Secant Walls, formed by installing overlapping CFA or Rotary bored concrete piles, are used to retain water as well as prevent the ingress of soil as oppose to contiguous piled retaining walls which simply retain the earth and don’t overlap at all. In hard/soft secant walls, only the hard, also referred to as male or secondary, piles are reinforced. In the case of hard/hard secant walls, every pile forming the 360° beneath ground structure is reinforced to spread both the horizontal and vertical loads.
Primary piles are installed on the first pass with secondary piles constructed in between the primary piles, once the latter have gained sufficient strength. Pile overlap is determined by a number of factors but primarily by the retained height. In a wall constructed of 600mm diameter piles it would typically be 150-200mm.
Secant walls offer a significant lateral strength advantage compared to that of sheet piles and are also often designed to take a load applied by the structure above.
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