Abstract: The impacts of pile length on end resistance of jacking pile, surrounding earth pressure after unloading, and bearing capacity of double piles were investigated through indoor model test of jacking double piles in modera-tely dense sand. Pile length in this paper is defined as the total length buried in sand when pile jacking is finished.Results demonstrated that the pressed end’s resistance of both piles increased approximately linearly with the deepening of pile jacking; at depth 600 mm at the finish of pile jacking, the end resistance of pile 1^# (which is pressed firstly) was larger than that of pile 2^# (referring to the pile pressed later) by about 0.2%, indicating that the end resistance of two piles reached the extreme value at this depth. Given the same pile length, the surrounding earth pressure of pile 1^# before the sinking of pile 2^# was about 90% of that of pile 2^#; after the unloading of pile 2^#, the surrounding earth pressure of pile 2^# was about 2~3 times of that of pile 1^#, and in the meantime increased with the increasing of embedded depth of pressure gauge. However, at the same buried depth, the surrounding earth pressure of the two piles both decreased with the increase of pile length. The ultimate bearing capacity of pile in single pile test was higher than that of pile 1^# in double pile test by about 2%~5%, but such increment attenuated with the increasing of pile length. The ultimate bearing capacity of double pile system in no consideration of pile interaction was about twice times of single pile. The research findings are of engineering significance in further clarifying the interaction mechanism between piles and improving the bearing capacity of double piles.

Key words: pile foundation, jacked pile, end resistance, surrounding earth pressure, bearing capacity, interaction between pile and pile, model test