TIME-DEPENDENT BEHAVIOUR OF PILED RAFT FOUNDATIONS ON SATURATED CLAY: EXPERIMENTAL INVESTIGATIONS

When designing a piled raft foundation (PR) on clay, it is necessary to understand the time-dependent behaviour of the foundation because stresses and strains in clayey ground will change for a long time after the construction, due to consolidation processes of the ground. This paper, therefore, aims to investigate the long-term behaviour of PRs through small-scale physical modelling. In the experiments, the model ground was prepared by consolidating a slurry mixture of Kasaoka clay and silica sand. The model foundations consisted of a square raft having a width of 125 mm and 4 or 9 piles having a length of 150 mm. For the loading test of PR, vertical load was increased by multiple steps and each load step was maintained to observe the long-term behaviour. The experimental results show that the piles effectively suppress the foundation settlement for relatively smaller loads. In the primary consolidation stage, the pile resistance increases with elapsed time while the raft resistance decreases. This is caused by the dissipation of pore water pressure and the corresponding increase of effective stresses of soils below the raft base. In the secondary consolidation stage, for the case of 4-pile PR, the resistances of both the raft and piles are stable although creep settlement continues. For the secondary consolidation stage of 9-pile PR, pile load reduces slightly under smaller applied loads and stable under larger applied loads. In general, the 9-pile PR reduces the foundation settlement significantly under smaller applied loads, compared to the 4-pile PR.