Secant piles are drilled shafts constructed in such a way that the shafts overlap each other to form a wall. A typically constructed wall would consist of 36-inch diameter shafts drilled at 30 inches on center. The construction sequence involves drilling every other shaft and then returning after the concrete has set in the initial shafts to drill and pour the shafts for the secondary piles. The concrete in the shafts can be all low strength, all high strength, or a combination of low strength primary and high strength secondary piles.
Secant piles are used to build cutoff walls for the control of groundwater inflow and to minimize ground movement in weak and wet soils. They are very stiff retaining walls 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 Foundation Company'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.