| Many technical problems and safety issues are often encountered during the construction of subway tunnels,And most of them are concentrated in geological formations,Such as land subsidence,ground fissures,loess collapsibility,water-rich formations,etc.In Xi'an,because of its geological environment is unique,different sections of tunnel construction process will be faced one or more geological problems with different geological environment,one of the premise for solving the technical problems and safety problems in construction is to find out the so-called “mechanical behavior” in tunnel construction.In this paper,based on physical engineering,the first stage of the project of xi 'an subway line 5(Heping-cun ~ Fangzhi-cheng railway station)through F3 fracture section of engineering of civil construction project between Xiyao-tou station and Hancheng-nan station.The section is in the water-rich yellow earth layer.Excavate while lowering the water level with crossing the f3 crack with a small angle of 29 degree is a typical "construction of an underground excavated subway tunnel through the crack in the loess layer".The main work and conclusions are as follows:Through on-site monitoring and analysis of the tunnel precipitation,surface settlement and dome deformation during construction,we can conclude :(1)Affected by dewatering,there is a dip offset between the upper soil and lower soil,and the settlement value of upper soil is larger than the lower soil,the difference between the two is 5mm.(2)In the process of construction,when the working surface is 20 m to 25 m away from the position of the ground fissure,the soil at the location of the ground fissure begins to be affected and the settlement deformation occurs.Then the upper and lower plates start to offset.By selecting three cross-section pressure boxes in the left-line tunnel can draw the following conclusions:(1)When the excavation face reaches and passes through the location of the ground fissure,the change in contact pressure between the initial support and surrounding rock of the first section is: the strength of the vault begins to decrease,inverse arch force began to increase.The change in contact pressure between the initial support and surrounding rock of the first section is: the strength of the vault begins to increase,inverse arch force began to decrease.Rapid increase in contact pressure between initial support and surrounding rock of the second section is: Contact pressures at the vault and invert positions are increasing,And the stress at the vault is greater than the position of the inverted arch.(2)The ratio of primary support to secondary lining load is 1:0.9,and the ratio of primary support to secondary lining load is 1:2.22,and the ratio of primary support to secondary lining load is 1:1.19.According to the actual engineering numerical simulation analysis,we have reached the conclusion:(1)During the forward construction of shallow-buried and dug subway tunnel passes through ground fissures from the lower plate to the forward plate,the range of influence of ground fissures to the lower plates along the longitudinal direction of the tunnel is 24.3 meters,to the upper plates is 41.3 meters.(2)The primary support structure of the ground fissure location is more stressful than the rest,especially the tensile stress is already close to the tensile strength.(3)Only when the left-line tunnel is excavated.ground settlement forms a settlement trough centering on the central axis of the left-line tunnel and settling groove width is 30 m.When the right line is connected,the surface settlement of the right tunnel is greater than the surface settlement of the left tunnel,and the maximum settlement occurs at the center of the model,and the differential settlement show at the ground crack location.(4)There was no fluctuation in stress during the construction of the secondary lining and the initial support construction.It maintains a more steady state of stress increase and reaches a gentle state in a relatively short period of time. |
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