This method of earth retention and shoring is the perfect choice for supporting an excavation on a site where underground utilities and the footings of adjacent buildings are present, and traditional earth-retention methods would encroach upon those adjoining sites.
Contiguous pile walls are constructed by drilling successive, adjacent piles, leaving only small gaps between piles. Both the optimal diameter and spacing of the piles are governed by the soil type and the level of ground water, however the nominal gap is usually between 100mm to 150mm. Large spaces between piles are generally avoided as they are more likely to result in ground materials from the retained side of the wall being lost through the gaps.
The piles are intended to bear tension loads and lateral bending forces, and the addition of a capping beam at the top of the piles helps to distribute downward pressure equally across the piles and ensures they act uniformly. This kind of retaining wall can also support an axial load (line load and/or point load), distributed through the capping beam.
Contiguous pile walls are formed by using rotary bored piles or Continuous Flight Auger (CFA) piles to construct a row of unconnected, cast in-situ concrete piles. The decision on whether to use bored piles or CFA piles is usually determined by the purpose for which the wall is being constructed.
Bored piles are constructed using a modern drill rig to bore a hole of the required diameter. Bored piles offer the largest pile diameter up to 4 metres – and to a depth of 90m. The process involves repeatedly drilling, then withdrawing the drill to remove ground material from the pile, until the required depth is reached. After the pile base is cleared of excavated material, reinforcement steel is placed in situ and concrete is poured.
CFA piles, otherwise known as grout injected piles, are constructed by using a continuous auger to bore a hole of the required diameter –up to 1.2 metres and to a depth of up to 36m while injecting a special concrete mix or a sand-cement grout down the auger’s hollow stem while the auger is being extracted. This method avoids the need for temporary casings and is ideal for the construction of contiguous pile walls in soils that are not cohesive or have a high-water content.