The Combined Effect of Buoyancy and Excess Pore Pressure in Facilitating Soil Liquefaction

Soil liquefaction is a devastating earthquake hazard, commonly causing tilting, sinking and floating of infrastructure. The classical mechanism for liquefaction requires undrained and loosely packed soil, that upon shear experiences elevated, lithostatic, pore pressure and consequently zero effective stress. However, some field and experimental observations cannot be explained by this mechanism. These include liquefaction of pre-compacted soils, liquefaction under drained conditions, repeated liquefaction events, and liquefaction triggered by small seismic energy density. A recent study suggests a new mechanism for soil liquefaction that arises only from buoyancy effects of fluids plus grain accelerations, where the term "liquefaction", used as its phenomenological field definition, refers to a macroscopic transition from rigid to fluid-like behavior. We extend that study and seek a unifying mechanism for field observed liquefaction that accounts both for the buoyancy effect and for elevated pore pressure, though not necessarily with lithostatic values. To achieve this goal, we use a coupled fluid flow and granular dynamics numerical model to study the effect of pore pressure on the sinking of a large object ("intruder") into a drained densely packed granular system, undergoing cyclic shearing. Results show that despite the drained conditions pore pressure rises during shaking. Although pore pressure remains well below lithostatic values, the soil liquefies, as identified macroscopically by intruder sinking to its isostatic position. Even simulations with buoyancy effects alone show liquefaction and intruder sinking under certain conditions, yet inclusion of pore-pressure effects add to the buoyancy effect, and is seen to enhance liquefaction and promote intruder sinking.