Bored pile foundation

A deep foundation that transfers the building's loads to firm soil strata at depth, bypassing the weak or compressible surface layers. The piles, cast in place inside a bored hole and reinforced with a cage, carry load by skin friction along the shaft and by end bearing at the tip; a cap or a ground beam ties them to the superstructure.

When the surface soil cannot carry the loads - because it is soft, compressible or prone to settlement - the foundation abandons the «contact» logic of the raft and the footings and reaches down for a firm stratum. A bored pile is a cylinder of reinforced concrete cast inside a hole drilled by a rig; it transmits the load to the ground in two complementary ways, skin friction along the whole shaft and end bearing at the base.

Construction starts by drilling the hole to the design diameter and depth. In unstable soils or below the water table the borehole walls are supported by a steel casing or by a bentonite slurry, which balances the pressure of the soil and the water. With the slurry in place, the reinforcement cage - longitudinal bars and a spiral - is lowered in and the concrete is poured from the bottom through a tremie pipe, which rises as it displaces the slurry. The continuity of the shaft depends entirely on the regularity of this phase.

Under load the pile resists sinking with two contributions: the shear stresses developed around the shaft (friction or bond with the soil) and the reaction of the ground beneath the base (end bearing). In «floating» piles friction dominates, spread over a large lateral surface; in «end-bearing» piles the shaft crosses the soft layers to rest on a stiff substratum. Geotechnical design balances the two mechanisms and sets the length, diameter and number of piles.

A pile rarely works alone: several piles are gathered by a cap or a rigid ground beam that shares the load and makes them act together. The deep foundation also takes horizontal actions and moments well - wind thrust, earthquake, sloping ground - which shallow footings could not resist. Two things must be checked, though: negative skin friction (when a settling soil «hangs» on the pile, adding load) and the chemical aggressiveness of groundwater on the concrete.