What are short bored pile foundations?

Answer 1

Short bored pile foundations

Where the subsoil is of firm, shrinkable clay, which is subject to volume change

due to deep rooted vegetation for some depth below the surface and where

the subsoil is of soft, or uncertain bearing capacity for a few metres below the

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 augured

(drilled) into the ground down to a level of a firm, stable stratum of subsoil

The piles that are excavated to a depth of 4 m below the surface 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 into the ground by machine. An auger is a form of drill

comprising a rotating shaft with cutting blades that screw into the ground.

The soil is either withdrawn and lifted to the surface as the shaft rotates or,

with small augers, once the auger has cut into the ground it is withdrawn and

the soil removed from the blades. The advantage of this system of augered

holes is that samples of the subsoil are withdrawn, from which the nature

and bearing capacity of the subsoil may be assessed. The piles may be formed

of concrete 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 underneath the corners and intersection of load bearing walls and at regular intervals between to reduce the

span and depth of the reinforced ground beam, which transfers the wall and

building loads to the foundation. A reinforced concrete ground beam is then

cast over the piles. The spacing of the piles depends

on the loads to be supported and on economic sections of ground beam, see