Bearing capacity - requirements of a satisfactory foundation

Requirements of a satisfactory foundation

To perform satisfactorily a shallow foundation must: Be properly located with respect to any influence which could adversely affect performance Be stable and must not fail (this also means the underlying soil or rock must not fail) Not settle or deflect sufficiently to damage the structure or impair it's usefulness The first requirement involves many factors which are determined more by engineering judgement than analytical methods. In particular, the depth at which a foundation should be placed will depend upon a number of things including, but not limited to: depth of frost penetration depth of swelling and shrinkage zones in clay soils the positions of adjacent structures, underground services, excavations and any planned future construction operations nearby geological features such as faults, caves and quarries The second requirement must be evaluated analytically from a knowledge of the applied pressures, the shape of the foundation and the shear strength of the subsoil.The magnitude and rate of settlement can also be assessed from a knowledge of the loading conditions and the deformation characteristics of the subsoil.Failure of the ground in shear can cause catastrophic failure or complete collapse of the structure whilst excessive settlement can give rise to a functional failure where the structure fails to fulfil the purpose for which it was designed by tilting, distorting and cracking.

Load Settlement Relationship When a load is applied to a foundation, the subsoil deforms and settlement occurs. For light loads the settlement will be small and approximately proportional to the applied pressure. As the pressure increases, a point will be reached at which beyond it, the settlement will increase much more rapidly and will continue to increase without any further increase in pressure.This is illustrated by a curved graph. The initial straight line part of the load-settlement curve is due to the elastic distortion and compression of the soil. The steeper, curved part is caused by plastic deformation. The ultimate bearing capacity is at the point where the maximum load can be carried. The bearing capacity is a function of the size, shape and depth of the foundation as well as the shear strength of the soil.In general, elastic displacement takes place immediately as the load is applied, but in clay soils consolidation settlement will take place over time.

Shear Failure Three principle modes of shear failure are described below: General shear failure - occurs when a clearly defined failure surface develops on one or both sides of the footing and extends to the ground surface. Failure is sudden and often catastrophic. A characteristic heave of the ground surface also occurs Local shear failure - the failure surfaces start at the edge of the footing but do not reach the ground surface. Some bulging of the ground may occur. When the load on the foundation reaches a certain value the foundation settlement will be accompanied by sudden jerks which are due to the failure surfaces extending outwards Punching shear failure - considerable vertical movement of the foundation will occur without any noticeable development of any shear planes or surface heave. Some drag-down of the soil may also occur. The failure mode depends on the compressibility of the soil. General shear failure occurs in sands and clays of low compressibility. Punching shear failure occurs in soft silts and clays and loose sands. Local shear failure may be considered as a transitional mode between general and punching shear.