Here we will learn about foundation settlement in cohesionless soil which is a nonplastic soil like gravels and sands. Some silts are also nonplastic when there have little or no clay content. We know, cohesionless soil especially gravel is free draining soil; so long-term settlement is rare in this type of soil.
In this case settlement of these granular soils primarily is due to compression of soil skeleton under foundation loads. Their compressive load results a rearrangement in soil particles to reach a dense arrangements.
So it is clear to us that very loose deposit of sand/gravel when subjected to foundation load suffers more densification due to rearrangement of particles and subsequent settlement than that of dense/very dense cohesionless soil of same type.
It is obvious to us that cohesionless soil usually not suffers long-term settlement. The settlement occurs immediate after/during construction and dense soil results less settlement. Now we have to find out why they are subjected to long-term settlement?
The exceptions where long-term settlement is happened are:
a. Collapsible soil (cohesionless)
b. Seismic loading
d. Fluctuation of loads
Dear reader we will learn about each case in next part; we will discuss them, briefly as most of them are related to other topics already published in our previous posts.