大型地下洞室围岩最佳支护时间的判定

Judgment of Optimal Supporting Time of Surrounding Rock in Large Underground Cavern

对新奥法在大型地下洞室施工中的二次支护时间的现有判据进行分析,发现其考虑因素欠全面,主要以变形量或变形速率为主,洞室位移相对值不适应大型洞室.以围岩变形为研究对象,提出大型洞室围岩变形达到总变形的80%、洞室周边水平收敛速度小于0.2 mm/d、顶拱或者底板垂直位移速率小于0.1 mm/d、隧洞周边水平收敛速度以及拱顶或底板垂直位移速度明显下降、位移加速度应基本接近于0作为大型地下洞室最佳支护时间的判据.为了验证该判据的合理性,选择了向家坝大型地下洞室一个断面,按照实际开挖顺序,计算了不同开挖层的最佳支护时间.向家坝地下洞室该断面处围岩拱顶、不同开挖层上下游侧边墙最佳支护时间在8～18 d之间,较好地符合了向家坝实际支护时间.

Traditional judgment of the second supporting time in large underground cavern by the New Austria Tunneling method has its weakness in its limit to deformation or strain rate, and relative values of cavern displacement poorly adapted to large caverns. The deformation of surrounding rock was studied, and the optimal supporting time criterions of large-scale underground cavern was proposed to be when surrounding rock deformation of large cavern accounts for 80% of the total deformation, horizontal convergence speed of surrounding rock is less than 0. 2 mm/d, vertical displacement rate on arch top or bottom is less than 0. 1 mm/d, horizontal convergence speed of surrounding rock and vertical displacement rate on arch top or bottom is remarkably falling, the basic displacement acceleration velocity approaches 0. It was then applied to a section of large underground cavern of Xiajiaba as verification. During the process, different layers of a section of large underground cavern were calculated according to the actual excavation sequence. The optimal supporting times of surrounding rock of vault and downstream side walls of different excavations on this section fell between 8 to 18 days, in consistence with the actual supporting time of the cite.