川滇金沙江大桥重力式锚碇非闭合式拱形咬合桩基坑支护体系设计

Design of Unclosed Curved Shoring System Composed of Interlocking Piles in Foundation Pit of Gravity Anchorage of Jinshajiang River Bridge on Border of Sichuan and Yunnan Provinces

沿江高速川滇金沙江大桥主桥为1060 m单跨双塔钢桁梁悬索桥,云南岸采用重力式锚碇(基础平面尺寸65.0 m×69.5 m,最大开挖深度约29.2 m)。受锚碇基坑坡面起伏影响,不同开挖面土压力差异巨大,各开挖面无法通过荷载环向效应实现轴向推力自平衡,故提出一种适应山区陡峭地形的非闭合式拱形咬合桩基坑支护体系(由拱形咬合桩、角点抗力桩、压梁及预应力系杆索组成)。该体系仅在东侧和北侧设置基坑支护结构,通过咬合桩拱形效应将坑外土压力转化为拱轴方向推力,最终由设置在西北、东北及东南侧的角点抗力桩及桩后被动土压力共同承担。为验证结构安全性,对基坑边坡稳定性、支护结构内力及抗力、支护结构刚度进行计算分析。结果表明:最不利工况下,基坑边坡稳定状态良好,支护体系抗力有较大富余,结构刚度满足要求。该基坑开挖历时35 d,开挖过程平稳顺利,现场实测支护结构最大水平位移40 mm,实测值与计算值变化趋势基本一致。理论与实践表明,该新型支护体系结构安全可靠,可为山区桥梁深基坑建设提供新思路。

Situated on the border of Sichuan and Yunnan Provinces,the Jinshajiang River Bridge carrying a section of the Expressway winding along the Jinshajiang River is a two-tower suspension bridge with a main span of 1060 m,the stiffening girder consists of steel trusses and the hanger cables are only arranged in the main span.The Yunnan-side anchorage is a gravity anchorage measuring 65 m by 69.5 m,with a maximum excavation depth of 29.2 m.The uneven slopes at the location of the foundation pit imposed great challenges for the excavation,specifically remarkable soil pressure differences among different excavation faces,and none of these excavation faces could achieve a self-balancing condition of the axial thrusts through load ring effect,to address these issues,a type of unclosed curved interlocking piles shoring system(similar to an arch)is proposed,which consists of interlocking piles rowed along the winding of the slopes,force resistant piles in corners,compressed beams and prestressed tie cables.By using this system,shoring is only required in the east and north parts,the pressure of soil outside the foundation pit is transformed to thrust along the axis of the interlocking pile line via the arching effect,consequently shared by the force resistant piles in the northwest,northeast and southeast corners.To verify the safety of the shoring system,multiple parameters were analyzed,including the stability of the slopes at the foundation pit,internal forces and resistance of the shoring and the rigidity of the shoring structure.It is shown that under the most unfavorable loading conditions,the stability of the slopes is sound,there is a large safety margin of the resistance of the shoring system,and the rigidity of the shoring system met the requirements.The foundation pit excavation was carried out smoothly and took 35 days to complete,the maximum measured horizontal displacement of the shoring structure was 40 mm,the variation of the measured and calculated values agreed well,proving that the proposed shoring system is safe and reliable,which provides additional insights into the deep foundation pit construction in mountainous area.