Concise Notes on Bearing Capacity and Settlement of Soils (PDF)

Bearing Capacity and Settlement of Soils (PDF)

When a soil mass is loaded, usually by loads transmitted from buildings to the foundations by columns, load-bearing walls or other load components of the structures, it undergoes a shear pressure and settlement. For the soil to remain stable under the load and to support the load causing shear pressure and settlement, it is expected not to fail in shear, and the settlement is expected not to be excessive which can impair the functioning of the buildings. This is the essence of the bearing capacity of the soils and it is usually the duty of geotechnical engineers and structural engineers who carry out the task of foundation design to evaluate the bearing capacity of soils. The bearing capacity of soil is defined as the load-carrying capacity of foundation soil or rock which enables it to bear and transmit loads from a structure safely without failure or excessive settlement. Bearing capacity is considered whether the foundation is a shallow foundation such as pad, strip, strap, raft etc. or deep foundation such as pile, caissons and well foundations. However, the methods employed in the determination of the bearing capacity of each type of foundation vary.

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SETTLEMENT OF SOILS

When a foundation or a soil deposit is loaded, for example by a structure or a man-made fill, deformations will occur. The total vertical deformation at the surface resulting from the load is called settlement. The movement may be downward with an increase in load or upward (e.g. called swelling) with a decrease in load.

Temporary construction excavations or permanent excavations such as highway cuts will cause a reduction in the stress, and swelling may result. Lowering of water table will cause increase in effective stresses within the soil which will lead to settlements. Other factors that cause settlement in a loaded soil are: underground erosion, structural collapse of soil e.g. saline, non-cohesive, gypsum soil, clay, loess soils, thermal changes, frost heave, vibration and shocks, mining subsidence, landslides, creep, changes in vicinity.

In the design of foundations for engineering structures, we are interested in how much settlement will occur and how fast it will occur. Excessive settlement may cause structural as well as other damage, especially if such settlement occurs rapidly.

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