Drilled pile walls are constructed as contiguous pile walls (no requirement of resistance to groundwater) or secant pile walls (when resistance to groundwater is required) using the CFA technique (470mm continuous auger) or the cased CFA technology (500mm). Pile walls are usually installed as retaining structures and support systems for deep foundations, retaining elements and at the same time load-bearing walls and deep foundations for engineering constructions (e.g. walls and foundations of underground storeys, stand-alone and framed bridgeheads, etc.). Contiguous pile walls are mainly constructed using the CFA technology and consist of reinforced piles (soft – reinforcement cage; firm – steel section). The dimension of the gap between the piles typically ranges between 50 and 150mm. A permanent wall can be created, taking the form of a structural concrete facing wall tied to the piles or sprayed concrete.
Secant pile walls can be constructed using the CFA (1)(2) or the CCFA technology (1)(2)(3), consisting of reinforced piles with the gaps between them filled with:
1. columns made of low-strength material of 1-3MPa (concrete or mortar, usually with bentonite), spaced approx. 620-670mm for drilling diameter of 470-500mm. This solution ensures resistance to groundwater (temporary support), but does not guarantee permanent water resistance. In order to ensure permanent water resistance, additional liners or sealing agents are required,
2. columns having a compressive strength of 10-20MPa and prolonged binding time (56-day strength instead of 28-day strength), spaced 700-750mm for a drilling diameter of 470-500mm. This solution ensures long-term water resistance, provided that a minimum overlap of 25mm is provided,
3. full-strength concrete piles with prolonged binding time, spaced approx. 400mm for a drilling diameter of 500mm. This solution also ensures long-term water resistance and is widely used as a support system for deep excavations with a pile wall constructed on circular plane, since waling beams and/or struts are not required.
The primary piles/columns cast first are called female or soft. The reinforced load bearing piles are drilled to intersect the female piles/columns and are called male or hard.
Cantilever pile walls can be used for protecting land faults up to 5m high or, if using anchors or struts, up to 20m high. Secant pile walls allow for protecting land faults up to 5-6m or, if using anchors or struts, up to 25m high. Standard rake of installed pile wall does not exceed 1:75, however, under special monitoring the rake can be reduced to 1:125, and in some cases, even to 1:200 (for secant pile walls made of hard piles and installed using the CCFA technology).
NOTE! Special attention should be paid when designing and constructing pile walls using the CFA techniques in sandy soils above the groundwater level due to the risk of a sudden outflow of batched water from the shaft of the constructed piles, which may cause problems with reinforcement installation. In such conditions it is advisable to apply the cased CFA technology.