Laboratoire Matière et Systèmes Complexes

Renaud Toussaint, DR CNRS

Institut de Physique du Globe de Strasbourg, CNRS / Université de Strasbourg

Soil liquefaction and fractures triggered by fluid flow : simulations and experiments.

In this presentation, two examples of studies performed at IPGS will be given, dealing with mixtures of fluids and granular materials in contexts related to natural hazards. During some earthquakes close to coastal regions, some soils can be subject to liquefaction, during which their resistance to shear collapses, and overlying structures can sink partly or totally in the underground. This causes regularly important damage, and is seen in particular in saturated and low-density soils, often composed of silts or sands. The classical criteria used to determine whether a soil is prone to liquefaction is based on tests, and often refers to a critical density of seismic waves. However, some far-field liquefaction events do not follow this criterion. Using laboratory experiments and discrete element simulations, we show that localized buoyancy effects can be responsible for a simple liquefaction mechanism that does not require the effective stress to vanish. With tests at various frequencies and amplitude of shaking, we show that a liquefaction not involving pore pressurization occurs at a critical peak ground amplitude. We also show that this criterion allows to explain most of the far field earthquakes. Next, we present results related to the modes of deformation and induced seismicity during pneumatic fracture. Using simulations and experiments in Hele-Shaw cells equipped with fast cameras and broadband accelerometers, we characterize the phase diagram of flow modes, the rheology of the gas/granular mixture, and the statistics of the associated seismicity.

Legend : Fracture by fluid injection in a granular medium and associated microseismicity – experiment

Legend : liquefaction experiment