Natural surface water and pore water in the subsoil is not an ideal, incompressible fluid. Small microscopic air (gas) bubbles are dispersed in the water, so the fluid shows a certain compressibility. Compressible pore water causes a delayed reaction of the pore water pressure on any pressure change at the boundaries if the subsoil has a permeability less than the velocity of the lowering of the surface water level Due to this phenomenon, bank stability is affected by the interaction of surface water and pore water. The effect of retarded pore water pressure release on slope stability is primarily dependent on the permeability of the soil, the compressibility of the pore water, i.e. the air (or gas) content in the fluid, and the velocity of the pressure change. The phenomenon is explained, and a simple approach to take into account this effect in stability calculations is proposed.


The failure of a river bank is often a natural process that, from the ecological point of view, may be desired, since it is part of the development of the natural environment which is changing continuously to a certain extent. All riparian landscapes have developed in such a way and still do. But since there have always been human settlements in river valleys, the failure of a river bank may endanger human life. So the prevention of bank failures will save lives. At the coast, namely with steep shores, a failure may be even more dramatic. During a storm surge some 10 metres of land may be destroyed and washed away into the sea. But erosion will take place at the toe of the failed soil body, thus again creating very steep slopes, and future storm surges will cause new sliding resulting in land loss.

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