Where the subsoil is of firm, shrinkable clay which is subject to volume change due to deep rooted vegetation for some depth below surface and where the subsoil is of soft or uncertain bearing capacity for some few metres below surface, it may be economic and satisfactory to use a system of short bored piles as a foundation.
Piles are concrete columns which are either precast and driven (hammered) into the ground or cast in holes that are augered (drilled) into the ground down to a level of a firm, stable stratum of subsoil.
The piles that are used as a foundation down to a level of some 4 m below the surface for small buildings are termed short bore, which refers to the comparatively short length of the piles as compared to the much longer piles used for larger buildings. Short bored piles are generally from 2 to 4 m long and from 250 to 350 mm diameter.
Holes are augered in the ground by hand or machine. An auger is a form of drill comprising a rotating shaft with cutting blades that cuts into the ground and is then withdrawn, with the excavated soil on the blades that are cleared of soil. The auger is again lowered into the ground and withdrawn, cleared of soil and the process repeated until the required depth is reached.
The advantage of this system of augered holes is that samples of the subsoil are withdrawn, from which the bearing capacity of the subsoil may be assessed. The piles may be formed of concrete by itself or, more usually, a light, steel cage of reinforcement is lowered into the hole and concrete poured or pumped into the hole and compacted to form a pile foundation.
The piles are cast below angles and intersection of load bearing walls and at intervals between to reduce the span and depth of the reinforced ground beam they are to support. A reinforced concrete ground beam is then cast over the piles as illustrated in Fig. 10. The ground beam is cast in a shallow trench on a 50 mm bed of ash with the reinforcement in the piles linked to that in the beams for continuity. The spacing of the piles depends on the loads to be supported and on economic sections of ground beam.
Fig. 10 Short bored pile foundation.