Liquefaction

Overview

Mechanism by which liquefaction occurs and sand boils and lateral spreading.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is liquefaction?

   Model answer

   Liquefaction is the process by which saturated, loosely packed sediment temporarily loses its strength during earthquake shaking and behaves as a liquid rather than a solid. Ground that had supported buildings for decades briefly becomes something closer to a heavy fluid, so structures sink, tilt or float, and the surface may flow sideways. When the shaking stops the sediment settles again, usually more tightly packed than before, and the ground surface subsides.

2. 2

   Explain the mechanism by which liquefaction occurs.

   Model answer

   In loose, saturated sediment the grains rest against one another with water filling the spaces between them, and the weight of anything above is carried by the grain-to-grain contacts. Shaking jostles the grains and they attempt to settle into a tighter arrangement, but the water in the pore spaces cannot escape quickly enough. The pressure of that trapped water therefore rises sharply, and as it rises it pushes the grains apart, so the contacts between them are lost. With no grain contacts left to carry the load, the sediment has no shear strength and behaves as a dense fluid until the excess water pressure drains away.

3. 3

   What ground conditions are needed for liquefaction, and where are they typically found?

   Model answer

   Three conditions must coincide: the sediment must be loose and unconsolidated rather than compacted or cemented; it must be of a suitable grain size, typically fine sand and silt, since gravel drains too freely to build up pressure and clay is too cohesive; and it must be saturated, with a water table close to the surface. These conditions are typical of river floodplains and deltas, coastal and estuarine flats, lake margins, and areas of reclaimed or artificially filled land, all of which are precisely the low-lying, flat sites on which cities and ports are commonly built.

4. 4

   What damage does liquefaction do to buildings?

   Model answer

   Buildings sink into the ground or tilt over, sometimes toppling almost intact because the structure itself is undamaged while the ground beneath it has given way. Foundations crack and settle unevenly, which tears the building apart even if it does not fall. Buildings on shallow foundations suffer most; those on piles driven down to firm ground below usually survive, though the ground around them may drop away, leaving them standing above the surface.

5. 5

   Why do buried tanks and pipes float upwards during liquefaction?

   Model answer

   Because a hollow, sealed structure such as a fuel tank, a sewage tank or a length of empty pipe is less dense than the liquefied sediment surrounding it. Once the ground behaves as a heavy fluid, buoyancy applies exactly as it would in water, and the object rises. Buried tanks are pushed out of the ground, manholes and pipes are forced up through road surfaces. The rupture of the pipes and tanks then causes leaks, contamination and loss of services.

6. 6

   What are sand boils and lateral spreading?

   Model answer

   Sand boils, sometimes called sand volcanoes, form where the high-pressure water in liquefied ground escapes upwards through a crack, carrying sand with it and depositing small cones of wet sand at the surface. They are the clearest field evidence that liquefaction has occurred. Lateral spreading occurs where liquefied ground lies on a gentle slope or beside an open face such as a river bank or a harbour edge: the liquefied layer flows sideways towards the free face, carrying the intact crust above it, which cracks apart and moves horizontally by metres, wrecking roads, bridge abutments, quays and everything built across the cracks.

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