Study On Deformation Mechanism Of Supporting Structure And Stability For Long And Narrow Deep Excavations

The deep excavations in subway stations,comprehensive pipe corridors,highway and railway tunnels and other construction projects belong to long and narrow deep excavations.With the development and utilization of urban underground space,the number of long and narrow deep excavations is increasing.However,the influence of the width and soil spatial variability is less considered in the design of long and narrow deep excavations,and the supporting structure has room for optimization.Therefore,based on a long and narrow deep excavation project and based on the research methods of mathematical statistics,theoretical research,model test and numerical simulation,this paper carries out the research on the influence of the width and soil spatial variability on the deformation behavior of the supporting structure and stability for long and narrow deep excavation.Then,the optimization design method of supporting structure of long and narrow deep excavations considering the width and soil spatial variability is proposed,and the research results are applied to the optimization design case of supporting structure of long and narrow deep excavations.The main contents and conclusions are as follows:(1)Based on the case of a long and narrow deep excavation,the field test of deep excavation deformation is carried out.Combined with theoretical calculation,comparative analysis and mathematical statistics,the deformation law of the deep excavation is obtained,and it is clear that the width and the spatial variability of soil have a significant impact on the deformation of long and narrow deep deep excavations.The results show that the maximum surface settlement is between 0.02%H and0.4%H,and the average value is 0.19%H.The maximum horizontal displacement of the retaining structure is between 0.08%H and 0.38%H,and the average value is 0.19%H.The narrower the width of the pit,the smaller the deformation of the long and narrow deep excavationgs.The deformation value of the pit presents a random distribution law under the influence of the spatial variability of the soil mass.The actual deformation value of the pit is small,the existing support structure designed in the code is conservative,and there is a large optimization space for the support structure.(2)Based on the similarity model theory,the delicate support structure is made by using 3D printing technology,the scale model test of the pit is designed.Combined with numerical simulation,the influence law of the width on the deformation and stress of long and narrow deep excavations is studied,and the influence mechanism of the width on long and narrow deep excavations is revealed.The results show that the width of the pit significantly affects the mechanical response of the long and narrow deep excavation.The narrower the width of the pit,and the smaller the surrounding soil disturbance area,the smaller the earth pressure borne by the retaining structure,and the smaller the deformation and internal force of the pit.The spatial effect,restraint effect and mutual extrusion effect of sliding soil are the internal mechanism of the influence of deep excavation width on the mechanical response of the deep excavation.(3)Based on the upper bound finite element theory and adaptive technology,the basal heave stability calculation program of long and narrow deep excavation is independently developed,which without assuming the failure mode.The influence law of deep excavation width on the basal heave safety factor and uplift failure mechanism of long and narrow deep excavation is explored.The research shows that the basal heave safety factor of narrow and long deep excavations can be roughly divided into exponential rapid reduction stage(narrow excavations B/H≤2),slow reduction stage(wide excavations 2<B/H<f)and stable stage(ultra wide excavations B/H≥f).When the width depth ratio B/H≤2(narrow excavations),the influence of width should be considered in the analysis of basal heave stability.When the width depth ratio B/H>2,the influence of width can be ignored in the analysis of basal heave stability.The width of pit affects the mutual extrusion effect between sliding soil,sliding radius and sliding area of soil in the process of uplift and instability failure of foundation pit.The space effect,restraint effect and mutual extrusion effect of sliding soil are the internal mechanism of the influence of width on basal heave stability.The basal heave failure of long and narrow deep excavation is in the "arc expansion" mode,which is obviously different from the failure mode adopted in the existing codes,and the basal heave safety factor calculated by the code is too small.(4)In the framework of random field theory and Monte Carlo stochastic principle,considering the spatial variability of soil,a set of stochastic finite element algorithm for automatically calculating the reliability of the pit is developed by using Python programming and ABAQUS software.The influence of soil spatial variability on the deformation behavior and reliability of long and narrow deep excavation is studied.The results show that for the narrow and long deep excavations with the width depth ratio B/H≤2 under the influence of soil spatial variability,the deformation curve of excavations retaining structure and surface settlement curve are discretely distributed near the deterministic results.At least 21.9% of the random results are greater than the deterministic results.If the influence of soil spatial variability is ignored,the deformation of the pit will be underestimated.Under the spatial variability of soil mass,the deformation characteristics of deep excavations are related to the width of pit.The larger the width of foundation pit is,the higher the dispersion degree of foundation pit deformation is.The influence degree of soil spatial variability on deep foundation pit is related to its variability index,which affects the deformation prediction and control of deep foundation pit design and construction stage.Thus,the influence of soil variability should be considered according to the soil variability index in the design of the long and narrow deep excavation.(5)Taking the cost and robustness of deep excavations as the optimization objective,based on the Pareto optimal optimization method,for the narrow and long deep excavations with width depth ratio B/H≤2,an optimization design method of supporting structure of long and narrow deep excavations considering the influence of foundation pit width and soil spatial variability is formed,which is successfully applied to the optimization design case of supporting structure of long and narrow deep foundation pit.Compared with the original support scheme,the optimized design saves the cost,shortens the period,weakens the impact of construction on the environment,and has certain popularization and application value.