| Along with the urbanization construction and the expansion of number and scale of urban subways, limited by the geological conditions, shield method is the main construction method of subway projects. Because of the influence of the existing structures, geological conditions, underground space and other factors, the phenomena of the new urban subway tunnel of close tunnel and crossing or approaching existing subway tunnels and other structures are inevitable. Complex mechanical problems and construction problems are existed between the new built subway tunnels and between the new built subway tunnel and existing structures. The subway tunnel’s buried depth is very shallow and the tunnel is very sensitive to the excavation disturbance. When the new built subway tunnel is closed to the existing tunnel or other structures and no control and protection measures are taken, there will be many destructive consequences to the existing tunnel and other structures. New built subway tunnel shield construction usually cause soil disturbance of surrounding formation and make additional stress to the existing tunnel structures. If the additional stress exceeds the strength of the existing tunnel, tunnel segment will be misplaced and cracked and lead to the ingress of groundwater and affect the normal use of existing tunnel. At the same time, the structural deformation of existing tunnel can cause disturbance deformation of surrounding formation and threaten the construction safety of the new built tunnel. So, how to reduce and control the interaction of the two tunnels of approaching construction is the core problem of approaching construction of subway tunnels, which has attracted growing attention in the field of geotechnical engineering.Thus the research on the construction mechanics and interaction of shield tunnel approaching existing tunnel is especially important. Although there are large numbers of researches in this respect at home and abroad, the researches on the approaching construction mechanics of shield tunnel are very few. The researches on approaching construction of shield tunnel of particular geological section in Suzhou are especially less. And the researches on the control measures of approaching construction of different approaching influence degrees are lacking. As a result, the research on the construction mechanics and control measures of urban subway tunnel approaching construction has important practical implications.This paper is based on the construction project of Suzhou metro tunnel line 4 from Anyuan road station to Chun Shenhu road station. The distance of the subway tunnel of this section is 4.5m-8m. According to the relevant researches and the standard, this section belongs to the strong influence range and the influence of the later excavated tunnel on antecedence tunnel is obvious. The researching subway tunnels are two approaching construction tunnel through lacustrine silty clay layer. The purpose of this paper is how to control the large deformation of the ground caused by the approaching construction effectively and how to control the too much stress-strain caused by the influence of later excavated tunnel on antecedence tunnel.The paper analyze the construction control technology of approaching subway tunnel and stress-strain of approaching construction of existing tunnel and the deformation of the surrounding strata by laboratory experiment, theoretical analysis, numerical simulation, field monitoring analysis and mathematical analysis. And different shield construction control measures are taken to maintain stability of the existing tunnel in different sections. The conclusions provide guide and reference to the subway shield construction and similar engineering constructions.Based on the field investigation of engineering geological conditions and distribution of various types of approaching objects of Suzhou metro tunnel line 4 from Anyuan road station to Chun Shenhu road station, a research area where minimum distance is 4.5 meters is selected. Engineering geological conditions of research area are analyzed. Soil sampling and laboratory experiments are conducted. Combined with similar engineering experiences, shield construction parameters in this condition are put forward. Then based on the analysis and research of all kinds of approaching objects’ characteristics and classifications, a research project of the influence of the later excavated tunnel on antecedence tunnel is put forward. On this basis, shield tunnel approaching construction’s mechanism and stress-strain are analyzed. In order to make a further research on shield tunnel approaching construction’s mechanism, three-dimensional numerical simulation software ABAQUS is adopted to simulate and calculate conditions when the later excavated tunnel construct approaching and passing antecedence tunnel’ monitoring section. In order to prevent serious deformation of the formation and inefficient of construction in shield construction, the shield segment’s stress-strain, ground deformation and shield construction parameters are monitored. Through the analysis and research on the numerical analysis and field monitoring, shield construction control measures are put forward.The main research contents and conclusions of this paper include:(1)The field soil of the research section is sampled and experimented of physical mechanics. The physical characteristics, compression strength, shear strength and other physical and mechanics parameters of this section are analyzed. The tunnel crossing soil is lacustrine sedimentary soil and includes humic matter. The soil is saturated, plastic flow and relatively homogeneous and does not contain sand and cobblestone. Combined with compression strength test, the soil stress-strain characteristics under cumulative step by step load are confirmed. Along with the increase of the pressure, the modulus of compression of the soil increase. The shear mechanical parameters of the soil are earned by the soil shear strength test. And the sensitivity degree of the change of shear strength caused by the change of the vertical load is analyzed. The ①3 plain fill’s shear strength is changed most by the change of the vertical load. Through the compression-unloading-compression cycle test and compression-unloading-shear cycle test, mechanical characteristics under the three axial loading-unloading cycle conditions of the section soil are analyzed. The compressibility and deformation of the soil under the compression-unloading-compression cycle test are reduced and the shear strength and shearing property are increased.(2)The shield type and main working parameters are confirmed. Based by the formation permeability coefficient, grain composition and underground water pressure, earth pressure balance shield machine is most reasonable from the environment and the economic efficiency. Through the theoretical arithmetic and the construction experience of the similar projects, the main working parameters of the process of shield excavation are confirmed, including equilibrium pressure, tunneling thrust, cutterhead torque, advance speed, the rotation speed of the cutter head, volume of excavated earth, routing parameters, etc.(3)Through the mechanical analysis of the construction of single shield tunnel, shield construction makes the extrusion-shear-unloading effect on the surrounding soil and makes a disturbance area around the tunnel. According to the analysis of formation displacement after excavation and support of single shield construction, support has important effect on the control of soil deformation. By the analysis of the mechanical mechanism of approaching construction, the influence of the later excavated tunnel on antecedence tunnel is also a process of extrusion-shear-unl oading.(4)Through the mechanical analysis of shield construction, the extra-stress of the later excavated tunnel on antecedence tunnel is simplified as the sum of soil and water pressure and propulsion component force. A two-dimensional analysis model is established of shield machine passing monitoring section of antecedence tunnel. A uniform load of 0.22MPa calculation soil pressure is exerted on the left boundary. And the maximum deformation of soil is about 3.8mm. Segment makes vertical deformation and the maximum deformation of soil is 1.8mm. The surface soil displacement is controlled effectively. So, the calculative soil pressure balance force is reasonable.(5)After working face of the following tunnel reaches the monitoring section, when the pushing force in the 0.22MPa 0.72MPa range, the deformation of the segment is relatively small within 10mm and changes little when the pushing force is in the range of 0.22MPa～0.72MPa. With the pushing force continues to increase, the segment deformation rate increased significantly and the inflection point appeared at 0.72MPa, then the deformation exceeds 10mm. When the pushing force is 0.72MPa, stress of the monitoring section is most close to the actual monitoring section, while segment deformation is within the allowable range. Considering the construction efficiency and construction safety,0.72MPa can be the basis of correction of the earth pressure parameters in the construction of the following tunnel.(6)Numerical analysis model when the following tunnel go through the existing tunnel is established, the analysis of three dimensional stress state of the monitoring section in the existing tunnel, such as the radial stress, circumferential stress and axial stress, shows that the influence on the existing tunnel caused by the construction of the following tunnel is mainly the uneven stress in the segment, and the circumferential stress shaped fluctuately which easily leads to the damage of segment. In addition, the segment deformation of proximity location is obviously larger than the region that is far away from the proximity location and it has a big difference..The construction conditions should be controlled to ensure the safety in the construction process. The construction of the following tunnel leads to the arch crown settlement and arch bottom uplift in the existing tunnel, the segment pressure the soil on both sides under the action of the vertical deformation, then the soil tend to move outside the tunnel. In the adjacent construction, The maximum deformation of the existing tunnel appears on the left side, and right side is minimum. After excavation of the left tunnel, the maximum displacement of the right tunnel segment monitoring section was 6mm on the left side, and the minimum displacement of right side is 1.5mm.(7)The field monitoring of the stress and strain of the existing tunnel segment turns out that the circumferential stress is larger than the radial stress and axial stress when the working face of the following tunnel is far from the monitoring section. The radial stress of vault and left waist place of the segment in the existing tunnel has the tendency of reduction along with the working face going, while the bottom of the segment increases and the right arch waist almost has no change. With the advance of the following runnel, in the plus or minus 45 degree range of the existing tunnel segment, circumferential stress keeps constant and axial stress showed an overall increasing trend. The influence on the level position stress in the right tunnel segment caused by the excavation of the left tunnel is more complex which the circumferential stress is wave shaped and axial direction stress shows an overall decreasing trend.(8) Results can be obtained through the field monitoring of surface displacement:The variation of total displacement of surface due to the construction of ahead of the tunnel and tunnel underwent conformed the regularity of surface subsidence of proximity holes parallel tunnel. Before the shield machine arrived in the monitoring section C3 and C4, tunnel mainly uplift deformation above the surface when the distance is 5 meters, above the tunnel underwent produced the greatest amount of uplift, whose values were 1.28mm and 1.65mm. This shows that the irrational of shield tunneling parameters lead to surrounding soil by squeezing effect before the adoption of shield which results in the surface uplift. After shield machine through monitoring surface C5, the surface of the first hole produce the maximum settlement 25.2mm,which Close to control standard 30 mm. This shows that there is a certain irrationality of shield construction parameters, which need to regulation and control.(9) Through analysis of numerical simulation and field monitoring data, the basic criterion of shield proximity control technology was proposed and the parameters of shield proximity construction were controlled analysis combined with relevant engineering experience. Analysis of surface uplift deformation mechanism When the shield machine through 5m off the front of the monitor C3 and C4, obtained that the cause surface uplift may be soil tank pressure or driving force setting too large. Which need to reduce soil tank pressure or driving force appropriately, the soil tank pressure could reduce OMPa～0.02MPa and the driving force could reduce 0kN-220.85kN.(10) After shield machine through monitoring surface C5, the surface of the first hole produce the maximum settlement 25.2mm, closed to control standard 30mm, which due to lack of grouting in the process of grout or excessive advancing speed, required increase the amount of grouting properly or decrease advancing speed. Grouting quantity can be further increased between 0～0.5 m3/ring according to the actual situation. To avoid "kowtow" phenomenon during tunneling in silty clay, Shield stance should always stay for a little rise trend and pitch angle should be controlled at between 3-5mm/m, at the same time the deviation between axis of shield machine and design axis in the horizontal and vertical directions should controlled within ± 20mm. If the deviation is too big in construction, shield stance should gradually slow adjustment immediately and avoid urgent correction, in other word should avoid too large one-time adjustmentThis paper is based on the construction project of Suzhou metro tunnel line 4 from Anyuan road station to Chun Shenhu road station. Subway shield tunnel approaching construction and the stress and deformation mechanism of approaching objects are analyzed by different research methods. Different measures are taken to maintain the stability of tunnel structure. Important technology is provided to guarantee the construction of subway tunnel and safe operation. The research of the paper has important value of theory research and practical application. |
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