超大跨度隧道预应力锚杆——锚索协同支护机理

Combined Support Mechanism of Pretensioned Rock Bolt and Anchor Cable for Super-Large-Span Tunnel

探索运用1种新型支护形式预应力锚杆—锚索协同支护体系,在京张高铁八达岭长城站大跨过渡段跨度极大(32.7 m)且围岩质量极差(Ⅴ级围岩)的情况下保障隧道安全建设。通过现场监测,研究预应力锚杆—锚索的力学行为,结合现场围岩微震监测,分析预应力锚杆—锚索协同支护机理。结果表明:预应力锚杆、锚索的轴力均由初始预应力、预应力损失和被动支护力3部分决定;在隧道开挖过程中,锚杆和锚索的轴力演化过程可分为预应力快速损失、轴力波动和轴力稳定3个阶段,且预应力锚杆轴力沿自由段非均匀分布,锚杆自由段存在多个中性点;预应力锚杆锚固于浅层围岩内部,与被锚固岩体组成组合拱结构承担围岩荷载;预应力锚索锚固于深层围岩区域,调动深层围岩的承载力承担围岩荷载,并提高锚杆组合拱的稳定性;预应力锚杆—锚索协同作用实现了超大跨度隧道围岩的有效支护。

This paper explores the application of a new support form, i. e., the combined support system of pretensioned rock bolt and anchor cable, in the super-large-span tunnel of Badaling Great Wall Station of Beijing-Zhangjiakou high-speed railway to ensure the safe construction of the tunnel. The span of the large-span transition section is very long(32. 7 m) and the quality of the surrounding rock is extremely poor(Class Ⅴ).Based on field monitoring, the mechanical behaviors of pretensioned rock bolt and anchor cable are studied.Combined with the microseismic monitoring of surrounding rock, the combined support mechanism of pretensioned rock bolt and anchor cable is analyzed. Results show that the axial forces of both the pretensioned rock bolt and anchor cable are determined by the initial pretension force, pretension force loss and passive support force. During tunnel excavation, the evolution process of the axial forces of rock bolt and anchor cable can be divided into three stages: the rapid loss of pretension force, the fluctuation of axial force and the stability of axial force. Moreover, the axial force of the pretensioned rock bolt is non-uniform along the free length, and there are multiple neutral points in the free length of rock bolt. The pretensioned rock bolts are anchored inside the inner layer of surrounding rock and form a reinforced rock arch with the anchored rock mass to bear the load of surrounding rock. The pretensioned anchor cables are anchored in the deep surrounding rock area to mobilize the bearing capacity of the deep surrounding rock to bear the load of rock mass and improve the stability of the reinforced rock arch. The combined action of pretensioned rock bolts and anchor cables realizes the effective support of the super-large-span tunnel.