Study On Mechanical Characteristics And Load Distribution Pattern Of Up-down Overlap Tunnels

the large-scale construction of the mountain tunnels has brought a lot of engineering examples on the newly built tunnel passing through the existing one at short range,the construction of a new tunnel is often caused stress adjustment and redistribution of surrounding rock and supporting structure of a new tunnel,which brought complex changes on areas nearby and supporting structure mechanics properties,it has seriously endangered the construction and structural safety.So,the issue of how to reduce the negative impact of the adjacent construction ensuring that the existing tunnel safety operation has become an urgent problem.To take newly-built Liutang tunnel in Xiamen city oblique crossing the existing one as the research background,utilizing the means such as field monitoring and numerical simulation,mechanics of up and down crossover and the load distribution patterns are studied,the main research results are as follows1)the convergence of the new tunnel presents a trend of "rapid change-slow change-stability",which proves that on the condition of a close measure section of tunnel face made in early stage,and the stress is not yet stable,the internal force distribution has not been completed,and with the advancement of tunnel face,the distance between measure section is increasing,and the stress of measure section is stable,and then make the convergence deformation smaller until the leveling off.Contact stress,the steel arch axial force and the strain of second lining surface monitoring also verify the above rule,at the beginning of the construction,it is greatly influenced by the adjacent construction section,but after reaching a certain distance,the influence began to decay,eventually to stabilize.2)The construction on new tunnel crossing section has lesser effects on the existing tunnel,and the existing one reached a new equilibrium soon after construction interference,which proves that the construction of the new one conforms to the specifications,achieving the expected result of construction.3)By using FLAC3D,the numerical simulation on Liutang tunnel is carried out,and compared with the numerical simulation analysis of value and monitoring result,it indicates that its horizontal constringency and arch sedimentation of section is roughly same with the change trends,numerical value analysis and monitoring is more close to,it indicates that the numerical simulation could correspond with the monitoring quantity,and reflect the construction displacement better,improving the reliability analysis for the analysis of construction mechanics.4)The paper proposes a double-fold linear load distribution mode for Liutang tunnel,and studies the load distribution mode of existing tunnels under unloading,focusing on net,cross Angle,the changing rule of the load distribution patterns when a new tunnel span is different.The results show that unloading will not change the load computation schema,but the size of the load.The smaller the net distance,the larger the clear distance,while the smaller unloading coefficient and the more obvious the unloading effect.With the increase of the new tunnel span,the unloading effect is obvious and the unloading coefficient decreases.