Soils can be divided into two types depending on their ability to keep the mass together. The soils that have this ability is called cohesive soil and those don’t have such ability is called cohesionless.
Adequate clay content in soil particles renders a property that glues them into mass. They are also able to be shaped or molded. This property of molding is often called plasticity.
We have discussed many post about filed identification of soil from one type to other. Most of which is based on this remoulding property when clay content is the primary issue.
The strength we are discussing above depends on cohesive property. But cohesionless soil have also strength; now question is- how do they derive strength?
They derive strength from interparticle friction and sometimes call frictional soil.
But pure cohesive or pure cohesionless soil is rare most of them have both sources of strength and these soils are called C-φ soil. Where C is cohesion intercept and φ is angle of internal friction.
Now what is the difference between cohesionless and cohesive soil in saturated condition with respect to settlement of foundation?
We know permeability of cohesive soil is very low and cohesionless soils are relatively more permeable. So cohesive soil consolidate with time which may be several years depending on loading and drainage condition. We, geotechnical engineers can change this time by many means like vertical sand drain, PVD etc. But time is the vital factor for consolidaton of cohesive soil.
In cohesionless soil, settlement of foundation soil is not usually time dependent as they have high permeability and settlement is observed while construction when dead load is being applied gradually as construction process goes on. So the main difference is time; in cohesionless soil long term settlement is absent. But there have some exceptions; dear reader in our next post we will learn about the exceptions in foundation settlement supported on cohesionless soil.