Disciplines

Bored CFA contiguous & secant pile

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These wall types are constructed as cast in situ pile walls as either conventional bored piles with kelly bar rig (with or without drilling support fluid) or alternatively as CFA piles.

Contiguous pile walls feature piles that are almost touching (ie. gaps between piles of 50-100mm). They are normally adopted in cohesion-less soils where the soil cannot temporarily arch across large spans between piles (as required in conventional soldier pile arrangements). Typically the basement excavation level is not below the ground water table. In the final basement wall condition, the contiguous piles would normally be shotcreted to prevent any loss of retained soil between the piles over the life of the structure.

Secant pile walls rely on the interlocking or overlapping arrangement of piles to achieve a higher level of soil retention than contiguous or soldier pile systems. Following the construction of a guide wall at working platform level a series of primary piles (typically unreinforced and cast with weak concrete or cement-bentonite mix) are installed, then secondary piles (reinforced with high strength concrete) overlap the primary piles. Typically the final basement level is below the groundwater table and the soils are soft/loose and/or cohesion-less.  In the final basement condition the secant piles are shotcreted to prevent any degradation of the primary piles over time and reduce the likelihood of inflows at the pile to pile interface. 

Whilst both contiguous and secant pile walls are common in basement construction, they are also frequently adopted in civil applications such as tunnel portals and deep shafts for ventilation and services.

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