浅埋超大跨隧道出口偏压段双层初期支护承载特征研究

Load-Bearing Characteristics of a Double-Layer Primary Support in Asymmetrically-Pressurized Entrance Section of a Tunnel With Shallow Cover and Super-Large Span

为研究双层初期支护在开挖跨度达26.3 m、拱顶覆土约10.6 m的下北山2号浅埋超大跨4线隧道出口偏压段的工程应用效果,采用数值模拟方法比较单层初期支护和双层初期支护方案的承载特征差异,采用现场实测方法研究出口偏压段双层初期支护的受力特征。研究结果表明:1)浅埋超大跨隧道在偏压侧的右拱脚~右拱肩范围内支护结构受力偏大,存在局部应力集中,采用双层初期支护技术时的结构承载安全性和对围岩拱顶沉降的控制效果均优于单层初期支护技术;2)出口偏压段的实测水平收敛最大值为18.5 mm,荷载偏压侧的围岩沉降值达49.5 mm,二次支护施作后在拆撑施工阶段能有效稳定围岩变形;3)由双层初期支护受力的实测结果可知,传递到二次支护上的接触压力与对应位置围岩/一次支护接触压力之比为16%~33%,荷载偏压侧的右拱脚及右拱肩位置的二次支护轴力约为一次支护轴力的15%,通过二次支护的施作补强了一次支护对偏压侧的承载作用,采用双层初期支护技术提高了浅埋超大跨隧道出口偏压段支护结构的安全储备。

In the construction of the Xiabeishan No.2 tunnel featuring a shallow cover depth of 10.6 m and an impressively wide span of 26.3 m,a novel approach is adopted to enhance its structural integrity.Specifically,a double-layer primary support system is implemented in the entrance section where the tunnel experiences asymmetric pressure.The effectiveness of single-versus double-layer primary supports is investigated through numerical simulations.Furthermore,the performance of the double-layer primary support is examined in real-world conditions via field measurements.The following results are obtained in this study:(1)In the area stretching from the right arch foot to right arch shoulder on the side facing asymmetric pressure,the support structure undergoes substantial stress,with remarkable local stress concentrations.Here,the double-layer primary support system offers enhanced load-bearing safety and has more effective control over crown deformation than its single-layer counterpart.(2)Field measurements indicate that the maximum horizontal convergence of the asymmetrically-pressurized entrance reaches 18.5 mm.Additionally,the settlement of the surrounding rock at the crown on the pressurized side measures 49.5 mm.These findings suggest that secondary support plays a critical role in stabilizing the deformation of the surrounding rock during the dismantling stage of the primary support.(3)The contact pressure transmitted to the secondary support accounts for 16%-33%of the total at the respective locations.Moreover,the axial force exerted on the secondary support at the right arch foot and shoulder on the asymmetrically-pressurized side is 15%of that on the primary support.This underscores the vital role of secondary support in enhancing the load-bearing capacity of the primary support under asymmetrically-pressurized conditions.In conclusion,the adoption of double-layer primary support technology substantially improves the structural safety of the entrance section in the Xiabeishan No.2 tunnel,which faces asymmetric pressure owing to its shallow cover and wide span.