Pore water compressibility and soil behaviour - excavations, slopes and draining effects

Soils under water may be considered as an unsaturated porous medium containing gas, water and solids. Observations and laboratory measurements have shown that under natural conditions pore water contains microscopic air bubbles which change pore water compressibility considerably. These bubbles are embedded in the pore fluid of the soil skeleton and may play a key role in soil behaviour. Soil failure and delayed soil deformation may be explained by pressure changes applied on such unsaturated submerged soils, causing heaving or shrinking. By using a three-phase model, the mechanical behaviour of such soils may be described by an extended consolidation equation in order to calculate transient pore water pressures, induced by external pressure changes. Selected case studies in low permeable soils are presented. Effects of rapid external pressure reduction are discussed, such as excavation, pore pressure reduction to improve slope stability and fully soil embedded draining structures. Results of numerical simulations, laboratory and field investigations are presented with special regard to such structures acting as draining facilities. For the covering abstract see ITRD E128041.