Secant Piled Walls

A secant piled wall is formed by constructing alternative interlocking secondary and primary bored piles as a retaining structure providing load bearing walls and deep foundations for engineering construction.

What is a secant piled wall?

A secant pile wall is a bored pile retaining wall formed by installing interlocking primary and secondary piles to form a homogeneous continuous retaining structure incorporating a capping beam. Secant pile walls are flexible in shape and design compared to diaphragm retaining walls and offer a significant lateral strength advantage compared to that of sheet piled walls.

Secondary piles include steel reinforcing cages and high slump concrete, while the composition of primary pile can vary depending on soil type, ground water profile, retained heights, available construction time, propping arrangement, cost and design life. The following three alternatives are adopted:

  • Hard / Soft – a concrete bentonite blended mix is used within primary piles
  • Hard / Firm – low strength concrete is used in the primary piles
  • Hard / Hard – full strength structural concrete is in primary piles

Cased secant piling offers the cost and speed benefits of CFA piling with the accuracy of rotary bored piling to create a near watertight secant piled walls.

Key capabilities:

  • Rig loadings: 6–110 tonnes
  • Pile diameter: 600–1,200mm
  • Pile Diameter: 1,000mm cased CFA

What are the advantages of Secant Piled Walls?

  • Flexibility to maximise land use, fit complex site boundaries and weave between underground structures
  • Can be installed as a cantilever section or design to facilitate permanent anchors or temporary propping
  • The higher vertical tolerances can be achieved when using segmental casing during the installation of the secondary piles
  • Provides water tight retaining wall

Common Uses

  • Temporary or permanent water-tight retaining walls
  • Slope stabilisation
  • Water cut off for drainage channels
  • Enable and support excavations
  • Basements and underground car parks
  • Alternative to diaphragm walls where there are ground obstructions

Prior to commencing with pile installation, a scalloped guide wall is installed to ground level providing line and level. Piles can be constructed using CFA, cased CFA or rotary bored techniques or a combination of methods.

Pile installation commences with the installation of a series of primary (female) piles installed through the guide wall. After the primary piles have gained sufficient strength, the interlocking secondary (male) piles are then installed cutting partially into the primary piles on either side. The primary piles contain full length reinforcement and are cast with high strength concrete. This installation process is continued until a continuous homogeneous structure is formed.

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