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

Abstract

When designing a piled raft foundation (PR) on clay, it is necessary to understand the timedependent 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.