Settlement of a foundation

Probably the most difficult of the problems that a soils engineer is asked to solve is the accurate prediction of the settlement of a loaded foundation.

The problem is in two distinct parts: (i) the value of the total settlement that will occur, and (ii) the rate at which this value will be achieved.

When a soil is subjected to an increase in compressive stress due to a foundation load the resulting soil compression consists of elastic compression, primary compression and secondary compression.

Elastic compression

This compression is usually taken as occurring immediately after the application of the foundation load. Its vertical component causes a vertical movement of the foundation (immediate settlement) that in the case of a partially saturated soil is mainly due to the expulsion of gases and to the elastic bending reorientation of the soil particles. With saturated soils immediate settlement effects are assumed to be the result of vertical soil compression before there is any change in volume.

Primary compression

The sudden application of a foundation load, besides causing elastic compression, creates a state of excess hydrostatic pressure in saturated soil. These excess pore water pressure values can only be dissipated by the gradual expulsion of water through the voids of the soil which results in a volume change that is time dependent. A soil experiencing such a volume change is said to be consolidating and the vertical component of the change is called the consolidation settlement.

Secondary compression

Volume changes that are more or less independent of the excess pore water pressure values cause secondary compression. The nature of these changes is not fully understood but it is apparently due to a form of plastic flow resulting in a displacement of the soil particles. Secondary compression effects can continue over long periods of time and, in the consolidation test, become apparent towards the end of the primary compression stage: due to the thinness of the sample, the excess pore water pressures are soon dissipated and it may appear that the main part of secondary compression occurs after primary compression is completed. This effect is absent in the case of an in situ clay layer because the large dimensions involved mean that a considerable time is required before the excess pore pressures drain away. During this time the effects of secondary compression are also taking place so that, when primary compression is complete, little, if any, secondary effect is noticeable. The terms 'primary' and 'secondary' are therefore seen to be rather arbitrary divisions of the single, continuous consolidation process. The time relationships of these two factors will be entirely different if they are obtained from two test samples of different thicknesses.

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