Ground Supported Floor

Summary

Ground Supported Floor - Floor supported on original or improved ground, where universal uniform support from the ground is assumed.

Technical Information

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 deflects under loading or as the subsoil settles from the effects of consolidation or ground movements at depth.

In his design concept, westergaard assumed 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 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.

A detailed discussion of k values is given in the comprehensive 1995 NCHRP Report 372, Support under Portland cement concrete pavements. The report makes the important recommendation that the elastic k value measured on the subgrade is the appropriate input for design. It has been suggested that the addition of a granular sub-base can enhance the value of k. However, the enhancement that can be achieved in this way is, in fact, dependant on the type and magnitude of the imposed load and the nature of the sub-base. In any event, in normal circumstances such enhancements have little effect on the thickness design for flexural stresses. It is recommended therefore that the design of the slab should be based on the k value of the sub-grade without enhancement.

It is recognised that in some cases the existing subgrade materials are 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 should however be ignored. It is recommended that the expertise of a suitably qualified geotechnical engineer be sought to advise on the appropriate value of k in such circumstances.