Sheet1When a saturated stratum of sandy soil is subjected to a stress increase, such as thatcaused by the erection of a building on the ground surface, the pore water pressure is increased.This increase in pore pressure leads to drainage of some water from the voids of the soil. Becauseof the relatively high permeability of the sandy soil this drainage process will occur quite quickly.In other words the pore pressure increase will dissipate rapidly. As a consequence of the drainageof some water from the soil, volume change will occur and settlement will take place.When a saturated stratum of clayey soil is subjected to a stress increase, the dissipationof the excess pore pressure generated will take place much more slowly because of the relativelylow permeability of the clayey soil. This means that the settlement, caused by the drainage ofsome water from the voids of the soil, will take place gradually over a long period of time.Fig. 10.1(a) represents a rigid but smooth walled container which is filled with saturatedsoil. The container is sealed by means of a membrane covering the upper surface of the soil. Auniform pressure of s is applied to the top of the soil. Since the soil is saturated and thecontainer is rigid no settlement of the soil will be observed. If the pore pressure change within thesoil was observed it would be found to equal the applied stress s. Since the applied (total) stressand the pore pressure both increase by equal amounts, there will be no change in effective stress.The absence of any observed settlement is therefore consistent with the principle of effectivestress, which requires that volume change will occur only as a result of an effective stress change.In Fig. 10.1(b) an opening has been provided in the membrane to enable water to beexpelled or drained from the container of soil. Under the effect of the increase in pore pressureu (=s), water will be expelled from the soil and this drainage of water will continue until thewater pressure decreases to the equilibrium value prevailing before the stress change of s wasapplied to the soil. This means that the pore pressure change finally will be zero. Since the totalstress has increased by s the effective stress will also increase by s. In response to thiseffective stress change, settlement of the soil will occur, the amount depending upon thecompressibility of the soil.These observations illustrate that in a one dimensional compression situation for a

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