Abstract: Secondary cleaning must be made for the boreholes of castinsitu bored piles in soft subsoil before underwater concrete is cast, because the thickness of the mucks on the bottom of the boreholes always exceeds the limit values specified in relevant specifications. Taking the construction of the bored piles of a tunnel in Shunde, China as an example, the author determines to adopt the slurry replacement method in the secondary cleaning of the boreholes of the bored piles. The author studies on the optimum duration of the secondary cleaning of the boreholes and achieves the empirical formula to calculate the secondary borehole cleaning duration. This paper can provide guidance for the secondary cleaning of boreholes of castinsitu soft bored piles in soft subsoil in the future.

Key words: subsoil, bored pile made by normal circulation, secondary borehole cleaning