Secant piles are drilled shafts constructed in such a way that the shafts overlap each other to form a continuous wall. The construction sequence involves drilling every other shaft (primary shafts) and then returning after the concrete has partially set to drill and pour the overlapping shafts between the primary shafts (secondary shafts). 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 is 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 piles are relatively impermeable and can be used to control the flow of groundwater into an excavation and limit draw down outside the excavation. Secant pile 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. When difficult drilling conditions are anticipated, secant piles can be a very economical method for cutoff wall construction. Another benefit is the relatively low cost of mobilization when compared to other cutoff wall types.