| With the rapid advancement of the national economy,urbanization has been developed dramatically with serious problems of traffic jam and environmental pollution.Urban rail transit construction is an effective way to alleviate the traffic pressure.And for the advantages of being able to adapt to complex and diverse geological conditions,small disturbance and high degree of intelligence,shield tunneling method has been widely utilized in urban rail transit projects.However,the instability of the tunnel face during the shield construction often leads to some engineering accidents,which include abnormal shutdown of the shield,water inrush of the tunnel face,ground subsidence and even the collapse of buildings.Especially when the large diameter shield is constructed in soft ground with high permeability,the appropriate support pressure on the tunnel face is difficult to control,thus the possibility that the tunnel face becomes instable is enhanced.What's more,the support pressure in the excavation chamber and the water-earth pressure in front of the tunnel face are always in a dynamic balance state during the shield tunneling.If the support pressure is set unreasonably,the dynamic balance will be easily broken,which is also the essential reason for the instability of tunnel face.Furthermore,the support forms with different support media are different.Whether the support pressure can be effectively applied on the tunnel face as intended needs to be further clarified and guaranteed.In addition,the engineering geological conditions and hydrogeological conditions of different excavation cross-sections are various,and there are uncontrollable manual operation errors when setting shield tunneling parameters.Ensuring the scientific control of shield tunneling parameters is essential to maintain face stability of shield tunnel.In conclusion,in order to maintain shield tunnel face stability,the support pressure in the shield excavation chamber should be effectively applied through the reasonable control of the tunneling parameters,which is the key problem that must be faced and solved during shield construction in soft ground.And it is also the basic requirement to continuously improve the automation and intelligence of the shield machine and even to realize the "unmanned driving" of the shield machine.In the respect of the above problems,face stability analysis during shield construction in soft ground and the research on the support mechanism were investigated.And the supporting effect with different penetration patterns in slurry shield tunneling was discussed.Combined with the seepage effect of the supporting slurry and the cutting disturbance of the cutterhead,the calculation model of face stability analysis and the limit support pressure was modified.Finally,based on the analysis of shield tunneling parameters in the actual project,the prediction model of key tunneling parameters was established,and the key tunneling parameters were optimized considering the control of face stability and reduction of energy consumption.The main research work and results are as follows:(1)Based on the slip line theory and the upper-bound limit analysis theorem,the potential slip line due to the shear failure in front of tunnel face is studied,then according to the assumption of the velocity field of the tunnel face instability,the velocity discontinuity of the slip system is constructed.Next,the morphology of the three-dimensional(3D)sliding failure model is determined.Finally,based on the soil plasticity theory and the flow rule,the spatial discretization technique is utilized to determine the position information of each point progressively on the velocity discontinuity of the slip system,thus the internal dissipation power and external load power of the slip system can be calculated,then the limit support pressure of the tunnel face is calculated.The construction of theoretical model is firstly based on pure clay layer under the ideal state,which uses Tresca yield criterion to construct regular circular-arc slip surface.On this basis,the extension of theoretical model to frictional soil and the discrete construction of 3D failure model are realized.Based on the limit shear strain theory,the numerical calculation model under the critical state of tunnel face instability is constructed.The strict convergence criterion of the numerical model is defined to obtain the limit support pressure on the tunnel face,and the limit shear strain failure model under the critical state of the tunnel face instability is extracted.Therefore,the rationality of the theoretical model is comprehensively analyzed from the two aspects of the failure model:morphology and the limit support pressure.The good concordance obtained by the comparison of the results with the numerical simulations and other existing researches shows that the theoretical model reasonably considers the horizontal and vertical soil arching effect in front of the tunnel face,and the stress distribution law can be reflected from the perspective of strain by using the limit shear strain theory.In order to analyze the face stability during shield construction in composite stratum,the limit equilibrium analysis model is established.Then case study and numerical calculation are investigated to verify the rationality of the theoretical model.(2)The slurry-soil interaction mechanism under slurry support pressure in slurry shield is studied,and the influence of different geological parameters and slurry parameters on the migration law of slurry particles is investigated.In addition,based on numerical simulation,the variation law of key parameters such as excess pore water pressure and porosity of the strata in front of the tunnel face during the slurry infiltration is studied,then the necessary conditions of filter cake formation for slurry infiltration are clarified.On this basis,in order to better describe the migration law of slurry in the stratum and the formation and development of filter cake,the deep penetration and surface penetration models of slurry are established.Then the influence of the key geological parameters and slurry parameters on slurry infiltration and filter cake development are analyzed.Based on infiltration effect of slurry,the different forms of slurry support pressure during the whole shield tunneling are analyzed,and the influence of key parameters of shield tunneling on filter cake formation and support effect is also studied.In view of the periodic influence of cutterhead rotation speed and cutter penetration on filter cake formation,the cyclic variation of slurry infiltration law during slurry tunneling is stuided,and then the slurry support effect under the influence of cutterhead cutting is analyzed.Considering the infiltration effect of slurry and cutting disturbance of the cutterhead,the stability analysis method and calculation model of limit support pressure are modified according to the variation of slurry support effect,which can meet the requirements of face stability analysis under different tunneling parameters.(3)In view of the complexity of shield tunneling process and the variability of soil parameters,the correlation between various tunneling parameters is investigated by analyzing the shield tunneling parameters combination obtained in actual projects,then the interaction law between multiple shield tunneling parameters is clarified.Moreover,the rationality of shield tunneling parameter configuration is evaluated by support effect of tunnel face and energy consumption index of shield tunneling,and the optimal tunneling parameter combination is selected.On this basis,the prediction model of shield tunneling parameters is established by machine learning method to predict the key tunneling parameters.At the same time,the tunneling parameters are optimized according to the stability evaluation and energy consumption index during shield tunneling.The optimiazation can not only ensure that the shield tunneling parameters are well configured,but guarantee the safe shield tunneling and low construction cost. |
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