Study On Support Structure Mechanics Characteristics And Applicable Construction Methods For Complex Structure Tunnels With Small Interval Tunnels In Squeezing Rocks

The large deformation problem of squeezing surrounding rocks has been a rather thorny problem in tunnel construction around the world.During the construction of small interval twin tunnels,the large deformation problem of squeezing surrounding rocks will become more prominent and its typical performances behave as “cracking in advancing tunnel,and cracking in right tunnel while driving in left tunnel,that not only large deformation occurs during excavation of advancing tunnel and also the driving of following tunnel will affect the safety of supporting structure in advancing tunnel,thus,the entire construction is becoming a hot potato.As a major difficulty in project,a large amount of research results and engineering experiences to resolve the large deformation of squeezing rocks has been accumulated at home and abroad.At present,the research on small interval is still restricted to general surrounding rocks and therefore,it has great significance and theoretical values to carry out the research on small interval of squeezing surrounding rocks,especially the engineering background based on typical cases of small-interval with large-deformation of squeezing surrounding rocks.In this paper,some useful attempts to study the mechanical properties and applicable construction methods for supporting structure of small interval twin tunnels within squeezing surrounding rocks are made.Taking the typical large deformation at trumpet section of exit portal of Xin Chengzi tunnel on newly-built Lanzhou to Chongqing railway as a studying case,the engineering trials in small interval section of twin single-track tunnels are successfully conducted by using advancing heading stress release method for F32-1 fault fractured rock at trumpet section of exit portal and the stress tests of lining structure,deformation measurement and numerical simulation analysis of primary support are carried out from the perspective of supporting structure mechanical properties.Through comparative analysis of supporting structure mechanical properties under different construction conditions,the effects of advanced heading stress release method are discussed and verified.On this basis,the experiment results are summarized and refined,the applicability of construction method is analyzed and advancing heading stress release method is verified and perfected.The mechanism of advanced heading stress release is that originally,the primary support is only under once stress release and because the second stress release is formed in theprocess of tunnel excavation,the stress could be partially released by advanced heading in advance and thus the stress on supporting structure is relieved,which it is especially important to resolve the long-term risk of secondary lining in squeezing surrounding rocks.Research results show: the pilot heading stress release method is feasible and effective in theory and practice for solving the engineering problem of small interval twin tunnels within squeezing surrounding rocks.Tests indicate that comparing with conventional three-bench method,the max stress on secondary lining and its max increasing rate or the long-term growth trend of three years will be significantly reduced by pilot heading stress release.Numerical simulation analyses demonstrate that comparing with experimental condition of pilot heading method and three-bench method respectively applied in twin tunnels,the effects of stress release by pilot heading method in both twin tunnels are becoming more significant and more balanced and the pilot heading method is relatively more perfect method for severe deformation of small interval twin tunnels within squeezing surrounding rocks.As far as the stress release method by pilot heading is concerned,the key construction techniques are presented in this paper.At present,the secondary lining at small interval section of Xinchengzi tunnel on Lanzhou-Chongqing railway by stress release of advanced heading has been applied for two and half year.In order to observe the long-term effects,study tests will be continuously carried out.