Soil Settlement

Settlements refer to the soil’s movement in the vertical direction typically induced by stress changes. The total settlement of the ground consists of 3 components: immediate settlement (commonly referred to as elastic settlement, although this is a misnomer), consolidation settlement (or primary settlement) and creep settlement (or secondary settlement). 

Immediate settlement occurs in the soil upon load application and involves reduction in void space and rearrangements of the soil particles in response to that load. The consolidation settlement is induced due to volumetric change. Since soil particles are practically incompressible, consolidation settlements is caused by a reduction in voids due to gradual squeezing out of water. Finally, creep settlement occurs under a constant load and is depended on the stress history, the type of soil and the anisotropy of the soil.

The settlement process may be completed almost immediately or may last for a significant amount of time (even decades) depending on the soil’s permeability and water drainage paths.  In particular, cohesionless soils have higher permeability than cohesive soils that have small voids blocking the water movement. 

In geotechnical design, the total settlement of the soil has to be properly predicted and must meet the requirements of the project. If the soil’s characteristics are not adequate to meet the aforementioned requirements and the design of the superstructure cannot be changed, ground improvement is used to reduce the settlements.