Fluid Flow During Earthquake Soil Liquefaction

For the last 50 years or so Geotechnical Engineers and Geophysicists have been investigating the causes of the Earthquake Liquefaction of soil. When the ground shakes from an earthquake, some soils change their behavior from elastic solid to viscous flowing liquid. Ground deformations can become enormous as embankments flow into rivers carrying bridges with them or sidewalks settle 5 inches breaking the gas mains buried under the streets and causing huge fires.

The central assumption expressed in every elementary textbook on soil liquefaction is that the duration of the earthquake is so short that fluid flow through the pores of the soil (drainage) is irrelevant to the problem. The "undrained condition" is so strongly believed that some researchers have developed complex feedback mechanisms to inject water that compensates for the elastic stretching of the rubber membrane surrounding soil samples in tabletop testing apparati.

Through his PhD research on soil liquefaction, Daniel Lakeland began with this undrained condition in mind, and investigated the role of heating during ground motion. In the process, he discovered that heating is only a small effect, but that fluid flow is not only non-negligible, it is equally important to the grain deformation that squeezes the water. For medium loose sands the timescale of pressure drainage via fluid flow is on the order of 0.14 seconds, so typical earthquake loadings that last 10 seconds or more bring the soil into equilibrium conditions before the end of shaking. In these conditions, vertical settlement of the grains balances with fluid pressure diffusion and the variation in permeability throughout the deposit and a simple steady state solution of the fluid flow equation is predictive of the water pressure that controls the liquefaction process.

These results were published in 2013 in The Proceedings of The Royal Society A and opened the doors to new research focusing on the interplay between fluid flow, permeability variation, and grain skeleton contraction. As part of this research the role of the artificial rubber membrane in tabletop experiments was clarified, showing that tabletop experiments on liquefaction are completely dominated by the interaction between the rubber membrane and the soil, leading to results that have little relationship with the actual behavior of soils in the ground. Instead, future experiments should focus on the established alternative method of scale-model testing in geotechnical centrifuges where rescaling laws for fluid viscosity, gravitational acceleration, and time give accurate predictions of the full scale results.