Research On The Large Deformation Mechanism Of Soft Rock And Support Countermeasures Of NPR Anchor Cable-Truss In Tayi Tunnel

During the construction of the Yunnan Jianshui to Yuanyang highway road Tayi tunnel,due to its complex geological structure,fractured surrounding rock,affected by active fractures,high ground stress level and fissure water in the surrounding rock and other factors,the initial support and second lining damage occurred many times.Deformation characteristics are mainly manifested as follows:large amount of surrounding rock deformation,long-term deformation,and serious damage to the support structure,with a comprehensive arch replacement rate of up to 70%.In consideration of the large deformation of the surrounding rocks of the Tayi tunnel,the exploratory proposal of the over-grouting+the NPR anchor cable+the double-layer three-dimensional truss support technology based on the concept of flexible regulation and rigid restraint was made.A series of studies have been carried out by a combination of field studies,theoretical analysis,numerical simulations and field applications,and the main conclusions reached are as following:(1)According to the analysis of regional geological conditions,the surrounding rock of Tayi Tunnel is mainly thin-layered mud slate,and shows different degrees of medium and strong weathering.Affected by multi-level fracture structure and fault fracture zone,the rock body is relatively broken,and the dominant joint dip angle of the surrounding rock is between 15 and 60°.Through the regional ground stress measurement,it is concluded that the regional ground stress characteristics of Tayi tunnel areσ_H>σ_v>σ_h,and the maximum horizontal principal stress direction is about 51°to 85°from the direction of Tayi tunnel.The magnitude and direction of ground stress has a significant effect on the stability of the rock surrounding the Tayi tunnel.The point load test apparatus was used to test the strength of the mud slate at the construction site.The average uniaxial compressive strength of mud slate is 13.42 MPa,and the strength is reduced into 40%after encountering water.The average compressive strength of strongly weathered mud slate is 7.29 MPa,and due to its clay minerals,such as illite and chlorite content of nearly34.5%,it is extremely susceptible to softening and disintegration when exposed to water.The strength stress ratio of the tunnel surrounding rock is 4.9,and the area is judged to be high ground stress according to the Engineering Rock Classification Standard.According to the soft rock engineering mechanics,the surrounding rock type of Tayi tunnel is jointed-high stress-expansive soft rock(JHS type).(2)Based on the H-B and M-C strength criterion,the analytical solutions of the extent and displacement of the plastic zone of Tayi tunnel under the elastic-plastic mechanics theory were obtained,and the variation of the radius of the plastic zone under the fractured rock conditions was compared and analyzed.Measured the surrounding rock structure of the tunnel under the original support by borehole peeping and low frequency geo-radar.The results show that the surrounding rock in the large deformation section of Tayi tunnel is mainly divided into the severely damaged area,the secondary fracture development area,and the minor fracture development area.Among them,the deterioration damage area of tunnel surrounding rock is between 6～10 m,among which the severe damage area reaches 6.0 m.The rock structure within 10～12 m of tunnel surrounding rock is relatively intact and is the main bearing structure area of surrounding rock.The original support design of anchors,locking feet and other support measures are within the area of severe damage to the tunnel surrounding rock,which is one of the reasons why the original support design is difficult to control large deformation.(3)The mechanical model of laminated slate was established,and the main controlling factors affecting the stability of tunnel surrounding rock were analyzed,including,tunnel radius,rock inclination,slate thickness,slate tensile elastic modulus and horizontal stress.Among them,the slate tunnel deflection large deformation damage increases with the increase of tunnel radius,dip angle and horizontal stress,and decreases with the increase of rock thickness and slate tensile modulus of elasticity.Under the condition that the radius of the tunnel,the stress level and lithology of the slab are approximately equal,the reinforcement of the surrounding rock at the palm face needs to be supported asymmetrically according to the dip angle of the exposed surrounding rock in order to effectively control the deflection damage of the surrounding rock.Jointed soft rock,high ground stress and the strength decay of the surrounding rock after water softening are the main factors causing the large deformation of soft rock in Tayi tunnel,whose mechanical deformation mechanism is the structural flexural extrusion large deformation damage of thin laminated mud slate.The NPR anchor cable+double-layer three-dimensional truss support system based on the concept of flexible regulation and rigid restraint is proposed through the mechanism of large deformation damage of the Tayi tunnel,and the concept and principle of action of its coupled support are analyzed.Among them,the control factors of constant resistance of NPR anchor cable were obtained by theoretical analysis,and the static tensile and dynamic impact static tests were conducted in the laboratory on the extraordinary mechanical properties of NPR anchor cable.The results show that the deformation of NPR anchor cable is in the elastic deformation stage when the deformation is around 0～50 mm,and enters the constant resistance deformation stage when the constant resistance anchor cable tension reaches350 k N,and the maximum constant resistance stretching length is about 500 mm.Compared to conventional strand and rebar of the same diameter,the working resistance of NPR anchor cable is 1.88 times higher than the breaking force of rebar anchor,and the elongation is 2.65 times higher than that of rebar anchor.The working resistance of NPR anchor cable is only 60.42%of the yield strength ofΦ21.8 mm steel strand,which can not only make use of the mechanical property of high yield force of steel strand,but also avoid the yield damage of steel strand through structural transformation,and realize the high constant resistance and large deformation mechanical property of support structure.Single NPR anchor cable is 5.7 times more energy absorbing than 21.8 mm strand and6.2 times more energy absorbing thanΦ22 mm rebar anchor.In large deformation engineering rock mass,NPR anchor cable can maintain effective and constant support resistance under the action of static sustained large deformation or dynamic impact.In the process of NPR anchor cable constant resistance large deformation,which can absorb a large amount of surrounding rock deformation energy,to effectively ensure the safety and stability of the support structure.(4)Through theoretical analysis,the mechanical model of the double-layered three-dimensional truss and its controlling equations of torque,shear force and bending moment were established.And the mechanical properties of double-layer three-dimensional truss and single-layer arch were modeled and analyzed by Rhino software+ABAQUS numerical software.The results show that compared with single-layer arch and double-layer arch,double-layer three-dimensional trusses show excellent mechanical properties under different loads.Under gradient loading,the shear resistance of double-layered three-dimensional joist is 1.6 times that of single-layered arch,and the torque resistance of double-layered three-dimensional joist is 2.7 times that of single-layered arch.Double-layer three-dimensional truss is 1.6 times of single-layer arch with different pressure coefficient,and double-layer three-dimensional truss is 2.4 times of single-layer arch in terms of torque resistance.(5)According to the soft rock type of Tayi tunnel,the mechanism is analyzed to belong to JHS type soft rock large deformation mechanics.Relying on the mechanism of large deformation mechanics of Tayi tunnel and NPR anchor cable truss coupling support mechanism,the transformation process of compound type mechanics deformation mechanism of Tayi tunnel to single type mechanics is also proposed.Exploratively proposed the support control countermeasures of overrunning tube shed grouting+NPR anchor cable+double-layer three-dimensional truss in Tayi tunnel.And relying on FLAC3D numerical simulation software,analyzed the surrounding rock control effect under different support measures.The results show that the flexible regulation of NPR anchor cable and the rigid restraint control countermeasure of double-layered three-dimensional truss are effective ways to solve the large deformation of the surrounding rock of Tayi tunnel.Compared with the metre-level deformation of the tunnel surrounding rock under the original support design scheme,the flexible control system of NPR anchor cable alone can control the metre-level deformation within 500 mm,and then the deformation of the surrounding rock can be controlled within 80 mm by rigidly restraining the double-layer three-dimensional truss to achieve the structural stability of the support body and the surrounding rock.(6)Anchor pulling tests were carried out in grouted and non-grouted conditions for the fractured surrounding rock of the Tayi tunnel.Compared to non-perimeter grouting,the ultimate pull-out force of NPR anchor ropes after perimeter grouting reaches a maximum of 475.89 k N at an anchor length of 1.8 m,an increase of 34.5%in anchorage force.The design of overrun pipe shed grouting+NPR anchor cable+double-layer three-dimensional truss was successfully tested in the field industry at the location of K63+540～560 in Tayi Tunnel.And based on the multi-source information cloud platform system to monitor the research and test section of the Tayi tunnel.The test and monitoring results show that the NPR anchor cable+double-layer three-dimensional truss control countermeasures can effectively control the surrounding rock within 250 mm.The NPR anchor cable at the position of the right arch shoulder of the surrounding rock first enters the constant resistance(350 k N)slipping state,and through the adaptive adjustment process of the NPR anchor cable with the surrounding rock,the load uniformity of the support body and the surrounding rock and the support integration are guaranteed.After the NPR anchor cable support,due to its high support resistance,the pressure of the surrounding rock on the arch is greatly reduced,the maximum initial support pressure also appears in the right shoulder of the arch,when the double-layer three-dimensional truss closed loop,the contact pressure between the initial support and the surrounding rock remains constant,the maximum value of the pressure on the double-layer three-dimensional truss is 0.27 MPa.The contact pressure between the primary support and the second lining is only 0.89 MPa.Through flexible regulation and rigid restraint,the primary support system can effectively share the load of the second lining,thus effectively guaranteeing the structural safety and stability of the tunnel.