Abstract: Underground space supplement by excavation beneath constructed basement usually encounters the situation that existing and newly-created retaining piles form a double layered retaining structure. This is a new topic that traditional excavation design methods have not involved. This study uses large-scale model tests to investigate the influence of row spacing between the two pile layers on the pile displacement, the pile force and the earth pressure acting on the front-row piles. The analyses show that as the row spacing decreases, both the displacement and the bending moment decrease in the existing piles but increase in the supplementary piles, making the supplementary piles be the principal bearing elements. After the existing retaining piles end service, an abrupt increase in the displacement and bending moment of the supplementary piles is detected while the location of the maximum bending moment is shifted to be deeper. The unit earth pressure imposed on the existing piles gradually decreases as excavation proceeds. When the active limit state is reached, the active earth pressure approaches the distribution mode of the Rankine active earth pressure and its magnitude increases with increasing row spacing. Through the analysis and fitting of the active earth pressure data of each test, a prediction method for the active earth pressure on the existing piles is established with considerations of the spacing of pile rows, the soil friction angle, the pile-soil interface friction angle as well as the soil arching effect.

Key words: underground storey supplement, model test, retaining structure, displacement, internal force, earth pressure