富水圆砾地层中，上覆结构深基坑开挖（基底距洞顶1.34 m）可引起下卧既有盾构隧道顶部地基回弹、应力释放、隧道隆起、管片开裂漏水等重大工程风险。针对此问题，首先, 计算出基坑底及隧底地基回弹量，通过隧道内加载反压使隧底地基产生沉降与基坑开挖后隧底地基回弹相抵消，从而计算出隧道内每延米加载质量。然后，利用 MIDAS GTS 有限元软件建立三维数值分析模型设计了3种工况，发现工况1不采取措施、工况2采取板凳桩基时，隧底最大隆起16.8、12.5 mm与理论计算14.95
mm接近；工况3采取板凳桩基+隧道内压载措施时，隧底平均隆起4.7 mm与计算压载后的结构差异变形量3 mm接近。按工况3实施后隧道竖向位移拱顶8.46 mm、隧底3.16 mm，完成“板凳桩”后隧道拱顶隆起7.64 mm、拱底2.56 mm，结构差异沉降最大2.96 mm，满足规范要求。得出结论： 1）隧道内压载对控制其隆起效果最佳； 2）基坑卸载隧道顶部基本临空后隧道受“内部压载、两侧挤压”整体呈“竖椭圆”趋势； 3）板凳桩基隔离效果不明显，但形成“板凳桩”后整体骑跨在盾构隧道上方彻底解决了上覆结构施工期及后期高铁运营动载对地铁隧道可能造成的重大影响。
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
highspeed railway operation can be effectively
reduced.
