Group of piles in sand: interaction effects from centrifuge modelling tests

The behavior of piles within a group is influenced by complex soil–structure interactions between the elements of the foundation and the subsoil. Two main interactions may take place in a group of pile in sand: (i) the decrease in the pile reaction modulus (i.e. the load–settlement ratio), (ii) the change in the pile bearing capacity. The increase in settlement of an individual pile in a group is due to the superimposition of the deformation fields of the neighboring piles (i.e. group effect). The variation in the ultimate capacity of a pile in a group can be ascribed to the overlapping of the stress field around the pile. These effects depend on installation method, on the sand state, on the load level, on the group dimensions and on the location of the pile within the group. This paper presents and analyses the results of centrifuge tests on groups of non–displacement piles in loose saturated sand, carried out to compare the load–settlement behavior and the bearing capacity of the piles in a group with that of an isolated single pile. The testing program included the models of the isolated single pile and groups of 3 and 7 piles with variable spacing. The adopted geometrical scaling factor was N = 100, the piles were close–ended and free headed. Some of them were instrumented with load cells to measure the axial load transmitted to each pile and to estimate the load transmitted through the shaft and the base.