The Soft-hard Formation Under Dynamic Analysis During The Construction Of Large Cross-section Tunnels By The Modified PBA Method

The geological condition of Dalian subway tunnel is very complex and the construction is difficult.it is of vital importance to choose a reasonable method for the construction of the subway station in numerous construction schemes.This paper takes Dalian school square subway station project as the background,according to the upper-soft and lower-hard geological characteristics,through the geological exploration and the research methods such as numerical simulation combined with field monitoring and propose the modified PBA method innovativly,simulated demonstration and scheme optimization are carried by the finite difference software FLAC^3D.it is very important in the construction scheme selection and the stability of surrounding rock by the research results.the main works and results are as follows:?1?For the upper-soft and lower-hard formation under the construction method of large cross section station,There is no mature specification at home and abroad.First,this paper analysis the limitations of traditional construction method in Dalian subway construction.The Center Drift Method and Center Cross-Diaphragm Construction process are multifarious,and can not meet the requirement of the deformation,While traditional PBA method is only applicable to soft soil layer,also has the problem of high cost and low efficiency.Then,according to the upper-soft and down-hard geological characteristics on the station,through argumentation and research,deciding to replace the side column with hard rock,omitting the both sides of the pilot tunnel excavation process,propose the modified PBA method suitable for the construction of school square subway station.finally,the modified PBA method construction steps are given.?2?Combined with the actual monitoring to the modified PBA method for dynamic analysis of construction,and then establish the three-dimensional numerical model of school square station,according to the modified PBA method to simulate the construction steps,and analysis the station's surface,vault and hole convergence displacement defonnation law.Research results show that the settlement values are in good agreement with monitoring results.summarizes the modified PBA method changing regularity of plastic zone,The method can control the deformation of the surrounding rock,but the midline of buckle arch is weak link and must be focused on the object.?3?In view of the large span station localization problem,the zone state index analysis method can reflect the local damage of surrounding rock.First of all,to optimize the pilot tunnel construction sequences of the modified PBA method,then,to grout simulation in the risky buckle arch area during the construction,combined with the zone state index to identify the optimal grouting schemes.Finally,to optimize the high sidewall of underground cavities anchor cable construction and determine the optimal anchor cable length,the optimized plan for the research adopted by the construction,which obtains the good economic effect.