Study On Response Characteristics Of Surrounding Rock And Structures Of In-situ Surrounding Expanding Excavation Of Tunnel By Blasting Method

With the development of society,the demand for highway transportation capacity is also higher and higher.In the past,the construction of municipal highway tunnel mainly takes four lanes in two directions,increasing traffic flow makes the traffic pressure of this kind of tunnel unbearable.At the same time,the original tunnels have been damaged to varying degrees over the years.That makes the reconstruction of old tunnel needs become more urgent,has gradually become the focus of engineering tunnel reconstruction project,compared with other engineering,tunnel reconstruction project process is more complex,the surrounding environmental factors restrict more,these characteristics to tunnel construction and engineering project management has brought a lot of potential risks.There are many differences between the excavation project and the previous extralarge span tunnel excavation project.Due to the existence of the original tunnel,the distribution of surrounding rock stress in the new tunnel is very different,and it also involves the dismantling of the original tunnel lining.Since the operation of the original tunnel after the first blasting excavation,cracks in surrounding rocks have developed and damage has accumulated.Some tunnels still have leakage and cavitation,and the mechanical properties of surrounding rocks are seriously deteriorated.In some projects,closed traffic is not allowed during excavation construction.This kind of problems can not directly refer to the previous tunnel excavation experience,so it is of great practical significance to study the response characteristics of surrounding rock and surrounding important structures in the excavation project of tunnel in situ.In this paper,based on the excavation project of Loushan tunnel,the response characteristics of surrounding rock and surrounding structures of the tunnel were studied by drilling and blasting method in situ.Luosan Tunnel in Wenling City,Zhejiang Province was completed in 2003.It is an important node of Luzetai First Class highway,which is the main road connecting Taizhou and Wenling,and has a large traffic flow.The tunnel is a two-way four-lane separated tunnel,which needs to be expanded into a two-way eight-lane tunnel around the original site due to factors such as terrain and displacement.At the same time,in order to ensure the demand of social traffic,traffic is maintained in the adjacent tunnel during the reconstruction and expansion of the first tunnel.According to the field investigation,some sections of the tunnel are class ⅴ surrounding rock,with relatively developed cracks and weak bearing capacity of surrounding rock.In this paper,the homogeneous constitutive model of fractured rock mass is improved by combining theoretical analysis and laboratory experiments.Based on this,the response characteristics of surrounding rock and surrounding structures of in-situ excavation tunnel by drilling and blasting method are analyzed by numerical simulation,and verified by field monitoring.The main research contents and main achievements are as follows:(1)Based on the theory of linear homogenization of heterogeneous materials and the theory of continuous damage,the homogeneous elastic-plastic damage constitutive model of rock mass with fracture development is analyzed and derived under the assumption of isotropy.The constitutive model can be invoked in Flac3 D through the coupled correction of elastic and plastic damage.(2)The surrounding rock of Loushan Tunnel was sampled and standard specimens were made.The physical and mechanical parameters of surrounding rock were obtained through laboratory tests.The conventional triaxial compression test results of surrounding rock specimens show that the bearing capacity increases with the increase of confining pressure,and the elastic deformation stage extends with the increase of confining pressure.The results of triaxial compression test are compared with those of simulated triaxial compression.The correlation between simulation results and test results is low when confining pressure is low,and the deviation of simulation results is large when confining pressure is high.However,the homogeneous constitutive model of fractured rock mass considering fracture and accumulated damage has higher correlation and smaller deviation with the measured values under each confining pressure condition.(3)The response characteristics of surrounding rock and surrounding structures under different order of excavation are analyzed.The main factor affecting the stability of surrounding rock is the inconsistency of excavation progress on the left and right sides of the tunnel,and the continuous construction of upper and lower parts on one side magnifies the influence of this factor on the stability of surrounding rock.The uneven surface settlement and its influence range caused by different digging sequence are the main factors leading to the tilt of HV transmission lines.Different digging order has great influence on the vertical deformation of adjacent tunnel,but has little influence on the stress change process of adjacent tunnel lining structure.The middle and then the two sides,and the upper and then the lower parts are constructed first,so that the construction progress difference between the left and right sides of the tunnel expansion section is reduced,the temporary support structure can be closed into a ring faster,and the influence of the expansion construction on the surrounding rock stability can be greatly reduced.(4)The response characteristics of surrounding rock and surrounding structures under different excavation methods are analyzed.The settlement of tunnel vault and adjacent tunnel structure by using top arc excavation method does not meet the standard requirements.Due to the supporting effect of double straight wall and transverse support in horizontal and vertical directions,and the characteristics of the temporary support structure closed into a ring in each step of construction,the influence of this method on the tilt and deformation of the surrounding structure is in line with the specification.(5)The response characteristics of surrounding rock and surrounding structures under blasting are analyzed.Under blasting,the main factor affecting the peak vibration velocity and stress distribution of surrounding rock is the core distance,and the secondary factors affecting the peak vibration velocity of surrounding rock are slope reflection effect and cavity amplification effect.The tunnel diameter becomes larger after the rear excavation of the face.Influenced by the cavity amplification effect,the peak vibration velocity of surrounding rock increases first and then decreases with the increase of the distance from the blast core.The effect of elevation amplification is more obvious for the high-voltage transmission lines above the tunnel,showing that the peak vibration velocity component in the vertical direction increases with the tower height.Under blasting,the position of adjacent tunnel lining structure close to the explosion source is under the synergistic effect of maximum principal stress and shear stress.The shear stress is higher and the maximum principal stress is lower in the adjacent tunnel far from the explosion source than in the adjacent tunnel far from the explosion source,and the influence of shear action is larger.The maximum principal stress is higher and the shear stress is lower in the adjacent tunnel far from the explosion source.(6)The displacement deformation and blasting vibration of surrounding rock and surrounding structures were monitored on site,and the variation law of surface settlement,surrounding rock deformation and adjacent tunnel deformation with burial depth and time,the response characteristics of high-voltage transmission lines under excavation and blasting,and the attenuation law of blasting vibration in surrounding rock were analyzed.The monitoring results show that the simulated response characteristics of surrounding rock and surrounding structures are in good agreement with the actual situation.In the actual construction,the displacement,deformation and vibration velocity of surrounding rock and surrounding structures are within the control standard.According to the standard of electric power industry,the deflection deformation of the high-voltage transmission lines tower caused by excavation construction is very small and can be neglected.