Modelling the Behaviour and Efficiency of Minipile Groups in Clay

Minipile groups have been used to create high load capacity foundations especially where significant space restrictions exist during construction. The interest in such foundations, particularly when the piles are arranged solely around a perimeter, initiated a research project in which geotechnical centrifuge model tests were used to investigate the key geometric parameters affecting the load-settlement behaviour and efficiency of groups of bored, slender, high aspect ratio piles. Circular and square perimeter pile groups were tested along with conventional grid groups. Two modes of failure were observed: either the individual piles pushed into the ground with no obvious settlement of the surrounding soil or as a block with the soil contained within the outer ring of piles settling by the same or similar amount as the piles. The change from block failure to individual pile failure generally occurred at a pile spacing of approximately 2 pile diameters. A complementary numerical modelling study gave insight into the pile-soil interaction and in particular the patterns of displacement as failure changed from individual pile to block mode. Overall, a grid group arrangement was found to be less efficient in terms of load carrying capacity than a perimeter group and the inclusion of a single target pile within a perimeter group could further enhance the group efficiency.