MODELLING OF PILED RAFT FOUNDATIONS IN SAND

A piled raft is a composite foundation in which the piles are used as settlement reducers and they share, with the raft, the load from the superstructure. The applied load is transferred from the raft to the shallow soil and to the pile heads, and from the piles it is diffused through the shaft and the base to deeper soil. The pile–raft and pile–pile interactions represent the distinctive aspect of the piled raft foundations since they modify the load–bearing behaviour of each foundation component, compared to an analogous isolated element, thus determining the overall foundation behaviour. The main aim of this thesis is to highlight the effects of the raft–soil–pile interactions on the resistance and stiffness of axially loaded piled raft foundations in sand. A series of centrifuge tests on models of rigid circular piled rafts in loose saturated sand has been performed to this end, employing both non displacement and displacement piles. The raft settlement and the load transmitted to the pile heads and bases were monitored during the tests, which also included unpiled raft and isolated pile tests. The test results have been analysed in terms of bearing capacity and stiffness; the former according to a load efficiency method, the latter by comparing the values obtained from centrifuge tests with those evaluated through a simplified analytical method. ii Modelling of Piled Raft Foundations in Sand D. Giretti In order to clarify the effect of the pressure transmitted by the raft to the soil on the behaviour of a single capped pile, some of the centrifuge tests were simulated via finite element numerical analyses, using an elasto–plastic strain hardening constitutive model for the sand. The geometry and the dimensions of the numerical models corresponded to those of the physical ones and the simulations were carried out applying an accelerated gravitational field to the mesh. The influence of a granular layer, interposed between the raft and the pile heads, on the load transfer mechanism has also been analysed, through an additional series of centrifuge tests which was performed on square rigid raft models on displacement piles in dry dense sand.