This paper presents model tests of slurry borehole wall stability at the borehole excavation for enlarged-bottom cast-in-place piles. The tests are performed to examine how the enlarged-bottom forms and the enlarged-bottom angles have effects on the stability of the surrounding ground of enlarged-bottom slurry borehole walls. This paper elucidates that it is advantageous with the bell type enlargedbottom form and the smaller enlarged-bottom angle to stabilize the slurry borehole wall.
Enlarged-bottom cast-in-place piles are effective to obtain a larger end-bearing capacity because of their enlarged diameter at the bottom where the bearing capacity is generated, comparing with usual cast-in-place piles whose diameter is uniform. In recent years, the enlarged-bottom cast-in-place piles are increasingly applied as the foundation pile for large-scale constructions of the urban development especially at seaside areas. Most Japanese constructions provide for the applied pile diameter of 900mm- 3000mm, the enlarged-bottom diameter of 1200n~n-4100n~ and the enlarged-bottom angle of under 12° for the enlargedbottom cast-in-place pile. The enlarged-bottom cast-in-place piles are characterized with their enlarged-bottom form, which is performed by the reverse-circulation method using slurry. The reliability of this method depends much on the stabilizing of the slurry borehole wall when the bottom is being enlarged. Most of former papers on the slurry borehole wall s t a b i l i t y at borehole excavation presented the study on the borehole excavation with an uniform pile diameter. The slurry borehole wall s t a b i l i t y of the enlarged-bottom at the borehole excavation has been hardly studied, and remains the unknown mechanical behavior. Consequently, the propriety to apply the enlarged-bottom borehole excavation can only be judged from construction experiences at present.
