Shake and sink: the physics of granular liquefaction
Earthquakes often trigger soil liquefaction: Usually saturated soils behave like elastic solids, supporting buildings and structures. But shaking induced by an earthquake may cause soils to undergo a rheological transition whereby they start flowing like fluids. Earthquake-triggered liquefaction causes sinking and tilting of buildings, floatation of buried structures (e.g. gas pipes), and ground settlement. This is one of the largest hazards from earthquakes: For example, liquefaction triggered during the 1995 Kobe earthquake in Japan caused 5000 deaths and $ 200 billion in damage.
The classical mechanism for explaining earthquake-triggered liquefaction invokes a poorly drained, loosely packed, saturated soil (i.e. granular media) undergoing cyclic shear. The cyclic-shearing causes pore collapse. The pressure of fluid trapped in the pores (pore pressure) then rises till the fluid fully carries the weight of the grains, permitting complete loss of shear resistance. This view is what guides current engineering practices and building codes.
Field observations from around the world show however that this only part of the story: in contrast to this widely held view, liquefied soils may be fully drained, and initially densely packed. We develop a physics based theory for treating granular and fluid deformation, and use it simulate liquefaction in a coupled multi scale discrete element + fluid code. Our theory predicts the conditions for pore collapse and explores the conditions when subsurface fluid flow can cause liquefaction. It also explains why buildings sink during liquefaction, pipes float, how remote earthquakes can trigger liquefaction, and why liquefaction can occur some time after the earthquake has passed. These results may greatly impact hazard assessment and mitigation in seismically active areas.