Land subsidence and soil ripening

Land subsidence is due to various causes. In reclaiming sediments, deposited by the sea, by rivers or in lakes, having a high porosity mainly, the two main causes for subsidence are the increase of the intergranular pressures, resulting from the lowering of the groundwater table, and the physical soil ripening in the topmost 1 to 1.5 m of the soil. A prediction of the subsidence and, hence, the future surface level of a polder as accurate as possible is very important. The effect of the increasing intergranular pressures is calculated by a soil mechanics equation, which is discussed. Remarks are made on the relation between the applied consolidation constants and the porosity. The physical ripening process is dealt with, as is the resulting contraction of the soil, giving rise to subsidence and crack formation, and the resulting loss of soil moisture. The method of calculating the subsidence, either by a simulation model or by comparison of the density of soils, is explained. Examples of such calculations are given. Remarks are made on the subsidence of peat soils.