Three-dimensional numerical analysis of the stress transfer mechanism of XCC piled raft foundation

An X-section cast-in-place concrete (XCC) piled raft is a new type of foundation that uses piles with X-shaped cross-sections. Compared to a traditional circular cast-in-place concrete (CCC) pile, an XCC pile of the same cross-sectional area has a larger side resistance due to its larger cross-sectional perimeter. Although the pile capacity and load transfer mechanism of the XCC piled raft have been studied, the influence of this X-shaped geometry on the stress transfer between pile and surrounding soil is still not fully understood. To investigate the effects of the cross-sectional geometry, three-dimensional numerical analyses are conducted on an XCC and a traditional CCC piled rafts using the finite element method. The numerical results are verified by a field test of an XCC piled raft. Computed results reveal that lateral soil arching develops to a distance of approximately twice pile diameter surrounding the XCC pile, which can be demonstrated by the rotation of the principal stresses, leading to a non-uniform effective normal stress across a given cross-section and along the pile depth. The magnitude of the unit side resistance that acts on the concave surfaces of the XCC pile is up to twice the magnitude of the unit side resistance that acts on the flat surfaces. For a given applied load, the total side resistance mobilised on the XCC pile is usually larger than the total side resistance of the CCC pile, by a factor that ranges from 0.5 to 10, depending on the pile depth. Therefore, approximately 66% and 46% of the applied load is carried by the XCC and CCC piles respectively, and simultaneously, approximately 45% and 24% of the applied load is taken by the side resistance of the XCC and CCC piles, respectively. The larger effect of the XCC piled raft contributes to the cross-sectional geometry, which results in a larger perimeter and arching effects.