| Muzhailing super-long highway tunnel and its inclined shaft pass through many fault fractured zones and carbonaceous slate strata.The geological conditions are relatively complex.The main manifestations are as follows:strong fault tectonics,high in-situ stress,large tunnel depth,groundwater development and so on.Taking the No.2 inclined shaft of Muzhailing Tunnel as an example,the initial support failure occurred many times during the construction period.The main deformation characteristics are asymmetric deformation,large deformation of surrounding rock,long deformation duration and serious damage of initial support structure.The comprehensive arch replacement rate has reached 27%.The existing supporting methods have been difficult to meet the needs of tunnel construction under this geological condition,which has a serious impact on the construction period and safety of the project.At present,in the field of soft rock support in coal mine,the control technology of high pre-tightening force constant resistance anchor cable has been popularized and achieved good application results.Constant resistance anchor cable is caused by friction between the constant resistance body and sleeve in the constant resistance device,resulting in large deformation of constant resistance.Because of the extrusion of the constant resistance body,the sleeve expands horizontally,thus realizing the macroscopic structure effect of NPR(Negative Poisson's ratio),so the constant resistance anchor cable is also called NPR anchor cable.Aiming at the large deformation of surrounding rock of Muzhailing Tunnel during construction,this paper explores the application effect of high pre-tension constant resistance support technology in tunnel.Based on the 2#inclined shaft of Muzhailing Tunnel,the engineering characteristics and deformation mechanism are analyzed,and the control mechanism of high pre-tension constant resistance anchor cable to surrounding rock in deep-buried thin slate tunnel is expounded.Aiming at the asymmetry of surrounding rock of tunnel,this paper discusses the control mechanism of high pre-tension constant resistance anchor cable to surrounding rock.Based on the deformation characteristics,the asymmetric high pre-tightening force constant resistance anchor cable support strategy is proposed,and the reliability of the support strategy is verified by means of similar simulation,numerical simulation and field measurement.The main conclusions are as follows:(1)Based on the field investigation and field and laboratory experiments,it is clear that the surrounding rock of No.2 inclined shaft of Muzhailing Tunnel is mainly thin-layered carbonaceous slate,and the dominant joint dip angle is distributed in 0 to 90 degrees.The maximum horizontal principal stress direction is about 37?42 degrees between the inclined shaft of No.2 inclined shaft of Muzhailing Tunnel and the inclined shaft of Muzhailing Tunnel.According to the Classification Standard of Engineering Rock Mass,it is judged that this area belongs to extremely high ground stress,and the stratum rock mass is relatively fragmented,and the possibility of large deformation of tunnel surrounding rock is relatively high.Through rock mechanics experiments on surrounding rocks with different joint tendencies,the influence of joint dip angle on the strength of surrounding rocks is obtained.Carboniferous slate was tested by point load test instrument at construction site.The test results show that the average uniaxial strength of natural carbonaceous slate samples is 29.5 MPa,and the average saturated water absorption uniaxial strength of carbonaceous slate is 11.7 MPa.(2)Statistical analysis of 2#inclined shaft of Muzhailing Tunnel shows that the depth of the tunnel is more than 500 m where the deformation exceeds 1 m.The structural mechanics model of slate tunnel based on thin slab theory is established,and the areas which do not meet the analysis based on thin slab theory are analyzed by Mohrku's ethics model.The research shows that the main controlling factors affecting the deformation of thin slate tunnel include:tunnel tunneling;tunnel burial depth;joint inclination;water absorption and softening of surrounding rock.A numerical modeling method of equivalent ubiquitous joints with dominant joint groups is proposed.By using GSI geological classification method,rock strength is transformed into rock strength.Based on the inversion analysis of field measured data,the relevant parameters of numerical simulation are obtained.By means of numerical simulation,the effects of joint inclination angles of 15,30,45,60 and 75 degrees on tunnel stability are comprehensively analyzed.At the same time,the stability of surrounding rock under different burial depths and saturated water-bearing conditions is analyzed.The results show that the deformation of surrounding rock is more obvious at 45 degrees joint inclination than other inclination conditions.With the increase of burial depth,the deformation and failure of tunnel become more serious,when buried depth is 500 degrees.At m,the instability of the tunnel occurs,not only the bending failure of the thin plate at the left shoulder socket,but also the sliding instability of the surrounding rock mass of the tunnel.After water absorption and softening of surrounding rock,the plastic zone also extends obviously.(3)Through the static tension test of constant resistance anchor cable,it can be seen that the deformation of constant resistance anchor cable is elastic when the deformation is about 0-50 mm,and when the tension of constant resistance anchor cable reaches 35 t,it enters the stage of constant resistance deformation,and the constant resistance tension length is about 350 mm.The energy absorption mechanism of constant resistance anchor cable is analyzed,and the energy release mode of surrounding rock is analyzed.The energy absorption of steel arch frame and initial shotcrete is obtained by using thin plate elastic foundation beam calculation method.The energy generated by surrounding rock is different from that of steel arch frame and initial shotcrete,and the calculation method of energy absorption of constant resistance anchor cable is obtained.The composite beam and suspension of rock mass under the interaction of long and short anchor cables with high pretension force are analyzed.The research shows that under the pretension force,short anchor cables form composite beams of shallow surrounding rock,and high pretension anchor cables extrude thin plates to form thick plates to improve their flexural rigidity.According to the suspension theory of long anchor cable,the composite beam formed by short anchor cable is suspended in the stable rock stratum to avoid the instability of composite beam.The numerical calculation shows that the stress field generated by the long and short constant resistance anchor cables under high pre-tightening force can effectively cover the roof in the support area and play an active role in roof reinforcement.(4)Aiming at the phenomenon of asymmetric deformation of 2#inclined shaft in Muzhailing Tunnel,on the basis of previous research,the initial support system of high preload constant resistance anchor cable+anchor net+W steel belt+steel arch frame+grouting is proposed,and the support control strategy of asymmetric support is adopted to control the deformation of key parts of tunnel surrounding rock.The 3D printing technology is used to make the constant resistance anchor cable with equal ratio,and on this basis,the comparative study of similar simulation experiments is carried out.The experiment shows that the surrounding rock with anchor cable support can significantly increase the overall strength of surrounding rock and effectively control the deformation of surrounding rock.A numerical model considering the length and length of constant resistance anchor cables,steel frame and concrete is established.The cable element under Mohr-Coulomb constitutive model is used to simulate the constant resistance anchor cables.The steel arch frame+initial spray+steel mesh is equivalent to the solid model for numerical modeling.The research shows that under the condition of no constant resistance anchor cables support,the left shoulder of the tunnel undergoes obvious deformation and the asymmetric deformation characteristics of the tunnel as a whole.Obviously,under the initial support structure with constant resistance anchor cable,the deformation of surrounding rock at the left shoulder is slightly larger than that at the right shoulder,and the asymmetric deformation is obviously reduced,and the deformation is obviously smaller than that of the case without constant resistance anchor cable.(5)By means of deformation monitoring of surrounding rock,stress monitoring of anchor cables,stress monitoring of steel arch frame and multi-point displacement monitoring of deep surrounding rock,the surrounding rock of tunnel supported by high pre-tension constant resistance anchor cables is monitored and analyzed.The monitoring results show that under the action of asymmetric high pre-tension constant resistance anchor cable support system,the deformation of surrounding rock of tunnel is in a controllable stage,and the monitoring point is the largest.The convergence is 230mm,which is less than the reserved deformation.The loss of pretension force of anchor cable is about 5 t(pretension force is 35 t),and the force acting on surrounding rock by anchor cable is about 30 t.With the increase of surrounding rock deformation,the force of anchor cable increases until the constant resistance value is reached.The stress of steel arch truss exceeds the yield strength.According to the monitoring of deep displacement of surrounding rock,the separation layer of shallow surrounding rock is smaller under the action of high pre-tension anchor cable,and the separation layer mainly distributes in the range of 5m-10m. |
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