Soil Compressibility

From previous blogs we talked about 2 factors we need to consider when designing of a foundation, first one is Bearing capacity, and second criteria is Settlement of the foundation. We already deeply discussed about Bearing capacity. from this blog on wards let's talk about settlement and related topics.

Settlement of foundations can be divided in to the following main 3 categories,
(1). Immediate (elastic) settlement - Si
(2). Consolidation settlement - Sc
(3). Secondary compression (creep) settlement - Ss

The toal settlement of a foundation St is the sum of the above three settlement components,

St = Si + Sc + Ss


For settlement calculations we need to consider influence zones,
For squire footings it is upto 2B (B-least dimension of the footing) and for strip footing it is upto 4B. 

Immediate Settlement (Si)

Occurs instantaneously after the load application during, or immediate after the erection of a structure. It is primarily a consequence of soil movements as a result of grain distortion and reorientation. Basically for,

Cohesive soils ------------ due to change in shape
Cohesion less soils--------due to expulsion of water

Note : Predominant in sandy and gravelly soil types.

Consolidation Settlement (Sc)

Occurs in saturated soils when the application of loads creates a state of excess pore water pressure that can only be dissipated by the gradual expulsion of water out of the soil particles which results in volume changes. this settlement generally takes several years to occur and is the major concern for cohesive soils.

Note : Predominant in clay, sity soils

Consolidation Settlement (Sc) 

Occurs due to decomposition of soil particles with no change in applied stress. very small settlement this settlement is usually negligible for most soils. but it may be significant for organic soils and high plastic clays. this will occur after the consolidation settlement.