| The surrounding environmental impact caused by shield tunneling and effective monitoring and early warning are hot issues in urban subway construction.Anhydrous sand layer has its own unique rule in the construction of shield tunnel.Based on the construction data of several subway sections of Shijiazhuang,this paper uses statistical analysis,numerical simulation and theoretical analysis to carry out the research on the environmental impact law of this kind of stratum.At the same time,the systematic research on the environmental impact monitoring of the shield construction is carried out by the close range photogrammetry.Finally,the paper puts forward and build an early warning framework for the environmental impact of shield construction for the anhydrous sand layer.The detailed contents and conclusions of the study are listed below:1)The main factors affecting the ground settlement of shield tunneling are analyzed.Based on the shield construction data of the water sand layer in Shijiazhuang subway,the shield construction data of the anhydrous sand layer are evaluated in several intervals,and the influence of the shield type selection on the ground surface settlement is studied.The selection rules are summarized to find out the key influencing factors.A large number of shield construction parameters are statistically analyzed with SPSS mathematical statistics software,and the influence factors and rules of different shield construction parameters are studied in detail.The influence degree of shield construction parameters on ground surface settlement is revealed by two means of grey correlation and measured data correlation coefficient statistics.The research results show that the shield machine should be given priority to using composite cutter disk,small opening rate and torque of small cutter head in tunnel excavation in anhydrous sand layer to improve the stability of the excavation face.The correlation between the total thrust F of the shield and the grouting volume Q,the torque T of the cutter disk and the driving speed S is better,and the correlation between the average pressure P and the torque T of the cutter disc is better than that of the other parameters.In the anhydrous sand layer,the influence degree of the shieldconstruction parameters to the ground surface settlement is S,the grouting amount Q,the average soil pressure P,the shield total thrust F,the torque T of the cutter disk and so on.2)The analytical solution for ground settlement of shield tunneling with longitudinal gradient section is derived.The physical and mechanical parameters of soil layer are carried out by combined numerical simulation and BP artificial neural network.The training function adopts the elastic gradient descent algorithm to make the neural network quickly enter the convergence state,and the results of the soil physical and mechanical inverse analysis can meet the requirements of the presupposition precision.In view of the shield driving in the longitudinal gradient section,based on the theory of spherical hole expansion and principle of mirror image method,and by integrating the the displacement formula in longitudinal direction of the shield and in the direction of the change of the depth of the tunnel respectively,the formula of ground displacement is obtained under the condition of the shield tunnel heading under the longitudinal slope line.The numerical simulation is used to calibrate the range of influence coefficient of double track tunnel in anhydrous sand layer.And the specific value was obtained by grouping experimental analysis with field measurement data.The results show that the settlement value increases gradually with the increase of theta,and the surface subsidence value caused by the upward heading is less than the ground subsidence value caused by the lower heading.The approximate value range of the double line influence factor correction factor of the soil layer of the anhydrous sand layer is-0.2~0.22,and when the fitting factor is-0.2 and 0.22,the double line influence coefficient is the most accurate.3)The empirical prediction formulas for ground settlement of shield tunneling suitable for Shijiazhuang’s specific geological characteristics are established.Based on a large number of regression analysis and Peck formula correction,the empirical prediction formula suitable for the specific geological characteristics of Shijiazhuang is obtained,and the regularity between the surface settlement curve and the design parameters of the double track tunnel is studied.The results show that the variation range of the maximum settlement correction parameter of the surface subsidence of the sand layer in Shijiazhuang area is approximately 0.32 ~ 0.57,and the variation range of the correction factor of the width of the sink width of the sand layer is approximately 0.35 ~ 0.9.The maximum settlement value of the double line tunnel is relative to the one side of the central axis of the double line tunnel,and the deviation distance is 1.7~4m.The surface subsidence curve of the rear excavation tunnel is no longer consistent with the normal distribution,and the maximum settlement value is less than that of the first tunnel.4)The analytical solution for the influence of underground pipelines in shield tunneling is derived.Based on the Pasternak double parameter elastic foundation beam model,the displacement field caused by the excavation of the shield tunnel is taken asarbitrary form.The load of the surface displacement field on the underground pipeline is expressed by Fourier series,and the form of the deflection and internal force expression of the underground pipeline under the action of arbitrary load is derived.Through the comparison of literature and engineering examples,the correctness of the derived formula is fully proved.By the sensitivity analysis of the shear modulus G of underground pipelines,it is found that the deformation of underground pipelines decreases with the increase of shear modulus G.Moreover,the analytical results obtained in this paper are more reasonable than those in the Winkler model for the analysis of underground pipelines in shield tunneling with larger shear modulus.5)A formula for deformation of adjacent buildings in shield tunneling is proposed.Taking full advantage of the data of six interval tunnels of Shijiazhuang No.3 line,the statistical analysis of the width parameters of the settlement trough in the anhydrous sand zone is carried out,the correction coefficient of the stiffness correction is corrected.By the single line tunnel correction Peck formula and the correction coefficient of the double line,the prediction formulas of the settlement and tilt of buildings adjacent to a double line tunnel are established.Considering the influence of design parameters,material parameters and geological parameters,the numerical simulation of the adjacent buildings under multi conditions of shield tunneling is carried out.The results show that the width parameter of the surface subsidence trough is 0.65 that conforms to the geological characteristics of the Shijiazhuang anhydrous sand layer.Because of the influence of the building,the maximum value of the settlement maximum is smaller than the Peck formula,the width of the settlement trough is larger,but the location of the maximum settlement point is not obviously changed.The position of the inflection point of the prediction deformation curve of the building and the measured settlement curve is basically consistent.6)The application of close range photogrammetry in shield tunneling monitoring is discussed,and an early warning mechanism suitable for the environmental impact of shield construction in the anhydrous sand layer is constructed.Through the orthogonal simulation test and the field test,the shooting parameters suitable for the ground monitoring of the shield tunnel construction are summarized.By analyzing the geometric conditions of the two dimensional changes of the building cracks,the analytic formula for the observation of the building cracks is derived.In view of the complex interaction system of tunnel soil layer,underground pipeline and adjacent building,on the basis of control of the respective deformation of the surrounding environment supplemented by the corresponding field deformation monitoring,a multi-stage fortification early warning framework for the surrounding environment of the shield construction is constructed.The research results show that the parameters of the surface deformation observation suitablefor close range photogrammetry are the focal length of 28 mm,the shooting distance of0.3m,the angle of 30 degrees,the length of the shooting height of 1.55 m.And the common precision of the close range photogrammetry crack observation can satisfy the requirement of the observation precision.The derived crack observation formula can be used to obtain two-dimensional development of cracks with high accuracy. |
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