An investigation into the vertical axial capacities and groundwater cut-off capabilities of secant pile walls

Secant pile walls are increasingly becoming popular groundwater cut-off systems of choice on underground construction sites, as they offer a number of advantages over traditional sheet piling and diaphragm walls. However, some elements of uncertainty surround the design and construction of secant pile walls. Vertical and horizontal installation tolerances are major issues, especially in the case of multi-level basements, as required tolerances are difficult to achieve with deep piles associated with deep basements. In addition, while conventional foundation piles are routinely statically or dynamically load-tested, hard piles in secant pile walls are rarely axially tested and this does not allow for the routine verification of existing design methods and assumptions. This paper presents the design, construction and testing of the secant pile wall on a 3-4 level basement construction scheme in Central London. Simple empirical correlations for estimating the geometry of secant pile walls for effective groundwater cutoff are derived and validated. An alternative approach for estimating the vertical capacities of secant pile walls is also presented and compared with a common routine method in the industry. The alternative method is shown to be less conservative than the common industry approach, while predictions are also in good agreement with results of a recent series of model tests on perimeter pile groups.