Study On Construction Technique Of Long Span Tunnel Under Shallow And Weak Surrounding Rock

Based on Kaoyishan Tunnel engineering in the Jingzhu(Beijing--Zhuhai) freeway, the thesis primarily studies construction mechanics behavior and the holistic stability of peripheral surrounding rock which is based on long span tunnels in the shallow and weak country rock ,which adopts very mature elastic mechanics theory and finite element method(FEM).On the base of it, the thesis studies the reasonable construction technology and construction preparation of Kaoyishan Tunnel, and it provides theory basis and practical significance for safe, rapid , effective construction of the whole tunnel, and saves on investment.Firstly, compared with two-lane section of approaching circle, the long span tunnels have the following basic mechanical characteristics: â‘  stress redistribution after excavating becomes very harmful, powerful support structure is needed to control its deformation;â‘¡stress concentration in the tunnel's feet is too large, great bearing capacity of soil is demanded;â‘¢owing to the large flat ratio of the tunnel and rather unstable rock of arch crown, generally speaking, assistant measures are needed for reinforcing rock around arch crown;â‘£ Large loosening crustal stress will come out after the tunnel excavation.Secondly, in the shallow and weak surrounding rock, the prime factors influencing the stability of long span tunnels include: crustal stress, construction technology, excavation technics, groundwater, time factor, flat ratio, and so on. In the actual engineering application, because of the differences between time and space, a certain or several affection factors usually have the call, so relevant measures are needed in order to reduce its effect on the surrounding rock.Thirdly, by researching on the construction mechanics behavior of long span tunnels under two different excavation modes which are parallel partial excavation with three-bench and center diagram method, the settlement of arch crown in the course of construction caused by parallel partial excavation method with three-bench is obviously less than caused by CD method, the former is very good for keeping surrounding rocks in shallow and weak strata totally stable. It is suggested that it is prior to apply the parallel partial excavation method with three-bench when long span tunnels are built in shallow and weak strata, it is proved by successful applying to build the Kanyishan tunnel.Then, by calculating and analyzing ,in the center diagram method, displacement field, stress field and plastic zone which is caused by "demolishing center diagram posteriorly" in the surrounding rock around the tunnel is less than by "demolishing center diagram first" ,so it is suggested that the centerdiagram should be demolished after the whole cavern is excavated and the support shape holistic closing ring .Finally, based on studying the stability of surrounding rock around excavated tunnel and construction mechanics of the long span tunnel, the article propose the reasonable construction scheme of Kanyinshan tunnel, and the scheme supply credible theory foundation for the whole tunnel's secure construction and ensure that the construction is well carried out.