既有?新增排桩双层支挡结构开挖模型试验研究

在既有地下室以下增设地下空间，常面临既有支护桩与新增支护桩构成双层支挡结构的开挖工况，这是传统开挖设计方法未涉及的新问题。通过大比尺模型试验，以内外双层支护桩排间距为主要参数，研究其对支护结构性能演变、排间土压力的影响。试验结果表明，随排间距减小，新增支护桩位移与弯矩逐渐增大，由嵌固作用转变为主要受力结构。开挖至既有支护桩失效退出工作后，新增支护桩位移与弯矩突增，最大弯矩点下移。既有支护桩背单位土压力随开挖逐渐减小，达到主动极限状态后主动土压力随排间距增大而增大，并接近朗肯主动土压力分布线。通过各试验桩背主动土压力数据分析拟合，建立了以排间距、土体内摩擦角、桩土摩擦角为参数并考虑土拱效应的既有支护桩背主动土压力预测方法。

Model excavation tests on double layered retaining structure composed of existing and supplementary soldier piles

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.