If ground conditions are poor, such that they are inadequate for a ground-supported floor, usually due to unacceptably high potential settlement values, the floor may be constructed on closely spaced piles to transfer the load to firmer strata. Pile grids are normally in the range 3 x 3m to 5 x 5m, depending on the pile capacities and the intensity of loading. It is assumed that the ground gives no support but simply acts as in situ formwork for the floor slab. The design is the same as for suspended floors, using the approaches in BS 8110, now withdrawn and superceded by Eurocode 2.
From a construction point of view, it is economic to transfer the slab loads directly to the piles without any beam supports or local thickening around the pile heads. However, punching shear around the piles may be the critical design criterion and hence some local thickening may be required. Compared with normal suspended slabs, spans are short. It is recommended that the ratio of slab span (measured diagonally between the faces of the piles or enlarged pile heads as appropriate) to overall depth should not exceed 20, to avoid the need for detailed deflection calculations. Experience has shown that there can, in some cases, be a high risk of surface cracking on piled slabs due to differential shrinkage effects.
Pile-supported floors may be reinforced with conventional bar reinforcement, fabric, steel fibres or a combination of materials. Some guidance on design, and the provision of joints, is given in Technical Report 34, Concrete industrial ground floors.