Research On The Key Technology For The Construction Of Shallowly Buried Large Span Metro Interval Tunnel With Double Layer Primary Support Arch Cover Method

With the booming development of metro construction,there are more and more cases of shallowly buried concealed large span tunnels in the metro interval,for which the CD method,CRD method are mostly used.But the based on engineering was innovatively constructed using the double layer primary support arch cover method,with the consequent problems of a large number of pilot holes,many temporary supports and complex structural type conversions.In order to solve these problems and to investigate the key technologies suitable for the construction of shallowly buried large span metro interval tunnels,this paper relies on the construction of large-span return line tunnels between Huajing Road Station and Huashi Station on Guangzhou Metro Line 11,through data research,numerical simulation,field monitoring and other means to study the key construction technologies,to investigate the changes of the surrounding rock displacement field,stress field and internal force of the support structure during the construction process.The main research work is as follows:(1)A three-dimensional numerical finite difference model was established for the construction of the based on project and the changes in the displacement and stress of the surrounding rock during the whole construction process were calculated and analysed with the characteristics of "3 stages and 2 processes",according to which the key steps of the construction of the shallowly buried large span tunnel with double layer primary support arch cover method were the construction of the pilot tunnels and the removal of the temporary support.(2)For the construction of the pilot tunnels,single and multi-factor analyses were carried out on the main construction influencing factors: construction sequence,construction step distance,and construction footage,to obtain the influence characteristics of each factor on the displacement of the surrounding rock,stress and internal force of the support structure,and then obtain the construction influence significance of each factor,and finally determine the best construction sequence is determined to be 3’-4-1-2-5-6 in line with the principle of "first down,then up,first side,then middle",and the influence of the step distance of the construction of the pilot tunnels on the construction effect is non-single,and the influence of the construction footage of the pilot tunnels on the construction effect is most significant.Selecting the construction offset 6m,construction footage selection 0.5m is the best.(3)Taking the incremental displacement of the surrounding rock and the safety of the support structure as the judging criteria,the temporary support removal plan was optimised in terms of the sequence of transverse removal of the support,the length of one removal and the sequence of longitudinal removal of the support respectively,and the change law of the incremental displacement of the surrounding rock and the internal force of the support structure with the removal process was analysed to clarify the safety weakness of the support structure.The final optimized dismantling plan is "first two sides and then middle" transverse dismantling order,one dismantling length of 2m,and the longitudinal dismantling order of every other dismantling one.(4)The monitoring data were analysed in conjunction with the site construction conditions,and the changes in the site construction effects were summarised and compared with the numerical simulation results to verify the reasonableness of the numerical simulation and to determine the key construction techniques of the double layer primary support arch cover method for shallowly buried large span metro interval tunnel.