ABSTRACT

As explained in Chapter 3, consolidation is the gradual reduction in volume of a fully saturated soil of low permeability due to change of effective stress. This may be as a result of drainage of some of the pore water, the process continuing until the excess pore water pressure set up by an increase in total stress has completely dissipated; consolidation may also occur due to a reduction in pore water pressure, e.g. from groundwater pumping or well abstraction (see Example 4.5). The simplest case is that of onedimensional consolidation, in which the stress increment is applied in one direction only (usually vertical) with a condition of zero lateral strain being implicit. The process of swelling, the reverse of consolidation, is the gradual increase in volume of a soil under negative excess pore water pressure. Consolidation settlement is the vertical displacement of the soil surface corresponding to the volume change at any stage of the consolidation process. Consolidation settlement will result, for example, if a structure (imposing additional total stress) is built over a layer of saturated clay, or if the water table is lowered permanently in a stratum overlying a clay layer. On the other hand, if an excavation (reduction in total stress) is made in a saturated clay, heave (upward displacement) will result in the bottom of the excavation due to swelling of the clay. In cases in which significant lateral strain takes place there will be an immediate settlement due to deformation of the soil under undrained conditions, in addition to consolidation settlement. The determination of immediate settlement will be discussed further in Chapter 8. This chapter is concerned with the prediction of both the magnitude and the rate of consolidation settlement

under one-dimensional conditions (i.e. where the soil deforms only in the vertical direction). This is extended to the case when the soil can strain laterally (such as beneath a foundation) in Section 8.7. The progress of consolidation in-situ can be monitored by installing piezometers to record the change in pore water pressure with time (these are described in Chapter 6). The magnitude of settlement can be measured by recording the levels of suitable reference points on a structure or in the ground: precise levelling is essential, working from a benchmark which is not subject to even the slightest settlement. Every opportunity should be taken of obtaining settlement data in the field, as it is only through such measurements that the adequacy of theoretical methods can be assessed.