Tuesday, December 11, 2012

Foundation Safety Criteria.

It is a statement of the obvious that the function of a foundation is to transfer the load from the structure to the ground (i.e. soil) supporting it – and it must do this safely, for if it does not then the foundation will fail in bearing and/or settlement, and seriously affect the structure which may also fail. The history of foundation failure is as old as the history of building itself, and our language abounds in such idioms as ‘the god with feet of clay’, ‘build not thy house on sand’, ‘build on a firm foundation’, ‘the bedrock of our policy’.

The foundation must also be economical in construction costs, materials and time.

There are a number of reasons for foundation failure, the two major causes being:

(1) Bearing capacity. When the shear stress within the soil, due to the structure’s loading, exceeds the shear strength of the soil, catastrophic collapse of the supporting soil can occur. Before ultimate collapse of the soil occurs there can be large deformations within it which may lead to unacceptable differential movement or settlement of, and damage to, the structure. (In some situations however, collapse can occur with little or no advance warning!)

(2) Settlement. Practically all materials contract under compressive loading and distort under shear loading – soils are no exception. Provided that the settlement is either acceptable (i.e. will not cause structural damage or undue cracking, will not damage services, and will be visually acceptable and free from practical problems of door sticking, etc.) or can be catered for in the structural design (e.g. by using three-pinned arches which can accommodate settlement, in lieu of fixed portal frames), there is not necessarily a foundation design problem.

Problems will occur when the settlement is significantly excessive or differential.

Settlement is the combination of two phenomena:

(i) Contraction of the soil due to compressive and shear stresses resulting from the structure’s loading. This contraction, partly elastic and partly plastic, is relatively rapid. Since soils exhibit non-linear stress/strain behaviour and the soil under stress is of complex geometry,  it is not possible to predict accurately the magnitude  of settlement.

(ii) Consolidation of the soil due to volume changes. Under applied load the moisture is ‘squeezed’ from the soil and the soil compacts to partly fill the voids left by the retreating moisture. In soils of low permeability, such as clays, the consolidation process is slow and can even continue throughout the life of the structure (for example, the leaning tower of Pisa). Clays of relatively high moisture content will consolidate by greater amounts than clays with lower moisture contents. (Clays are susceptible to volume change with change in moisture content – they can shrink on drying out and heave, i.e. expand, with increase in moisture content.) Sands tend to have higher permeability and lower moisture content than clays. Therefore the consolidation of sand is faster but less than that of clay.

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