四车道大跨度浅埋黄土隧道开挖方案数值分析

为保证四车道浅埋黄土隧道的施工安全,优化施工方案,采用数值模拟手段对大断面黄土隧道开挖常用的三台阶七步开挖、中隔壁、交叉中隔壁和双侧壁4种方法进行系统的对比和分析。结果表明:(1)四车道浅埋黄土隧道与普通的两车道隧道变形存在较大差异,拱顶下沉显著增大,水平收敛较小且拱脚有向外挤出的趋势,故变形监测应以拱顶下沉为主;(2)地层竖向位移随深度的增加线性增大,距拱顶4m范围内呈指数型增大,且与洞周形成塑性区有关;(3)围岩应力均未超过其强度极限,初期支护应力较大,起到了主动承载和控制围岩松弛的作用,其与围岩共同承担大部分荷载,二次衬砌应力普遍较小,可为隧道结构提供安全储备;(4)变形控制要求严格的地区推荐使用双侧壁法,允许较大变形的情况优先采用三台阶七步开挖法。结论可为黄土隧道设计和施工方法的选择提供理论依据。

Numerical analysis on excavation scheme for four-lane large-span shallow-buried loess tunnel

To guarantee the construction safety of four-lane shallow-buried loess tunnel and optimize construction schemes, four methods commonly used in large-section loess tunnel, named as three-bench seven-step excavation method(TSEM), central diaphragm, cross diaphragm,and double side-drift method (DSDM), were systematically compared and analyzed using the numerical simulation method. Compared with the common two-lane road tunnel, crown settlement significantly increases, horizontal convergence is relatively small and the arch foot has the tendency to outward extrusion deformation, so crown settlement should be the focus of deformation monitoring. The stratum vertical displacement increases linearly with the increase of depth,but it shows exponential increases within 4 m away from the vault,which is closely related to the formation of plastic zone around the hole. Surrounding rock stress does not exceed the ultimate strength, stress of primary support is relatively large and it bears the most loadings coupled with surrounding rock, and loosening and excessive rock deformation must be controlled. Stress of secondary lining is relatively small and it provides safety storage. DSDM is recommended for the strict deformation requirement, giving priority to TSEM for allowing the large deformation. Conclusions can provide theoretical basis for the design and construction methods of loess tunnel.