Secant (tangent) piles are interlocking piles that form a continuous watertight wall. A continuous reinforced concrete guide wall is constructed to pinpoint the location of each overlapping pile. Piles are spaced a distance which is a little less than one pile diameter. The exact spacing depends on construction tolerances.
Alternate piles (known as "female", "intermediate" or "primary" piles) are drilled inside temporary steel casing and constructed without reinforcement using a slow setting concrete. Temporary casing is extracted while the concrete has not fully set and heavy casing is then driven into the intervening pile location cutting into the fresh concrete of the adjacent female piles. The "male", "secondary" or "king" piles are then promptly drilled. The male pile steel cage reinforcement is inserted and structural concrete is poured adhering to the female pile concrete on either side to form a watertight continuous wall.