Surface Spread Foundations.

Surface spread foundations consist mainly of rafts and are generally used where the normal ground bearing
sub-strata is relatively poor and the depth to suitable loadbearing soils is excessive or the load-carrying capacity of the soil deteriorates with depth. Surface spread foundations are therefore employed to distribute the superstructure/substructure loads over a large area of the ground thus reducing the contact bearing pressure. Since most structures also require a ground floor slab it is usually  economic to incorporate it with the foundation into one structure/element. This can be done by making the upper surface of the raft foundation coincide with the top surfaceof the floor slab. A simple example is shown in Fig. 9.22.

Surface spread raft foundations are often adopted in areas of active mining as the best means of resisting excessive distortion, tensile and compressive forces, etc., resulting from the ground subsidence. These and other types of surface spread foundations are discussed in the following sections.

It should be noted that rafts do not necessarily distribute the loads as a uniform contact pressure to the sub-strata, on the contrary, most rafts are relatively flexible foundations and will have higher contact pressure under loaded points and edge thickenings than below the main slab areas.

Fig. 9.22 Typical raft foundation.

⇒ Nominal crust raft
⇒ Blanket raft
⇒ Slip-plane raft
⇒ Cellular raft
⇒ Lidded cellular raft
⇒ Beam strip raft
⇒ Buoyancy (or ‘?oating’) raft
⇒ Jacking raft

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