Research On Surrounding Rock Degradation And Deformation Control During Blasting Construction Of Tunnels Crossing Fault Fracture Zones

With the rapid development of urban rail transit,more and more tracks need to be built below the ground,which involves tunnel engineering construction issues.During the tunnel construction process,crossing fault fracture zones is an unavoidable problem.As a common method of tunnel excavation,drilling and blasting excavation will inevitably cause damage and deformation to the fault surrounding rock,and in severe cases,it will cause tunnel structural damage and instability of the surrounding rock,This will result in significant economic losses.In response to the above issues,it is necessary to conduct research on the deterioration and deformation control of surrounding rock during tunnel blasting construction through fault fracture zones.This thesis takes the Tongluoshan Tunnel of Chongqing Metro Line 15 Phase I project as the engineering background,uses a combination of theoretical research and numerical simulation methods,and uses finite element software ANSYS/LS-DYNA to establish a three-dimensional simulation model.The blasting parameters are optimized and analyzed from the spatial layout of the cut holes,radial decoupling coefficient,and directional blasting devices.The blasting load is calculated based on the cut hole blasting parameters and applied to the tunnel excavation contour,In order to explore the impact of different blasting construction methods on the dynamic response law of surrounding rock and lining when the tunnel crosses the fault fracture zone,and finally use the restart function of LS-DYNA to simulate the cumulative damage effect of surrounding rock under three excavation methods,in order to explore measures to control the deformation of surrounding rock.The main research results are as follows:(1)Through theoretical research on the mechanism of rock blasting and the impact of faults on blasting action,a rock blasting damage model was determined.Combining engineering geological and hydrogeological conditions,three blasting schemes for excavation methods were designed,laying a theoretical foundation for threedimensional simulation of fault tunnel blasting excavation.(2)Using finite element software ANSYS/LS-DYNA,a three-dimensional simulation model was established to verify and optimize the blasting parameters of the cut hole.Starting from the spatial position of the blast hole,radial decoupling coefficient,and directional blasting device,it was found that using oblique cut has advantages such as low drilling difficulty and good blasting cavity formation effect.By using decoupling coefficient,the effective stress can reach 75 MPa,which can break the rock and mainly form crack zones,Reducing the damage to the reserved rock mass,the simulation results of ordinary and slit charge show that the slit charge has a good energy gathering effect,making the rock mass damage concentrated around the slit mouth.(3)Calculate the blasting load based on the optimized cutting hole parameters,apply the blasting load to the tunnel excavation contour line,and simulate the dynamic response of the fault tunnel surrounding rock and initial support only by changing the excavation method under the conditions of equal excavation footage,blasting load,and blasting cycle number.From the vibration velocity,stress,and displacement curves of monitoring points at different positions of the initial support,it can be concluded that the changes in the arch crown and arch shoulder are the most significant.The vibration velocity,stress,and displacement curves are basically the same when using two step and three step excavation methods,with the exception of the CD method.The results of surrounding rock monitoring points indicate that when the explosion source is located within a fault,the presence of the fault greatly changes the dynamic response state of the surrounding rock.Compared with the V-level surrounding rock area,the vibration velocity and displacement of the monitoring points in the fault area increase,while the stress decreases.Especially at the junction of the V-level surrounding rock and the fault fracture zone,the vibration velocity,stress,and displacement undergo sudden changes.As the blasting excavation progresses,The vibration speed and displacement curves show a trend of first increasing and then decreasing,while the stress curve changes in the opposite direction,which is consistent with the attenuation law of stress waves.Through comparison,it was found that the dynamic response of the three step method and the two step method is the most obvious.The reason may be that on the same excavation surface,the larger the excavation area,the greater the impact on the dynamic response of the surrounding rock and initial support,while the CD method is the least affected.Therefore,it is recommended to use the CD method for blasting.(4)The restart method was used to calculate the cumulative damage of the surrounding rock of the three excavation methods.The results showed that there is a certain range of rock damage.Beyond this range,the lateral damage of the tunnel surrounding rock will no longer expand,only the damage along the excavation direction occurs.The rock near the excavation contour line is the most severely damaged,while the rock in the distance is affected by stress waves,and its internal original cracks expand Development deteriorates the performance of rock mass.Compared with the surrounding rock damage variable D of three excavation methods,the damage of the three step method is slightly greater than that of the two step method,and the damage of the CD method is the smallest.However,the damage degree of the excavation step meets the excavation requirements,but attention should also be paid to the problem of over excavation and under excavation.In summary,combined with numerical simulation results,optimizing the spatial position and charging structure of blast holes,using directional blasting technology,changing excavation methods,and adding support can achieve the goal of controlling the deterioration and deformation of surrounding rock.