Study On Entry Technology Of Shallow Unsymmetrical Pressure Tunnel In Dashanwan

With the rapid construction of transportation infrastructure in mountainous areas of Southwest China,the safe connection of mountain tunnels is very important.Based on the entrance of the left line of dashanwan tunnel of Chuda Expressway in Yunnan Province,aiming at the topographic conditions such as weak surrounding rock and shallow buried bias pressure in the portal section,this paper studies and analyzes the deformation law of surrounding rock through numerical simulation and on-site monitoring measurement,so as to provide some guidance for the safe construction of the portal section.(1)According to the existing theories,the causes of asymmetric deformation of biased tunnel are summarized,and the definition methods of shallow buried and biased pressure are summarized;At the same time,the advance prediction,advance support,tunnel excavation method and support method of tunnel entry technology are sorted and analyzed.(2)The three-step method of aqus and the three-step method of aqus are used to analyze the structural change law of the tunnel excavation and the three-step method of aqus.(3)Explore the new advanced support scheme of "transverse pipe shed + asymmetric longitudinal pipe shed",and study its influence on surrounding rock during tunnel construction.The results show that the key construction technology of "crossing" pipe shed advance support into the tunnel is optimized by 58.28% in terms of vertical displacement of deep buried side arch shoulder;The initial support stress of shallow buried side spandrel is optimized by 43.53%.In addition,through the comparative analysis of 27 support parameter schemes,the following conclusions can be obtained,which are applicable to most shallow buried tunnels under eccentric pressure:a.When the terrain gradient is 50 °,the transverse pipe shed with a diameter of Φone hundred and fifty-nine × 6mm,the length is the diameter of the excavation surface,the spacing is 1 times the excavation step,and the thickness is 2 times to 3 times the step,the effect is the best;b.When the terrain gradient is 40 °,the transverse pipe shed with a diameter of Φone hundred and fifty-nine × 6mm,the length is the diameter of the excavation surface,the spacing is 1 times the excavation step,and the thickness is 2 times the step,the effect is the best;c.When the terrain gradient is 30 °,the transverse pipe shed with a diameter of Φone hundred and eight × 6mm,the length is the diameter of the excavation surface,the spacing is 1 times the excavation step,and the thickness is 3 times the step,the effect is the best.(4)Combined with the actual project of dashanwan tunnel,the field measured data of surface exploration,vault subsidence,surrounding rock pressure and steel arch stress are sorted and analyzed,and the numerical simulation results are verified,which provides an important basis for judging the stability of surrounding rock.