The Study Of The Distance Between Primary Support And Tunnel Face Based On The Damage Of The Rock Mass

In the tunnelling construction,the drilling and blasting method is widely used due to its advantages of economy,high efficiency,and adaptability.However,due to the restrictions imposed by the construction space when the support was immediately following the construction of the face,it was inevitable that the surrounding rocks would be over-excavated or undercut under the conditions of the existing drilling tools,it will resulting in greater economic losses.In the case of a high grade of surrounding rock and good integrity,the initial support lags behind the face by a certain distance,which can not only control the over-under-excavation of the surrounding rock,but also obtain a good smooth blasting effect and make full use of the surrounding rock.The self-supporting ability to reduce support costs is of great benefit to the rational and efficient construction of tunnels.This paper studies the stability of the tunnel when the initial support stays behind the face at different distances under the conditions of surrounding rock damage,using the Jiangshuiquan Tunnel as the engineering background.Using sound waves to detect the disturbance damage of surrounding rock after tunnel blasting excavation,the damage range of surrounding rock is about 40cm.Combining the timing of tunnel support with the distance between the initial support and the lagging face,the FLAC3D numerical simulation was used to analyze the displacement,stress,and plastic zone size of the surrounding rock under different initial branch lag distances.Under this engineering geological condition,it will came to a conclusion that with the increase of the distance between the face and the face of the initial support,the volume of displacement of the surrounding rock and the volume of the plastic zone increase slightly,the stress of the surrounding rock decreases,and eventually it tends to be stable.According to the law of change,it can be obtained that the surrounding rock can be self-stabilized when the initial support is 9m behind the lagging face.For the sake of safety,two excavation cycles of the initial postponed face are carried out on site and monitoring of the surrounding rock pressure,concrete strain and steel arch strain are performed.By analyzing the blasting effect and test data,the light explosion effect is good when the two excavation cycles of the first hand face are delayed.The semi-porosity rate of the light burst hole is more than 90%,and the over-excavation of the surrounding rock is controlled effectively.The spray volume is reduced from 300%of the original design value to 180%to 190%.The stress of the surrounding rock on the supporting structure is effectively reduced,and the stress on the surrounding rock of the construction arch roof immediately below the initial support is reduced by 60%.,greatly increase the safety reserve of tunnel support.When the initial support must immediately follow the face of the hand,a long-short-hole smooth blasting technique is proposed and field tests are performed to obtain a good light explosion effect.The over-under-excavation problem of surrounding rocks is effectively controlled,and tunnel construction is conducted rationally and efficiently.Bigger benefits.