What Is a Secant Pile Wall Design?
Secant pile walls form watertight excavation support from overlapping drilled shafts — alternating primary (unreinforced) and secondary (reinforced) piles, the secondary cutting into the primary, creating the continuous wall. The design sizes the piles for the earth and water pressures, the groundwater cutoff, and the construction sequence. The excavation support where the water table demands watertightness: basements, subway stations, pump stations.
Best price · Quotes back fast · PE stamps backed by Apex Grid Engineering
How secant walls work
Primary piles (the 'female' — unreinforced or lightly reinforced, installed first at wider spacing), secondary piles (the 'male' — reinforced, drilled second, cutting into the adjacent primaries — the overlap creating the interlock), forming the continuous wall. The wall resists the lateral earth and hydrostatic pressures (designed as the continuous beam spanning between the bracing levels — or cantilevered for shallow cuts), and the overlapping concrete provides the groundwater cutoff (the wall's permeability orders of magnitude below the soil's).
Design considerations
The pile diameter and overlap (typically 3-6 inch overlap — the construction tolerance consuming it; the design specifies the minimum effective overlap), the reinforcement (the secondary piles' cages — designed for the bending moments; the primaries sometimes reinforced for the water cutoff redundancy), the bracing (internal struts/walers or tiebacks — the support system designed with the wall), and the toe embedment (the wall extending below the excavation — the basal stability and the groundwater cutoff depth).
Construction and verification
Guide walls (the surface template ensuring the pile positions — the accuracy starts here), the drilling sequence (the secondary cutting the green primary concrete — the timing window; too-hard primaries damage the equipment), verticality control (the wall's watertightness depends on the piles staying overlapped at depth — the drilling tolerances are the design's assumption), and the integrity verification (the records, the CSL where specified, the excavation observation). Cost: $80-$150+/sq ft of wall — the watertight premium over soldier-pile walls.
The bottom line
Secant pile walls overlap drilled shafts forming watertight excavation support — design the pile overlap, reinforcement, bracing, and toe embedment; control drilling verticality.
Torch Engineering PLLC · (480) 490-0064 · info@torchengineering.com — PE stamping backed by Apex Grid Engineering.