Drilled Minipiles

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.

Drilled minipiles

Drilled minipiles can be positioned as close as 500 mm to adjacent structures from the pile’s centre to a vertical face. When they are required close to underground structures, such as foundations, tunnels or services, permanent sleeves can be used to prevent load transfer from the piles to these structures.

This technique requires comprehensive ground investigation information to design the pile for the prescribed loads. The ground conditions can be verified during the installation process when the pile bore arisings can be observed and compared with the soil and rock descriptions indicated in the ground investigation boreholes.

High pile load-carrying capacities can be achieved with this technique. Capacities do vary significantly depending on rock strengths and the structural capacity of the pile section. Maintained load tests can be carried out on preliminary or working piles for design verification purposes.