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Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Wales SY23 3DB, UK
Geological sediments tend to strengthen during progressive burial but the interplay of porosity and permeability, strain and effective stress gives rise to numerous circumstances in which the strength increase can be temporarily reversed. The sediment becomes capable of bulk movement — sediment mobilization. Most explanations involve overpressuring, which results from additional loading being sustained by pore-fluid that is unable to dissipate adequately, leading to frictional strength reduction. The processes are highly heterogeneous, areally and with depth. The loads can be external (‘dynamic’) and both monotonic (e.g. a rapidly added suprajacent mass) and cyclic (e.g. the passage of waves), internal (e.g. the result of mineral reactions) and hydraulic (e.g. injection of external fluid). The sediments may become liquidized — that is, lose strength completely and behave as a fluid — through temporary fabric collapse (sensitive sediments) because loads are borne entirely by the pore-fluid (liquefaction), or by the grains becoming buoyant (fluidization), typically due to the ingress of externally derived fluids. In response to hydraulic gradients, buoyancy forces and reversed viscosity or density gradients, the weakened sediment may undergo bulk movement, though this requires failure of the enclosing material and sustained gradients. Mobilized but non-liquidized sediments retain some residual strength but can attain large shear displacements under critical state conditions.
