Study On The Key Technology Of PBA Method For Large-spanned Metro Air Duct Under Complicated Geological Conditions

With the rapid development of social economy and the speeding up of urbanization, city problem such as traffic jams are more and more serious. The emergence of the urban rail traffic have effectively solved the problem of urban traffic congestion, subway engineering in our country is vigorous developing. Stratum deformation caused by the construction will bring to the infrastructure such as surrounding buildings and underground pipelines great potential safety hazard, as a result, designers and construction personnel must pay attention to construction of stratum deformation effect problem. In numerous underground construction method, PBA method for the high flexibility and strong adaptability, large section utilization rate, time limit control more easily and less impact on the environment, obtain attention and favor from the industry much. Studying the PBA method the influence law of stratum deformation, to search the optimum construction scheme and to reduce the disturbance of the method to the formation of construction, etc., have the important practical value for the design and construction.This paper takes Beijing subway line 8 three phase of the da hong men qiao station-heyi station project as the research backing. Through the collection of field monitoring data and numerical simulation research method, studied the PBA method the key technical problems in the process of building long-span metro air duct, from the deformation of tunnel surrounding rock stability, structural stress state and formation of construction scheme optimization, give each construction stage recommended construction scheme, provide reference for design and construction. The main research results are as follows:(1) By studying the pilot tunnel construction sequence, come to the conclusion: "first excavation upper drift, the lower pilot tunnel after excavation" has a better solution.(2) By studying the main structure construction plan, come to the conclusion:the stress at the beginning of the arch foot of completely suitable approach as well as upper pilot tunnel and lower pilot tunnel according to the order of the construction respectively significantly greater than completely top-down method, well, the stress at second lining arch foot much smaller than completely top-down method. Three options for arch stress difference is not obvious.(3) By studying the subject construction of the water penetration, it is concluded that the upper pilot tunnel of appropriate increase the effects of a construction penetration on the surface subsidence, lower pilot tunnel increase of construction is relatively upper pilot tunnel can increase the impact on surface subsidence.(4) From the perspective of numerical calculation, the choice of a single period of alternating remove the timbering is the better solution, a single removing 15m for the surface subsidence control effect is better than other solutions. As the change of construction stage, the width of the subsider settlement curve inflection point basic remains the same, maintain at about 15 m.(5) Advanced deep hole grouting pre-reinforcement measure for ground subsidence control effect is significant, puts forward the measures to improve the quality of buckle arch grid node connection technology and a kind of combination supporting scheme.