Ground-supported slabs are not rafts and do not have the ability to span over soft zones or poor-quality subsoil. They will tend to conform to the shape of the subsoil as it settles under loading. This means that differential settlement is potentially problematic for ground supported slabs.
In his design concept, Westergaard assumes that a slab acts as a homogeneous, isotropic, elastic solid in equilibrium with the reactions from the subgrade; which are vertical only and are proportional to the deflections of the slab.
The subgrade is assumed to be an elastic medium whose elasticity can be characterised by the force that, distributed over unit area, will give unit deflection. Westergaard termed this soil characteristic the ‘modulus of subgrade reaction’ (k), with units N/mm2/mm. This value is a key input in the structural design of the slab.
It is recognised that in some cases the existing subgrade materials can be improved by stabilisation or the addition of a designed capping layer. Where this has occurred, it is considered appropriate to base the design of the slab on the k value arising from a regulating layer immediately beneath the slab.
All geotechnical investigations are carried out by a suitably qualified geotechnical engineer.
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