Secant Piling Construction System
Secant piling is a robust, rigid system which can be used to construct combined earth retention and groundwater cut-off walls. The continuous wall is constructed by drilling overlapped concrete piles with diameter typically between 2 and 4 feet. A wide range of drilling techniques can be employed allowing secant pile walls to be constructed in variable ground conditions. In many cases a wall is constructed to penetrate through water bearing overburden and seal into dense till or rock below excavation subgrade.
Piles are drilled and concreted prior to start of excavation. The initial or “primary” piles are drilled into existing ground at the selected center spacing. The wall is completed by drilling structurally reinforced “secondary” piles which cut into and overlap with the adjacent primaries. Secant walls overlap individual piles which allows for flexible layouts accommodating linear or curved alignments with multiple corners. Vertical reinforcement is typically installed only in secondary piles and may be either a steel pile or rebar cage.
The key to secant pile wall construction is maintaining overlap between adjacent piles. A guide wall or template at ground surface ensures that every pile is accurately located. Drilling with heavy wall steel casing provides an extremely stiff drill string which can achieve vertical tolerance typically better than 0.5% of pile depth. Downhole surveys are performed to verify that actual drilling verticality meets the design requirements. Under certain soil conditions and for moderate excavation depths, alternative installation methods, such as auger-cast or soil mixed piling, can be highly productive and may be employed in lieu of cased drilling to construct secant walls at a lower unit cost.
Amongst other consideration, lateral support systems are selected to minimize risk of water infiltration. Options include tieback anchors and internal bracing, with tiebacks usually located above groundwater table. For circular excavations, self-supporting systems rely on internal hoop stresses and may not require steel reinforcing. The exposed face of secant pile can be mechanically ground and leveled to improve the substrate for waterproofing purposes.