Dynamic Risk Assessment On Arch-cover Tunnel Construction Under The Condition Of Rock-soil Stratum

Nowadays, the problem of urban traffic congestion is becoming more and moresevere. The subway has the advantages of less environmental pollution, run faster andpunctuality, strong carrying capacity, low energy consumption etc., which isconsidered to be the most powerful means to solve the problem of urban congestion.There are more than20cities where subway construction is in proceeding in China.Qingdao has been planning19urban rail transit lines, and there are3lines which isunder construction. But troubled with the lack of construction experience andcomplex formation and the environment, the construction period cannot be guaranteed.Because the defect of the risk of underground construction is too big and the workingface is fewer and smaller, such as face few and small faults, several Qingdao subwaymining stations time lagged. Compared with the other method of undergroundexcavation, the arch cover method can make more use of reasonable characteristics ofrock composition with soil formation in Qingdao region, and achieve the effect ofefficient construction, saving cost, shorten the construction period. But at the sametime, the arch cover method of construction's risk factors exist in the each link ofdivision of the excavation, the erection of temporary support, demolition of thetemporary support and so on. Once accident occurs, there must be seriousconsequences. In order to popularize arch cover method in Qingdao area, it isnecessary to conduct a comprehensive and detailed risk analysis and evaluation for itsconstruction process. Based on the combination of soil and rock formation of Qingdaoas the research background, the dynamic risk assessment of tunnel construction ofarch cover method under complicated formation environment are studied. The mainresearch contents and conclusions are as follows:(1) The applicability research of arch-cover method under the condition ofcomplicated stratum of soil-rock assorted environment. The research reveals that:from the perspective of control the risk of surrounding rock's deformation, the arch cover method is superior to the double wall pilot tunnel method and the steps method.By means of numerical simulation, the main risk factors of the arch coverconstruction were analyzed, and the results show that if the surrounding rock of archfoot has the same conditions, the vault subsidence decreases with the increase ofburied depth; When buried depth are all the same, the conditions of surrounding rockof arch foot has little impact on the surface settlement; If the arch foot located inmoderately weathered rock strata, arch and the surface subsidence are all in a saferange.(2)The selection of main risk resources in the arch-cover method construction.The expert investigation method is adopted to screen the main risk resources in thearch-cover method construction, which include excavation steps, support removal andthe influence on surroundings in the process of construction.(3) Risk assessment of support removal in arch-cover method construction.Numerical methods and model test are adopted to do researches about the changes ofsurrounding rock pressure and the characteristics of soil deformation. The researchreveals that the displacement causing by middle pilot tunnel excavation during theupper cross-section excavation, the rock deformation is rarely changing through thelower profile excavation, and the most sharp deformation development occurs duringthe second arch lining finished stage of middle pilot tunnel excavation. The researchalso shows that there is large difference in the rock load releasing process of each partof monitoring sections during the process of tunnel face excavation, and the rock loadof arch part releases faster and larger comparing with arch feet and side walls'load.Use the fuzzy comprehensive evaluation method for risk assessment duringexcavation?The result is that the risk level of middle pilot tunnel excavation is A,removal of vertical support and the second arch lining is B, the right pilot tunnelexcavation is C, the left pilot tunnel excavation is D, and the lower profile excavationis identified as E.(4)Risk assessment of support removal in the process of arch-cover construction.Firstly, aiming to the characteristics of arch cover method construction in soil-stoneassorted areas in Qingdao, safety control method of initial support in the process of dismantling temporary shoring is summarized to apply in arch cover tunnelconstruction. Then, according to the results of numerical calculation and experts'feedback, the risk level of each support removal program is classified. It is suggestedthat the third program should be taken in site support removal because of its lowestrisk, where the support is removed symmetrically and alternately from two sides tothe middle.(5)Research about influence on surrounding environment and risk assessmentduring arch cover tunnel construction under the condition of complicated stratum ofsoil and rock combination. First of all, aiming to the characteristics of arch covertunnel construction in soil-stone assorted areas in Qingdao, displacement controlmechanism applied to partial excavation in arch cover tunnel construction issummarized. Then, model test method is adopted to do research on tunnel rockprogressive collapse failure mechanism in large cross-section arch cover construction.The research clears that the failure areas of tunnel rock is gradually becoming largeand the finial failure areas is changing by descending. The result also includes that thetunnel failure areas mainly focus on each dome part. It is the main cause of liningstructural damage and rock collapse. The arch feet and side walls are not damagedcleared. Next, according to the results of analysis of measured data, model test andexperts' feedback, monitoring program risk warning value and warning standardapplied to arch cover tunnel construction of Qingdao are summarized.(6) Engineering application of integrated risk assessment model. AnalyticHierarchy Process is utilized to assign weights of main risk factors in the process ofarch-cover tunnel construction. And take all rick factors into considerationsynthetically, the integrated risk model is to be established and applied in a stationsuccessfully.