| With the rapid economic development of our country,the scale of subway construction in all major and medium-sized cities is increasing steadily.With the implementation of the "Belt and Road" strategy,the construction of urban underground space in China has been ushered in a new development climax.As the construction edge tool of underground engineering,shield machine is exerting its unparalleled advantage.It not only has the advantages in high degree of mechanization,less disturbance to strata,fast speed of driving and less influence on the surrounding environment,but also its safety is more reliable than the traditional mining method for tunnels excavation.Therefore,the application of shield method construction in underground engineering of our country is more and more.However,because the shield method is started late in China,there is still a large gap in the technical method and operation personnel's proficiency and management level compared with other advanced countries.In recent years,engineering accidents such as ground subsidence,collapse,water gushing in excavation face and soil instability in shield end and so on,have been occurred from time to time,resulting in casualties and economic losses,which has been caused great attention to the research of shield construction technology in academic and engineering circles.Through practice and investigation for shield construction in site of Fuzhou Metro Line 2,in view of the geological and hydrological conditions,which are mainly composed of mealy sand soils which is and rich in high water pressure,the research on key technical research has been carried out,in order to solve the difficult problems these are encountered in shield tunnel construction,such as shield launching,stability of excavating face,the control of ground settlement and groundwater level,and so on.By the combination of engineering practice,theoretical research,field monitoring and numerical analysis,the relevant technical researches have been carried out.The main research works and research progresses achieved are as follows.1.The main research works(1)Through the technical investigation on site,the general situation of the project,the engineering geological and hydrological conditions,the main contents of shield construction,the flow of work and the overall construction plan are all analyzed in detailed.The difficult problems in the construction of earth pressure balance shield method are studied,and the technical scheme to solve the related construction difficulties is discussed too.(2)Through the engineering practice and theoretical analysis,aiming at the key technical problems,such as shield launching,shield excavation,reinforcement length of shield tunnel end,stability of counterforce frame,stability of launching frame,controlling of tunneling parameters,sealing for shield tunnel portals and so on,a comprehensive discussion and analysis are carried out under the condition of full section is sandy layer.The technical mechanism of earth pressure balanced shield construction has been revealed,and the relevant technical schemes have been demonstrated.(3)Based on the study of the stability mechanism on excavating face in shield construction,the technical measures to be adopted to maintain the stability of excavating face in shield construction are proved from three aspects that are earth pressure controlling,improving residue soil and waterproof technology.(4)Based on the analysis of the monitoring data on site,such as settlements of ground,settlements of underground pipelines,the inclinations of the buildings and the three-dimensional deformations of the tunnel,the key technologies of settlement control used in the construction of this project are discussed.The effects of implementation for these technologies are analyzed.The influence laws of shield construction on the settlement of ground and buildings have been ascertained,and the influence mechanisms of relevant technologies on settlement and deformation controlling have been revealed.(5)Based on the analysis of pumping test data on site,the influence law of groundwater level falling on ground settlement during the process of pumping is ascertained,and the initial parameters in 3D model are determined.Using the established three-dimensional numerical model,the drawdown scheme for shield station foundation pit is numerically simulated.The influences mechanisms of drawdown on the settlements of strata and surrounding buildings during the excavation,in full-section are sand soils by shield method have been revealed.The reliability of numerical analysis model has been verified,by comparing the calculated results with tested results which are obtained by pumping test.Finally,the settlements of surrounding buildings have been predicted and analyzed,which are caused by the variation of foundation pit water level after drawdown.2.The major research advances(1)Based on the research on shield launching technology in full section are sand soils,the calculation method for shield end reinforcement is put forward.The theoretical calculation results are close to the actual reinforcement length and the practical application effect is good,which proves the reliability of the theoretical calculation method.Through the analysis of the tunneling parameters in the process of shield tunneling,the calculation method for the determination of shield parameters is put forward in the course of tunneling in full-section is sand soils.On this basis,the key technologies and construction essentials involved in the process of shield launching of have been determined.(2)In view of the special geological conditions that the full-section is sand soil layer mainly,and combining with the existing theory and the actual situation of engineering construction,the key techniques and methods to maintain the stability of excavating face are put forward from three aspects these are controlling the pressure in soil bin,improving the residue soils and water-proof and plugging.Furthermore,the effects of implementing the related technologies are analyzed,and the rationalities of the proposed construction technical measures have been verified.(3)Through the analysis of the settlement data obtained from the actual monitoring in this project,the variation laws of strata settlements have been found out,in the process of shield tunneling when the full-section is sand soils by using the soil pressure balance shield.In view of the abnormal settlement in the course of shield construction,the settlement control measures which should be adopted in the construction are put forward.By analyzing the effect of implementing of settlement control,the rationality of the proposed settlement control technology is verified.(4)Based on three-dimensional seepage analysis theory and field pumping tests results,a three-dimensional numerical analysis model is established.The numerical simulation analysis on dewatering scheme of the foundation pit in the subway station is carried out.Through numerical analysis,the comprehensive permeability parameters of soil layers and the influence radius of strata permeability are obtained.Using the established three-dimensional numerical analysis model,the groundwater levels and the settlements of surrounding buildings in subway station where the strata are water-rich sand layers are forecasted and analyzed.It provides a theoretical basis for the settlement control in shield construction.Moreover,the reliability of the three dimensional permeable model is verified by comparative analysis on the results obtained from model calculation and pumping test.From the foregoing,through researching on the key technologies for shield construction from Juyuanzhou Station to Hongwan Station of Fuzhou.Metro Line 2,the difficult problems encountered in the construction of earth pressure balance shield method are analyzed,under urban complex geological environment.And the key construction techniques to solve these difficult problems are explored.Through researching on the key technologies,such as shield launching,the stability of excavating face,the controlling of ground settlements and groundwater level and so on,the technical ideas to solve the difficult problems for this project construction have been put forward.These research results not only play an important role in promoting the development of construction technology and theory of earth pressure balance shield method,but also provide a useful technical reserve for similar shield construction.At the same time,it will also lay a technical foundation for the normalization and standardization of earth pressure balance shield construction in the full section is sand soil layers. |
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