Components Contributing Settlement of Soil
Total settlement ρ in mm, which is the response of stress applied to the soil, may be calculated as the sum of three components
ρ = ρi + ρc + ρs
ρ = Total settlement, mm
ρi = Immediate or distortion settlement, mm
ρc = Primary consolidation settlement, mm
ρs = Secondary compression settlement, mm
Primary consolidation and secondary compression settlements are usually small if the effective stress in the foundation soil applied by the structure is less than the maximum effective past pressure of the soil.
1. Immediate Settlement
Immediate settlement ρi is the change in shape or distortion of the soil caused by the applied stress.
- Calculation of immediate settlement in cohesionless soil is complicated by a nonlinear stiffness that depends on the state of stress. Empirical and semi-empirical methods are used for calculating immediate settlement in cohesionless soils.
- Immediate settlement in cohesive soil may be estimated using elastic theory, particularly for saturated clays, clay shales, and most rocks.
2. Primary Consolidation Settlement
Primary consolidation settlement ρc occurs in cohesive or compressible soil during dissipation of excess pore fluid pressure, and it is controlled by the gradual expulsion of fluid from voids in the soil, leading to the associated compression of the soil skeleton.
Excess pore pressure is the pressure that exceeds the hydrostatic fluid pressure. The hydrostatic fluid pressure is the product of the unit weight of water and the difference in elevation between the given point and elevation of free water (phreatic surface). The pore fluid is normally water with some dissolved salts. The opposite of consolidation settlement (soil heave) may occur if the excess pore water pressure is initially negative and approaches zero following absorption and adsorption of available fluid.
- Primary consolidation settlement is normally insignificant in cohesionless soil and occurs rapidly because these soils have relatively large permeabilities.
- Primary consolidation takes substantial time in cohesive soils because they have relatively low permeabilities. Time for consolidation increases with thickness of the soil layer squared and is inversely related to the coefficient of permeability of the soil. Consolidation settlement determined from results of one-dimensional consolidation tests also includes some immediate settlement ρi.
3. Secondary Compression Settlement
Secondary compression settlement ρs is a form of soil creep which is largely controlled by the rate at which the skeleton of compressible soils, particularly clays, silts, and peats, can yield and compress. Secondary compression is often conveniently identified to follow primary consolidation when excess pore fluid pressure can no longer be measured; however, both processes may occur simultaneously.