Three-Dimensional Analysis of Stress Interaction between Closely-Spaced Bored Piles in Rock

Abstract

Large-diameter bored piles founded on rock are prevalent in Hong Kong. Where the piles are positioned at close spacing, there is a concern regarding the overlapping of bearing stress between adjacent piles that would overstress the rock formation. A conventional 45° stress-dispersion envelope is commonly adopted to estimate the stress distribution and assess the interaction of bearing stress. Some practitioners surmise that the overstressing condition could be avoided by restricting the foundation level of adjacent piles to below a 45° envelope projected upward from the deepest pile. However, the imposition of this envelope may force adjacent piles to be extended to deeper depths, which has a significant impact on the cost and complexity of the piling works. This paper presents a study that critically examines the interaction and stress distribution between closely spaced piles at different founding levels, with the aid of three-dimensional numerical analyses. The Hoek-Brown failure criterion was adopted in the analyses to characterize the engineering properties of the rock formation. The study findings reveal that the stressed zone beneath a pile base is more localized than conventionally assumed, and it is more accurate to adopt a stress-dispersion angle of 26.5° for estimating stress increments with depth. Numerical results also confirm that interaction of bearing stress is minimal and does not lead to an exceedance of the applied bearing pressure, even when piles are spaced as close as 0.5 m apart. The analyses are applicable to end-bearing piles and piles mobilizing shaft and end-bearing resistance. The study also assessed the effect of inclined rockhead surfaces on the mobilization of shaft resistance. Based on the study findings, improved design practices are recommended when placing piles at levels higher than adjacent piles, which include (a) eliminating the check on overlapping bearing stress and the 45° envelope used to restrict founding levels between adjacent piles, and (b) focusing on the stability of plausible wedge failure under the foundation load from closely spaced piles and the load-transfer effect on the structural capacity of closely spaced piles. These improved practices contribute to optimizing the cost and constructability of bored piles while safeguarding safety standards.