Research On Surrounding Rock Deterioration And Deformation Control Of Soft Rock Tunnels With Small Clear Distance During Drilling And Blasting Construction

In recent years,the construction of underground projects in our country has made rapid progress.Small clear-distance tunnels have been widely used because of their unique advantages in saving underground space resources and conducive to the overall linear planning of the project.The stress situation of the surrounding rock and supporting structure of the small clear distance tunnel is more complicated.Due to the small clear distance between the tunnels,when the small clear distance tunnel is excavated by the drilling and blasting method,the fissures in the surrounding rock continue to expand and extend,and the damage to the surrounding rock continues to increase,The stability of surrounding rock gradually deteriorates.At the same time,the surrounding rock stress carried by the supporting structure will continue to increase,causing instability and failure of the supporting structure.Therefore,it is of great practical significance and reference value to carry out research on the deterioration and deformation control of surrounding rock in the construction of soft rock tunnels with small clear distance by drilling and blasting.Through theoretical analysis,numerical simulation and field monitoring and measurement,this paper uses ANSYS/LS-DYNA software to establish a three-dimensional model,and drills and blasts tunnels with small clear distances under different surrounding rock grades,different clear distances,and different excavation footages.The calculation and analysis are carried out;the cumulative damage of the surrounding rock by the drilling and blasting method is simulated and the blasting parameters such as different charge coefficients,blast hole spacing,and charge structure are analyzed and optimized.Finally,combined with small-distance tunnel vibration monitoring and rock sonic velocity testing,representative monitoring data are selected for statistics and analysis.The main conclusions are as follows:(1)Comprehensively consider the influence of surrounding rock grade,net distance,excavation footage and other factors on the dynamic response of tunnel blasting,and obtain its change law through numerical simulation analysis.The results show that: the vibration velocity,stress,and displacement have a good correlation in time and space,and are negatively related to the mechanical properties of the surrounding rock;when the net distance is less than 1.5B(B is the width of the tunnel),consideration should be given to optimizing the drilling and blasting construction plan to set shock absorption If the lithology is poor,the surrounding rock on the front side of the explosion(ie,the middle rock)should be reinforced;when the clear distance is less than 1B,the drilling and blasting construction cycle excavation footage of the small clear soft rock tunnel shall be controlled at the clear distance Less than 1/3 is more appropriate.(2)Established a 12 times drilling and blasting method construction surrounding rock deterioration damage model for a tunnel with a small clear distance,and analyzed the relationship between the damage variable D of the preceding and following tunnels and the number and distance of drilling and blasting,and obtained the evolution of surrounding rock damage.law.In 1～6 blasts,as the number of blasting increases,the increase rate of the damage variable D of the advance hole gradually slows down.During 7 to 12 blasting times,the damage to the surrounding rock of the advance tunnel was significantly worsened,and the damage became more serious as it was closer to the excavation contour line.The damage variable D on the tunnel rock wall reached 0.8.The damage pattern of the trailing hole is basically the same as that of the leading hole1～6 times;the larger the drilling and blasting center distance,the smaller the impact of blasting.When the distance from the monitoring point to the tunnel face exceeds 1.5times the hole width,the cumulative damage variable D tends to be stable.The accumulation of surrounding rock damage is an irreversible process,showing an obvious "step effect".When the excavation method of each blasting cycle is the same,as the number and distance of drilling and blasting increase,the damage range of the surrounding rock remains basically unchanged,and the damage degree increases.(3)The influence models of different charge coefficients,blast hole spacing,and charge structure on retained rock mass damage were established to optimize the blasting design of surrounding holes.The analysis shows that the method of using uncoupled charges,reducing the distance between the surrounding holes,and using the slotted charge to control the blasting can improve the efficiency of explosive rock breaking,reduce the damage of blasting to the retained rock mass,so as to control the deterioration and deformation of the surrounding rock.(4)Based on the Chongqing Railway East Loop Tunnel Project,the actual measured value and the simulated value are compared and analyzed.Because the material parameters selected in the modeling process are different from the actual engineering geological conditions,the calculated value is too large.For the sake of calculation safety,it shows that the numerical simulation is more reliable.