Assessment And Prediction Of Surrounding Rock Stability Of The Cross Section Of Inclined And Main Tunnel In Ngong Hill Tunnel

According to the needs of the tunnel function,some tunnels have secondary wells such as inclined shafts,shafts and branch holes in addition to the main holes.The intersection between the main hole and the secondary hole forms the tunnel intersection.Stress concentration and large deformation often occur at the intersection of the tunnel,which seriously affects the stability of the surrounding rock of the tunnel because the tunnel intersection is a structural abrupt change,it is also a place where the force is abruptly changed.Therefore,it has a important theoretical and practical significance to strengthen evaluation and prediction of the stability of the surrounding rock of the tunnel intersection.The paper takes the excavation construction of the surrounding rock of the inclined shaft and the main section of the Ngong hill tunnel in the first phase of the Kenya railway from Nairobi to Malaba as the research object by the theory and method of field test,elastoplastic theory analysis,artificial intelligence technology,numerical simulation and etc.The evaluation and prediction stability of the surrounding rock of the tunnel intersection of the inclined shaft and the main section of the Ngong hill tunnel are studied.The main research results are as follows:(1)The stability of the intersection and non-intersection of the Ngong hill tunnel is evaluated by the monitoring data of surrounding rock deformation of the intersection and non-intersection of the Ngong hill tunnel and combined with the stability analysis and evaluation index of the surrounding rock of the tunnel: tunnel vault settlement,surrounding rock excavation stabilization time and surrounding rock deformation convergence speed.Both of the stability of the intersection and non-intersection meet the stability requirements of tunnel surrounding rock.However,the deformation of the vault and the horizontal convergence deformation rate of the deformation sequence around the surrounding rock have abrupt changes and the convergence time is relatively long.Further analysis and prediction are needed.(2)For the general tunnel surrounding rock deformation calculation theory,when the tunnel structure changes in the mutant part of the intersection surrounding rock deformation is considered less.The prediction formula of the surrounding rock deformation of the tunnel intersection based on the thin shell theory method is proposed.At the same time,the Mindlin formula is used to correct the surrounding rock deformation of the tunnel intersection of the inclined shaft and the main hole.The prediction of the surrounding rock vault and horizontal displacement of the tunnel intersection is compared with the actual monitoring deformation of the tunnel surrounding rock.By the theoretical analytical method,the prediction error of tunnel surrounding rock is small and it meets the requirements of tunnel surrounding rock deformation prediction accuracy.(3)In order to further analyze the surrounding rock deformation at the intersection of the tunnel,the numerical simulation analysis of the intersection of the tunnel section is carried out.The settlement and horizontal convergence deformation of the surrounding rock displacement of the intersection and non-cross section of the Ngong hill tunnel are simulated.The results of simulation is compared with the monitoring values of actual tunnel surrounding rock deformation.And the surrounding rock of the tunnel crossing section is in a more unstable state by comparing with the vertical stress,maximum shear stress and plastic zone of the two tunnels.At the same time,when studying the influence of blasting excavation dynamic load on the tunnel,comparing the vibration velocity and deformation of the surrounding rock of the tunnel intersection and non-intersection section,the vibration velocity and convergence deformation of the tunnel intersection section under the dynamic load are also larger and more dangerous.(4)For the problem that the surrounding rock deformation sequence of the intersecting tunnel contains the mutation point,the artificial bee colony algorithm and the ordered clustering method are used to search the mutation point in the deformation sequence of whole and part.And the mutant point in the dome settlement and horizontal deformation of the surrounding rock of the intersection of the Ngong hill tunnel occurred on the 15 th day after tunnel excavation.And there is no mutation in the surrounding rock sequence of the non-cross section.At this time,the maximum Lyapunov exponent method in phase space reconstruction is used to predict the deformation sequence after the sudden change point.At the same time,the exponential function is used to predict the deformation sequence of the non-crossing section surrounding rock without the mutation.Both prediction results satisfy the surrounding rock of the requirements of tunnel deformation prediction accuracy.