Foundation, Concrete and Earthquake Engineering

Contact pressure on sand

The soil reaction beneath footing produce a upward pressure which is assumed uniform in deriving different relationship for soil-structure interaction problem. This pressure is called contact pressure. 

Flexible footing
In this case, the edges of flexible footing undergo a large settlement than at the centre. The soil at the centre is confined and, therefore, has a high modulus of elasticity and deflects less for the same contact pressure. The contact pressure is uniform. 

Rigid footing 
If the footing is rigid, the settlement is uniform. The contact pressure increases from zero at the edges to a maximum at the centre. The soil, being unconfined at edges, has low modulus of elasticity. However, if the footing is embedded, there would be finite contact pressure at edges.
Contact pressure on sand
Fig: Qualitative contact pressure distribution under flexible and rigid footing resting on sandy soil and subjected to a uniformly distributed load q.

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