| As an important auxiliary construction method,the support of advanced small pipes plays a vital role in maintaining the stability of the tunnel face,which is used in the construction of shallow-buried tunnel of Harbin Metro with a plastic silty clay layer.However,there is currently no clear understanding of the supporting and reinforced mechanism of advanced small pipes.During the construction process of the tunnel in the bench method,excavation steps have an important influence on the construction progress and total duration of project,and too large excavation step will affect the safety of tunnel.In order to solve these problems,a large-scale model test was designed based on similarity theory,and laboratory tests and model tests were carried out to study the influence of different working conditions on the stability of tunnel surrounding rock and supporting structure;furthermore,by using FLAC3D,a numerical calculation model was established to analyze the influence of different working conditions on the stability of surrounding rock and mechanical characteristics of primary support structures,and the results were compared with the model test results;finally,based on the pipe-roof effect induced by the support of advanced small pipes,the calculation model was proposed to evaluate the stability of the tunnel face,and results obtained by the model test and numerical simulation were validated.(1)In order to reduce the influence of boundary effect on results obtained by model tests,a large-scale model test was carried out based on similarity theory.Through laboratory tests,the physical and mechanical parameters of the soil were obtained,and the proportion of gypsum used to simulate the lining was determined;what’s more,the monitoring data obtained by model tests was analyzed.The results show that,when applying advanced small pipes under working conditions with different excavation steps,the values of ground settlement and earth pressure are reduced;for working conditions with or without advanced small pipes,the values of ground settlement and earth pressure increase;for the working condition where the excavation step is 48 mm,small pipes of 10 cm are relatively short,and the support effectiveness under this working condition is worse than that where excavation step is 32 mm;with the increase of excavation step,the internal force values of the arch increase for working conditions with or without advanced small pipes.(2)Based on parameters of model tests and the similarity ratio,using FLAC3D software,the numerical calculation model was established to analyze the influence of different working conditions on the stability of surrounding rock and mechanical characteristics of primary support structures.Moreover,the variation law of numerical calculation results was analyzed.The results show that the application of small pipes can support the surrounding rock effectively,and the increase of the excavation step will enlarge the disturbance to the surrounding rock during the process of excavation.Small pipes with the length of 2.5 m have better supporting effect under the working condition where excavation step is 0.8 m.Therefore,the small pipe with appropriate length should be used for advanced support according to the excavation step.(3)Based on the pipe-roof effect induced by the support of advanced small pipes,the stability of tunnel face was regarded as the stability of slope when pipes transmit and release load.The plastic shear limit state of the tunnel was regarded as the critical state,and the calculation model for evaluating the stability of tunnel face was proposed.Two working conditions with and without small pipes where excavation step is 0.8 m were selected to verify the accuracy of the results obtained by model tests and numerical simulation.It is found that for the VI grade plastic silty clay in the test section,the advanced small conduit support must be applied to ensure the stability of the tunnel face.It is found that for the VI-grade plastic silty clay,the advanced small pipes must be applied to ensure the stability of the tunnel face. |
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