This paper reveals several factors influencing the shape and the quality of the bottom concrete in cast-in-place concrete piles by model tests. A steel cylinder filled with a slurry was used to model a borehole where sediment so called "slime" settles on the bottom of the cylinder. Concrete placing follows by a tremie as practical manner. Through the experiment following conclusions were obtained, l) The shape of the concrete at pile bottom is deeply related to the soil type and stiffness of the sediment. 2) The effect of the thickness of the sediment on the compressive strength of the concrete at pile bottom is very small. 3) The stiffness of the sediment becomes small in the cases when the viscosity of the slurry is high. 4) The increasing stiffness of the sediment with depth is obtained by a sediment penetration test.
