Key Technologies Research For Construction Control Of Shallow Buried Railway Tunnels In Loess Area

Taking Hengshan railway tunnel as the engineering background,combined with engineering practice and numerical simulation,the control effect of different construction schemes on the stability and deformation of the surrounding rock of the tunnel is compared and analyzed.Reasonable and effective construction methods,auxiliary construction technical measures and construction techniques are proposed,and a complete set of key technologies are summarized.The main contents and achievements are as follows:(1)The stability of the open cut tunnel slope in aeolian sand stratum is analyzed by strength reduction method.The stability of the open cut tunnel slope in aeolian sand stratum is analyzed by strength reduction method.The stress and deformation characteristics of the open cut tunnel structure are analyzed and the strength of the lining structure is checked.The results show that the safety factor of exit slope stability is 1.95,the slope is stable,and the minimum safety factor of open cut tunnel structure is 2.68,the structure is safe.(2)The numerical simulation of entry in tunnel construction results show that the deformation of surrounding rock is well controlled by the large pipe shed and double-layer small pipe in advance.After working face reinforcement,the maximum vault settlement of the tunnel is about 8.5 mm,the horizontal displacement of surrounding rock in the tunnel is not more than 5 mm,and the maximum surface settlement is 6 mm.Finally,the method of shotcreting + ?25 glass fiber mortar anchor(6 m)is adopted to reinforce the excavating face.According to the field monitoring data,all monitoring points have not exceeded the limit,the maximum vault settlement is 19.7 mm,the maximum deformation in the tunnel is 14.89 mm,and the maximum surface settlement is 16.8 mm,and the entry in tunnel construction is safe.(3)Numerical simulation is carried out for the tunnel crossing the boundary between rock and soil and the steep terrain,the results show that the construction method of advance small conduit,two-step method + temporary cross brace + enlarged arch foot + excavating face reinforcement can effectively control the deformation of surrounding rock and ensure the safe construction of the tunnel.The calculation results and field measured data show that when the tunnel passes through the boundary between rock and soil,the surface settlement and surrounding rock deformation are less than 15 mm;When the tunnel passes through the steep terrain,the surface settlement and surrounding rock deformation are less than 10 mm.(4)According to the national standards and the requirements of cultural relic protection,combined with similar projects,relevant literature and design requirements,in the section of the tunnel through the Great Wall site of Ming Dynasty,the maximum allowable value of ground surface settlement is 30 mm,and the deformation rate is not more than 5 mm/d,and the local inclination is not more than 1‰.The numerical simulation results show that when the tunnel adopts the construction method of tunnel body pipe shed,advance grouting small conduit,two steps temporary cross bracing method + excavating face reinforcement + expanding arch foot,the maximum ground settlement of the section under the Great Wall is 21 mm,the local inclination is less than 1‰,which conforms to the settlement control standard,and the effect of ground settlement control is good.This method is also used in the construction of the tunnel under the Great Wall of Ming Dynasty,which effectively protects the site of the Great Wall.(5)The method of open dark cave and cover-cut excavation in the entrance section of tunnel is studied.The stability of the retaining pile + crown beam + cross brace structure and the stress characteristics of the lining structure are calculated and analyzed,and the displacement monitoring results of the retaining pile are compared and analyzed.The calculated vertical and horizontal displacement of the top of the retaining pile are 4 mm and 3.16 mm respectively.The measured settlement and horizontal displacement of the top of the pile are not more than 3 mm,and the stress and deformation of the retaining pile are in good condition.The structural strength of the retaining arch is checked and the minimum safety factor is 12.21.The construction method of open dark cave and cover-cut excavation can effectively ensure the safety of tunnel construction.The problem of tunnel's emerging hole is solved.