Contiguous and Secant Piled Walls
These two different methods of retaining wall construction employ bored piling techniques, usually CFA or Rotary. Used when the available workspace dictates that basement excavation is vertical, both methods offer technically effective and cost efficient solutions for retaining the sides of bulk excavations, even in water bearing soils.
The techniques are most common in large commercial basement projects as they avoid excessive excavation and assist in the control of ground movement. The walls can be designed to accommodate vertical loads in addition to horizontal movement and forces. The construction of both methods is the same as for foundation bearing piles.
The quickest and most cost effective form of bored pile walls, contiguous walls can be constructed in dry, stable soils. This technique is suitable to retain stiff and cohesive subsoils and where ground water levels are below the eventual depth of excavation. The walls are constructed in a phased sequence, with intervening gaps between adjacent piles. The edge to edge spacing between piles, usually 150mm, is determined by the nature of the soils.
Secant piles are a well-established method of constructing retaining walls and are formed by a series of interlocking bored concrete piles. We have the specialist equipment, experience and expertise to design and build hard/soft, hard/firm and hard/hard secant walls using continuous flight auger techniques or segmental casings for applications in excess of 1200mm pile diameters.
Powerful high torque drilling equipment allows secant walls to be installed in the most challenging urban environment. Piles in a secant wall are spaced at 0.8 to 0.9 pile diameters and are constructed with alternate primary piles and secondary piles to provide the necessary closed structure. The structure will then act as a barrier in water bearing soils and prevents the ingress of soil between the piles.
Hard/Soft or Hard/Firm
In scenarios where soils are saturated and water tightness is required, this system offers the most cost-effective and rapid solution. The wall consists of interlocking bored piles, with primary reinforced concrete (hard) piles used in combination with secondary piles of using either bentonite cement materials (soft) or weaker concrete (firm) . These walls may need a reinforced concrete lining for permanent works applications, depending on the particular requirements of the project.
Permanent retention can be provided with all reinforced piles, which can be incorporated within the final structure. Hard/hard wall construction is similar to a hard/firm wall, however the secondary piles are constructed in higher strength concrete and may be reinforced. Structural concrete is used throughout which means there is no need to provide a lining wall. The end product provides a fully concreted face and can be an effective alternative to diaphragm wall construction.