Secant piles are part of a rigid, robust system that combines earth retention and groundwater cut-off walls. Secant pile walls are constructed by drilling in a series of alternating primary (drilled first) and secondary (drilled second) overlapping shafts to form a continuous secant wall. The concrete in the shafts can be all low-strength, all high-strength, or a combination of low-strength primary and high-strength secondary shafts. Steel piles or reinforcing steel are typically placed in the secondary shafts. Unreinforced high-strength shafts can also be constructed in a self-supporting circular ring, such as those frequently used for access shafts to tunnels.
Secant pile walls are relatively impermeable and can be used to control the flow of groundwater into an excavation and limit draw down outside the excavation. Secant walls are also very stiff and are often used when there are sensitive structures behind the wall.
An advantage of using secant piles for cutoff wall construction is the ability of Schnabel’s high torque drill rigs to penetrate hard-drilling conditions while simultaneously casing the drill hole. These drills can advance a cased hole through man-made and natural obstructions including cobbles, boulders, and reinforced concrete.