Load transfer from a raft to a pile with an interposed layer

The load transfer mechanisms between a pile and a raft with an interposed granular layer have been studied by means of centrifuge physical model tests. The tests were carried out with an instrumented single model pile in two configurations: in direct contact with the raft and separated from the raft by a coarse granular layer. As the model tests show, a granular layer beneath a raft changes the pile shaft behaviour significantly compared with that exhibited by a pile in contact; it allows downward relative raft-pile and soil-pile displacement and the consequent development of negative skin friction around the upper pile shaft perimeter. Hence, the raft load is partially transferred to the pile through the pile head and partially through the negative skin friction of the upper shaft. The contact pile reduces the raft stress on the soil, and then a piled raft stiffness results that is close to that of a free-standing pile up to yield load and close to that of an unpiled raft after the yield load has been reached; the non-contact pile increases the piled raft stiffness over the whole investigated settlement range with respect to the unpiled raft.