This technique is particularly effective where rock penetration is required or where cobbles, boulders and mass concrete are in the overburdened material. The components are generally between 200mm and 450mm in diameter and installed using pneumatic hammers mounted on minipiling rigs, typically weighing between 4.5 and 12 tonnes. They can be installed in restricted-access areas and low-headroom environments, depending on the rig and diameter requirements.
They are constructed using a combination of drilling techniques to penetrate the overburdened material and form rock sockets. Concentric, eccentric or duplex systems penetrate and stabilise the overburdened soil with casings before down-the-hole hammers penetrate the rock. This technique uses either temporary or permanent casing. The arisings can be flushed from the bore using air, water, foam, grout or polymer. When the predetermined rock socket length has been achieved, the steel reinforcement is placed, and the pile bore is filled with either concrete or grout.