关键词: 盾构隧道, 地下结构, 深基坑开挖, 隧道配重, 板凳桩, 保护技术

Abstract: The excavation of deep foundation pit (1.34 m from the top of the tunnel) in water-rich round gravel stratum often brings risks of foundation rebound, stress release, tunnel uplift, pipe cracking, structural leakage, etc., for the shield tunnel below. To solve these problems, the rebound amount at the bottom of foundation pit and tunnel foundation is calculated theoretically. As the foundation rebound amount caused by foundation pit exvacation is offset by the settlement caused by loaded pressure, the loading weight per running meter inside tunnel is calculated. And then a 3D numerical analysis model using MIDAS GTS finite element software is built and three working conditions, i.e., the method without any control measures (condition 1), that with bench pile foundation (condition 2), and that with bench pile foundation and loading pressure inside tunnel (condition 3), are simulated respectively. The results show that: (1)In conditions 1 and 2, the maximum uplifts are 16.8 mm and 12.5 mm, which is close to the theoretical calculation result (14.95 mm). (2)In working condition 3, the average uplift at tunnel bottom is 4.7 mm, which is very close to the structural difference after theoretical calculation (3 mm).  The method in working condition 3 is adopted in practical engineering, the vertical displacement of tunnel vault and invert is 8.46 mm and 3.16 mm respectively, and after bench pile constructed, the uplift of tunnel vault and invert is 7.64 mm and 2.56 mm respectively, the differential settlement of structure is 2.96 mm, which can meet the requirements of standard. It is concluded that: (1)Internal loading is the best way to control uplift. (2)After the foundation pit excavation, the upper part of the tunnel is free from load, its internal load and extrusion on both sides cause the tunnel to become a "vertical ellipse" . (3)Bench pile foundation has limited isolation effect, but if it rides cross the upper part of shield tunnel, the disturbance from upper structure construction and highspeed railway operation can be effectively reduced. 

Key words: shield tunnel, underground structure, deep foundation pit excavation, tunnel distribution weight, bench pile, protection technology