| The karst area occupies a large area in China’s vast territory,and the construction of tunnels in the central and western regions inevitably requires the construction of karst tunnels through this area.One of the biggest problems encountered during the operation of karst tunnels is the impact of karstification on the safety of karst tunnels.In the process of tunnel construction,bolt support is often adopted to achieve the effect of tunnel stability.From the above introduction,it can be seen that revealing the mechanism and regularity of long-term reliability weakening of bolts under Karst action is a prerequisite to ensure the safety of surrounding rock during the operation of karst tunnel.The long-term reliability of bolts(especially those filled with full-length bonded cement mortar)is affected by the mechanical properties of limestone in surrounding rock wall,the contact state between limestone and the hole wall of bolt;on the one hand,the dissolvable karst in surrounding rock leads to stress concentration and deformation of surrounding rock,induces stress adjustment of macroscopic surrounding rock,produces new damage and macroscopic deformation,and creates stability of surrounding rock itself.On the other hand,because of the continuous karstification,the surrounding rock fissures continue to develop and expand,especially the development and expansion of the fissures at the contact between limestone and anchor hole wall,which leads to the weakening of the effective interaction surface between limestone and anchor hole wall,and the failure of the supporting structure at the contact surface fissures,which leads to the continuous weakening of the long-term reliability of anchor over time.In this paper,full-length bonded bolt,cement mortar as the main filler,is selected to carry out simulation test on the long-term reliability Weakening Mechanism of bolt under Karst action.The main research results are as follows:1)The implementation scheme of long-term reliability simulation test for bolts is described in detail from the aspects of design idea of long-term reliability weakening simulation test for bolts,operation flow of long-term reliability weakening simulation test for bolts,improvement of long-term reliability weakening simulation test system for bolts,selection principle of samples and preparation,etc.(1)Aiming at long-term corrosion effect,an equivalent substitution method is adopted to design the long-term reliability simulation test scheme for bolts.According to the long-term corrosion effect equivalent theoretical calculation and long-term corrosion effect equivalent simulation test analysis,combined with theoretical calculation and equivalent simulation test,the corrosion PH value of PH=4 for this test and the corrosion effect of PH=6 for corrosion foundation time of 30 days for corrosion foundation time are equivalent to that of PH=6 for 2 years.2.The simulation test system for long-term reliability weakening of anchor rods is mainly composed of the following factors: The simulation test platform,the simulation bolt,the bolt drawing module,the auxiliary lifting equipment and the measuring system(drawing displacement,drawing stress)are composed of three parts.Thirdly,the typical limestone blocks obtained are classified effectively.Based on the classification conditions,the limestone blocks at different fracture development and expansion stages are finally prepared.2)Through the simulated test schemes of different anchorage angles,the same corrosion conditions were tested at different anchorage angles.The variation laws of pull-out force with corrosion at different anchorage angles were as follows:(1)The peak range of pull-out stress at different fracture development stages was different,the range of I and II stage samples was 265.2-294.4 MPa,and the peak stress norm of III stage samples was 265.2-294.4 MPa.The stress peak value of stage IV ranges from 75.2 to 102.4 MPa with the circumference of 196.1 to 247.7 MPa.Secondly,the simulated test phenomena of specimens at the same fracture development stage under different anchorage angles are similar,while the specimens at the fracture development stage I and II have no obvious phenomena at the beginning of the bolt drawing.With the increase of drawing time,the bolt is gradually pulled out,and the bolt is pulled out with a large amount of cement mortar.The specimen with stage III of fracture development has no obvious phenomenon at the beginning of bolt pulling out.With the increase of pulling time,the bolt is pulled out,and the bolt is pulled out with a small amount of cement mortar,and the pulling time is slightly shorter than that of stage I and II.The specimen with stage IV of fracture development appears loosening phenomenon at the beginning of bolt pulling out,and the bolt is pulled out with the increase of pulling time.When the bolt is pulled out,it appears that the bolt is pulled out almost without cement mortar.3)Through the simulation test schemes of different corrosion conditions,the simulation tests under different corrosion conditions were carried out.The variation rules of the simulation test for the long-term reliability weakening of bolts under different corrosion conditions were as follows: 1.The peak pullout stress of the specimen with I and II at the stage of crack growth and propagation after corrosion,the relative difference of the reference peak value is 33.7 MPa when the corrosion time is 0 D.After dissolution,the peak value of pulling stress of specimen with fracture growth stage III decreases obviously,and the maximum relative difference of peak value can reach 79.9 MPa.The peak value of pulling stress of specimen with fracture growth stage IV decreases continuously after dissolution stage,and the maximum relative difference can reach 42.1 MPa.2.After dissolution,the fracture develops further in samples with the same fracture development type.With the increase of dissolution time,the width and length of cracks increase,and the damage degree of specimens increases gradually.The specimen develops from a small amount of cracks on the surface to extending along the surface,from the stage of crack extension to crack penetration,and even fracture failure occurs.The phenomena of sample tests at different fracture development stages are consistent,showing a trend that the longer the dissolution time is,the wider the fracture range is.Through the study of the long-term reliability simulation test law of bolts,it provides important scientific guidance and technical guarantee for the design,construction and long-term safe operation of tunnel engineering in karst areas of China. |
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