Secant Piles

Secant pile walls can be used for temporary rigid support of excavation or to form permanent earth retention systems. This technique is suited for dense, urban environments. Walls are constructed by the sequenced drilling and construction of overlapping, reinforced concrete piles.

Tangent pile walls are constructed in a similar manner to secant piles walls except that individual elements are designed to abut the adjacent pile than overlapping it. Since tangent pile wall construction eliminates the need to drill through previously placed wall elements, construction is faster. This type of system is typically not used where a groundwater cut-off is required.

Applications

Secant pile walls provide both structural excavation support and earth retention as well as water cut-off. Although more costly than sheeted excavation support, this system offers greater alignment flexibility and increased wall stiffness. Piles can be drilled through most subsurface conditions, including bouldery or cobbly soils, depending on the type and power of the drilling equipment used. Properly constructed guide walls are essential to ensure accurate horizontal alignment and minimize pile deviation that could cause overlap problems with increasing pile depth.

Installation

Secant pile walls are constructed by the sequenced drilling and concreting of overlapping vertical elements. Primary piles are spaced at slightly less than the nominal pile diameter. Intermediate secondary piles are then installed (ideally before the primary pile concrete has achieved full strength) such that they cut into and interlock with adjacent piles, forming a continuous cut-off structure. Where the cut-off wall will also provide ground support, the piles are reinforced with H-piles or reinforcing cages. Typically only secondary piles are reinforced to avoid the risk of cutting the reinforcing members in the primary piles during secondary pile construction.

Advantages & Limitations

  • Can penetrate dense soils and soils with natural and man-made obstructions
  • Can be braced or tied back for greater excavation depths.
  • Stiffer lateral response than sheet pile or soldier beam and lagging walls which minimizes deflections.
  • Equipment used for installation is large, expensive and requires a large amount of space.
  • Complete replacement of the excavated soil is required, with a consequent increase in the volume of spoil generated
  • More expensive compared to other excavation support techniques.