Numerical and analytical modelling of pile-raft interaction in sands.

For many foundations in sands, a shallow raft alone is often able to provide an adequate bearing capacity; piles are then introduced in order to limit settlements to an acceptable level or to reduce the state of stress into the raft. It clearly follows the need to develop new methods that allow simplified piled raft designs. With this aim, the paper presents a numerical and analytical study of capped bored piles (rafts over a single pile) in sandy soils. Focus is given to the raft effect on pile capacity mobilization. Indeed, to better understand and quantify this effect, parametric analyses have been performed dealing with many capped piles embedded in different soil condition (loose/dense, fine/coarse sands). The results show that mechanical and physical soil properties influence this interaction, which becomes crucial in loose and fine sands. An analytical method, called 3M, is then presented to evaluate shaft capacity of piles underneath the rafts. Such a method is based on the fundamental mechanic behaviour of sands and keeps track of the major mechanisms occurring in the soil close to the pile during loading. It is very simple to be used and requires few, physically based parameters. In the paper, the 3M method is employed to modify the Burland’s method (1995), already available for the preliminary design of settlement reducing piles in clays, making it suitable for piled rafts in sands.