasticity index and liquid limit both appear in the criteria used to screen fine-grained soils. Non-plastic silt, by contrast, can be as susceptible as fine sand.
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Sand boils observed after an earthquake are evidence of
Answer: A
Water under excess pressure escapes upwards through cracks, carrying sand with it and depositing it in small cones at the surface. Mapping the boils after an event is one of the ways liquefaction susceptibility maps are compiled.
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Lateral spreading is
Answer: B
A surface crust riding on a liquefied layer can move metres towards a free face or down a very gentle slope, pulling apart everything founded on it. Pipelines, bridge abutments and quay walls are the structures most often damaged by it.
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A buried empty tank in liquefied ground tends to
Answer: C
The liquefied soil behaves as a heavy fluid, so anything lighter than it rises, which is why manholes and empty tanks are found protruding after an earthquake. Anchoring such structures against uplift is part of designing for a liquefiable site.
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A building on a shallow foundation over a liquefied layer typically suffers
Answer: D
With the effective stress gone, the soil cannot carry the bearing pressure, and the building settles and rotates while remaining structurally intact. The apartment blocks that toppled intact at Niigata in 1964 are the classic illustration.
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The 1964 Niigata earthquake is remembered in soil mechanics chiefly for
Answer: A
Correct answer: buildings that tilted intact on liquefied ground
The Kawagishi-cho apartment blocks at Niigata rotated bodily, some almost to the horizontal, while remaining structurally undamaged - the clearest possible demonstration that the ground rather than the structure had failed. Together with Alaska in the same year, it is where the modern study of liquefaction begins.
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The 1934 Bihar-Nepal earthquake produced widespread liquefaction over the
Answer: B
Thick young alluvium with a high water table is exactly the setting liquefaction requires, and the belt of sand and water ejection across the plain was described at the time. The same deposits underlie much of the region's population today.
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Ground settlement after liquefaction occurs because
Answer: C
Correct answer: the soil reconsolidates into a denser state as the pressure dissipates
During liquefaction the grains are momentarily suspended and lose contact; as the excess pressure drains away they settle into a closer packing than before. The volume of water expelled is the volume of settlement, which is why sand boils at the surface and subsidence go together.
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The factor of safety against liquefaction is defined as
Answer: D
Correct answer: the cyclic resistance ratio divided by the cyclic stress ratio
\[ FS = \frac{CRR}{CSR}. \] It is capacity over demand, as every factor of safety is. Inverting it - which is the first distractor - turns a safe layer into an unsafe one and is the commonest slip in the calculation.
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