Theory of settlement
If the stress change in the soil or in the currently build earth structure, caused by ground surface surcharge, is known, it is possible to determine the soil deformation. The soil deformation is generally inclined and its vertical component is termed the settlement. In general, the settlement is non-stationary dependent on time, which means that it does not occur immediately after introducing the surcharge, but it rather depends on consolidation characteristics of a soil. Permeable, less compressible soils (sand, gravel) deform fast, while saturated, low permeability clayey soils experience gradual deformation called consolidation.
Time dependent settlement of soils
Applied load yields settlement, which can be subdivided based on time dependent response into three separate components:
- Instantaneous settlement (initial)
- Primary settlement (consolidation)
- Secondary settlement (creep)
During instantaneous settlement the soil experiences only shear deformation resulting into change in shape without volumetric deformation. The loss of pore pressure in the soil is zero.
This stage of soil deformation is characterized by skeleton deformation due to motion and compression of grains manifested by volume changes. If the pores are filled with water (particularly in case of low permeability soils), the water will be carried away from squeezed pores into locations with lower pressure (the soil will undergo consolidation). The consolidation primary settlement is therefore time dependent and is terminated by reaching zero pore pressure.
When the primary consolidation is over the skeleton deformation will no longer cause the change in pore pressure (theoretically at infinite time). With increasing pressure the grains may become so closely packed that they will start to deform themselves and the volumetric changes will continue - this is referred to as creep deformation of skeleton or secondary consolidation (settlement). Unlike the primary consolidation the secondary consolidation proceeds under constant effective stress. Particularly in case of soft plastic or squash soils the secondary consolidation should not be neglected - in case of overconsolidated soils it may represent app. 10% of the overall settlement, for normally consolidated soils even app. 20%.