Study On U-shaped Steel Retractable Support Technique Of The Squeezing Surrounding Rock And Large Deformation Tunnel

With the continuous development of China's high-speed railway,highway construction,tunnel project already belongs to grow deep direction,it's liable to occur highland stress situation due to the complex geological conditions and other factors.The impact of highland stress on the tunnel project is significant,especially for large depth soft surrounding rock tunnel engineering,often lead to associated geological disasters,such as soft rock large deformation.Currently for highland stress surrounding rock large deformation,it exists two concepts of treatments"rigid support,strong support" and "soft support,after the first release anti-".Most of the time the industry's first thought is to use "rigid support,strong support" measures,but rigid support play an effective role in supporting just for small deformation,it is not very satisfactory for large deformationespecially for squeezed surrounding rock deformation.As a "soft support" concept,although advance stress release hole can be more effective to solve squeezing surrounding rock and large deformation problems,but it existmany deficiencies.For example,it causes repeated disturbance,due to limited space and time it could result in the release of little effect and since the pilot hole support weak,there are security risks,etc.Therefore,whether through a construction support to solve the problem of large deformation of surrounding rock has become an important research direction in the field of tunnel.Since crossing the complex geological conditions in the construction of long and large tunnel,it will inevitably encounter high stress and soft rock mass conditions,usually resulting in geological disasters such as large surrounding rock deformation and so on.It is a major problem puzzling the underground engineering whether solve the problem of surrounding rock large deformation,which is essential to safety and quality of construction.According to current engineering research on large deformation especially compressional large deformation problems,this paper briefly describes the determination of large deformation,the mechanism,evolution mechanism,and further squeeze deformation prediction and classification,providing the basis for design and construction of similar heights stress extruded tunnel.For the current extruded large deformation,which often resulting in geological disaster such as bracket twisted,concrete spalling and invasion limited to issues,engineering often used "strong support hardtop"measures and could not solve,often need to change the demolition of arch,enlarging excavation section to deal with.This paper proposes the use of a U-shaped retractable steel to deal,and research its compression capability,sliding properties,stress and deformation characteristics and so on,with theoretical analysis,numerical simulation,laboratory test combination in the form.Include the following:(1)The elastic plastic deformation mechanism of surrounding rock is studied,and the criterion of plastic deformation is analyzed,the range of elastic and plastic distribution,the stress state,the stress field and the stress field of elastic plastic zone after excavation and support are analyzed to explore the influence factors of the displacement and stress of the tunnel.Then,accord to the characteristic curve of surrounding rock to analyze the size of the supporting force on surrounding rock,and the effect of support structure.(2)By researching U-shaped steel supporting structure,this paper will analyze stent sliding conditions,discuss the factors affecting bracket anti-shrinkage ability,the relationship between compression capability and bracket slip amount while dragging,structure internal force calculation after the move,and research stent carrying energy change.(3)Establish numerical model of two sections(250km/h two-lane,250km/h singlet)of passenger dedicated railway tunnel,with U-shaped steel bracket supporting.According to analyze the force and deformation data,verify the relationship between carrying capacity and reduced dynamic and decision the supporting effect.(4)By indoor U-shaped steel joint anti-slip test,impose pre-tightening torque on card line nut step by step as well as ordinary wrench fastening,then add to pressure on the specimen.We can inferred the relationship between U-shaped steel fittings shrink momentum and Joint anti-sliding resistance P,and compare with the results of the theoretical analysis and numerical simulation.By studying we can draw the following conclusions:(1)For squeezing ground,plastic radius after the excavation is proportional to the radius of the tunnel,and positively correlated with the size of the initial stress state,but the smaller the value of the physical parameters of the surrounding rock,the larger the radius of plastic.After applying the supporting structure,the range of plastic zone is also affected by supporting the resistance,the greater the support resistance,the smaller the radius of the plastic zone.(2)The size of U-shaped steel frame adapter sliding resistance is related with the contact surface of steel in the form of ear or waist positioning.But when the stent reduced move,due to the difference of the contact area and sliding stability,ear-positioning card cable has better mechanical properties than waist-positioning.Joint sliding resistance value is also proportional to the card cable preload,and cable card number,etc.After the shrinkage,the anti shrinkage ability of the support is also linear with the joint shrinkage,the bigger the contraction momentum,the bigger the sliding critical load value.(3)According to the results of numerical simulation,the displacement distribution of surrounding rock is basically similar under different bracket reduction,and the maximum displacement occurs in the region which is the arch waist the to largest span of the side wall,but the displacement of vault and invert is relatively small.It can be seen from the distribution of surrounding rock plastic zone:the distribution of plastic zone in the hole week is roughly "Butterfly",and the plastic zone in the arch waist part is developed especially in the surrounding rock,which is about 2?2.5 times of the crown.(4)Under different bracket reduction,the size distribution of the principal stress of the supporting structure is:The phenomenon of stress concentration occurs in the side wall,the internal force distribution of the other parts is more uniform,and the numerical value of the vault is relatively large;the internal forces acting on the support are gradually reduced with the increase of the shrinkage,it means with the release of the stress and deformation of surrounding rock,the deformation pressure on the support is gradually reduced,and the effective control of the surrounding rock stress can be realized.(5)According to set parameters of grade V press surrounding rock tunnel,for the single track tunnel buried depth of 300 meters,the surrounding rock deformation amount can be designed to be 3 5cm,and the total reduction momentum of U steel can be designed as 100cm.For the double line tunnel,the surrounding rock deformation can be designed to be 50cm,and the design of the U type steel retractable support frame is 140cm,which can meet the requirements of the stability and strength of the structure.When the physical parameters of surrounding rock are changed at the same level,these values should be re calculated and analyzed(6)Different pre tightening torque is applied to the U type steel joint,and the maximum sliding resistance at the initial time is different,Torque value is 75N·m?100N·m?125N·m?150N·m,and the corresponding initial shrinkage time joint displacement is 15?20mm,70?78mm,60?80mm,60?108mm,respectively,the maximum sliding resistance is 100kN,202?275kN,198-200KN,180?205KN.The performance of the joint is affected greatly by the shape of the U steel and the cable,the surface roughness and stiffness.(7)According to the 205kN of the maximum slip resistance of the joint under the action of the tightening torque 150N·m,for single line tunnel,the loose rock mass height that the support can bear is 1.5m under the IV surrounding rock by reverse analysis,it's 1.75m under the V surrounding rock.For the double line tunnel,these two values are 1m and 1.15m.When the physical parameters of surrounding rock are changed at the same level,these values should be re calculated and analyzed.According to the analysis content and the research conclusion,the paper also puts forward some suggestions in the end:(1)According to the results of the numerical analysis,the squeezing surrounding rock large deformation displacement rules are the surrounding convergence than vault settlement.Therefore,the monitoring frequency and supporting measures of the side wall can be strengthened in the construction,such as adding anchor bolts or grouting to control the deformation of surrounding rock.(2)In order to reduce surrounding rock plastic zone depth development,it should timely support after tunnel excavation to avoid rock exposed for too long,it also can make use of the anchor grouting to improve the physical parameters of surrounding rock and improve the strength of surrounding rock and integrity,and improve the crushing of the surrounding rock stability.(3)In order to improve the working resistance of the support,the proposed:1)When applying the preload torque to the U type steel retractable bracket joint,it should avoid using artificial ordinary wrench fastening,but require the use of special electric or pneumatic wrench according to the torque design value;2)In order to avoid the influence on the working resistance improving of the deformation of the clamping cable when the bracket is contracted,the rigidity of the cable should be improved;3)After the bracket is contracted,the screw nut should be tightened to avoid the squeeze pressure drop when the nut is loosened.(4)Considering that the pressure and bending moment of the vault surrounding rock are larger,and the curvature is not conducive to the reduction of the support,two joints can be arranged on both sides of Arch waist and the side wall of the bracket.According to the specific parameters of surrounding rock grade V,for the single line tunnel,the total design of the bracket is 100cm and the design of each joint is 25cm;for the double line tunnel,the corresponding two values are 140cm and 35cm.When the physical parameters of surrounding rock are changed at the same level,these values should be re calculated and analyzed.Through the above theoretical research,simulation analysis,the combination of indoor test means,it can summed up the corresponding rules and results to provide reference and help for the practical application of U steel stent in the extrusion of surrounding rock.