Cohesionless soils are 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
c. Vibrations
d. Fluctuation of loads
Collapsible soil
Our concern is collapsible soils that are cohesionless. We have learnt earlier in this blog that collapsible soil suffer large settlement when water infiltrates through the soil under no or small compressive load.
In most cases soil deposit are stratified and different layer, as usual, may have different degree of settlement, here we use the term collapse potential as discussed in our previous post-about collapse potential of collapsible soil.
Total settlement of multiple collapsible layers of soil is derived by adding collapse value of different layers.
Seismic loading
Seismic shaking due to seismic waves traveling through soil layers or along ground surface, may also result densification of cohesionless soil, provided that the soil layer must be in saturated condition and be in loose conditions. This phenomenon is known as liquefaction we have discussed many posts about cause, potential, triggering and mitigation of liquefaction susceptible soil in previous post.
Vibrations
Another source of such settlement is vibration from machinery. Let us consider a machine like printing press supported on foundation soil of this type, here also loose sand is more susceptible. The densification of loose deposit is done slowly by vibration and this is also a long-term settlement.
Fluctuation of loads
These types of cases occur when loading and unloading of material results fluctuation of loads on cohesionless soil. Dear reader in our last part we have discussed this elaborately, please read this post for more clarification.
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