Concrete for female secant piles
The low strength concrete for the “female” piles has a number of functions but first and foremost it needs to retain the ground and groundwater from passing between the reinforced concrete “male” piles. However, to achieve this and facilitate construction the concrete requires the following properties:
- Sufficient strength to support and distribute the earth pressures to the “male” piles. In reality these piles are spanning less than a pile diameter and the forces are distributed by arching with the material in compression.
- Low permeability to minimise the seepage of groundwater through either the piles or secant wall as a whole.
- Low early strength gain to facilitate the boring of the “male” piles. The difference in strength between the adjacent “female” piles is likely to affect the drilling tolerances of the male piles and thus verticality and overlap achieved.
- Suitable for pumping without segregation and maintaining stability during and after the placing.
- Suitably durable for the intended life of the material. The “female” pile is typically unreinforced and effectively temporary if a permanent lining wall is constructed on the inside face of the piles.
For this reason durability is not normally a concern and a design chemical class does not need to be specified.
The relatively weak concrete in the female piles can lead to long term durability problems and the secant pile wall can only be considered to be water resistant in the short term.