Construction follows a top-down sequence with strict timing between primary and secondary pile placement. A continuous concrete guide wall, typically 12 to 18 inches deep, is cast first along the wall alignment to fix the position of every primary and secondary pile and to provide drilling-rig alignment during installation. Primary piles are then drilled at every other location at 24 to 48 inch diameter, advanced under temporary casing or polymer or bentonite slurry to maintain hole stability, and concreted by tremie. Soft secant walls place a low-strength concrete (1,000 to 2,000 psi) in the primaries to facilitate cutting; hard secant walls place full structural concrete (4,000 to 6,000 psi) with reinforcement cages in the primaries; firm secant walls use unreinforced medium-strength concrete (2,500 to 3,500 psi). Tremie placement follows EFFC and DFI Best Practice and the requirements of ACI 336.1.
Secondary piles are drilled in the gaps once the primary concrete has reached the design strength for cutting, typically 12 to 48 hours after placement. Rock augers, core barrels, or down-hole hammers cut through the primary concrete to create the design overlap, typically 3 to 6 inches face-to-face. Reinforcement cages and structural concrete (4,000 to 6,000 psi) are placed in the secondaries, which carry the long-term structural load while the overlaps deliver continuity and the watertight cutoff. Excavation then advances in lifts in front of the wall. Where the exposed face exceeds the cantilever capacity of the wall, generally 20 to 25 feet of unsupported height, tieback anchors are drilled through the secondary piles at design elevations and post-tensioned against waler beams, with each lift stopping at the next anchor row and the cycle repeating to final subgrade. Permanent walls receive a structural shotcrete or cast-in-place concrete facing once excavation is complete, often with a drainage composite and waterproofing membrane between the secant face and the finished interior wall.
1
Survey and Guide Wall Construction
Survey pile locations precisely, secant walls require tight tolerances. Construct concrete guide walls (typically 12-18 inches deep) to align drilling equipment and maintain pile spacing accuracy throughout installation.
2
Primary Pile Drilling
Drill primary (female) piles at every other location using casing or slurry to maintain hole stability. Primary pile spacing equals the design overlap plus secondary pile diameter. Install reinforcement if specified.
3
Primary Pile Concrete
Place concrete in primary piles using tremie method if below water table. For soft secant walls, use low-strength concrete (1,000-2,000 psi) in primaries to facilitate cutting. For hard secant walls, use structural concrete throughout.
4
Secondary Pile Drilling
After primary pile concrete reaches design strength for cutting (typically 12-48 hours), drill secondary (male) piles overlapping into both adjacent primaries. The drill cuts through the primary pile concrete to create the interlock.
5
Secondary Pile Reinforcement and Concrete
Install steel reinforcement cages in secondary piles. Place structural concrete. Secondary piles carry the structural loads while overlaps provide continuity and water cutoff.
6
Wall Completion and Excavation
Allow secondary piles to cure. Begin excavation in controlled lifts, installing tiebacks or bracing at design elevations. Expose and clean wall face. Apply waterproofing or architectural finish if required.
Drilling 
Shoring & Excavation Support 
