Research On Mechanical Properties Of Pile-Beam-Arch Subway Station Buckle Arch Construction In Loess Area

In the construction of urban subways in loess areas,due to the inherent large pores and water sensitivity of loess,it is easy to cause problems such as land subsidence and collapse during construction,resulting in large economic losses.In the construction of the Pile-BeamArch method station,the construction procedure and force conversion of the buckle arch are complex,which has a great impact on the stability and safety of the overall structure construction.In view of this,based on the Hejiaying Station project of Xi’an Metro Line 2;this paper adopts the research method combining indoor test,numerical simulation and field monitoring to study the force characteristics during the construction of the PBA method station in the loess area.The main work and conclusions are as follows:(1)Undisturbed soil samples were collected in Hejiaying area for uniaxial and triaxial soil compression tests.The influence of different water contents on the mechanical properties of the samples was studied,and the law of the influence of water contents on the physical and mechanical parameters of loess was obtained and fitting analysis was carried out.The test shows that the compressive strength,deformation modulus,cohesion and internal friction angle of loess all decrease with the increase of the moisture content of loess.It shows that the compressive capacity of soil in dry state decreases sharply when it encounters water,and it is prone to instability failure.Therefore,it is suggested that the monitoring of water supply and drainage pipelines and groundwater should be strengthened during the construction of the buckle arch.(2)The finite element software PLAXIS 3D is used to carry out numerical simulation of different buckle arch construction sequences.The research results show that it is a more reasonable construction sequence to construct the initial support of the middle span first,and then to stagger the safety step distance and then construct the initial support of the side span,which can effectively control the surface deformation,the internal force of the primary support structure,and the horizontal displacement of the top longitudinal beam.In addition,the staggered distance between the two sides of the span construction should be controlled within 3 meters,which is beneficial to the formation deformation and the stress of the structure.After the construction of the primary buckle arch is completed,the second lining buckle arch is applied in the order of "first middle and back side",and the side-span stagger distance should also be controlled within 3m.(3)The influence law of stratum deformation and buckled arch mechanical properties of soil layer under different moisture content conditions is analyzed,and the research results are as follows:with the increase of water content,the deformation of surface subsidence gradually slowed down.When the water content increases from 8%to 35%,the surface settlement increases by 357.26%,the bending moment of the primary support of the buckle arch increases by 69%to 94%,and the axial force increases by 20%to 30%.The stress of the secondary lining structure increases by 14%to 15%.Therefore,the moisture content of the soil layer has a great influence on the construction of the PBA method.In the construction of the project,the moisture content of the surrounding rock of the working face should be monitored in real time to avoid the decrease in the strength of the surrounding rock or the increase in the deformation caused by the increase of the moisture content.(4)In order to verify the reliability of the numerical simulation,on-site monitoring was carried out on the changes of the surface settlement,the surrounding rock pressure of the buckle arch and the internal force of the initial support at the construction site of Hejiaying Station.The monitoring results show that:after the completion of the primary support of the buckle arch and the construction of the secondary lining,the maximum surface settlement values are 11.01 mm and 15.37 mm,respectively.The measured value of the maximum settlement of the five main monitoring points differs from the numerical simulation by about 11.57%～17.29%.It shows that the research method of numerical simulation is reliable.The rapid rise of the surrounding rock pressure mainly occurs during the excavation of the upper bench of the buckle arch,and the maximum value reaches 53.2kPa;when the side span is excavated on the upper bench,the surrounding rock of the middle-span buckle arch is disturbed and the force is redistributed,and the surrounding rock pressure increases 50%.After the construction of the second lining of the buckle arch,the surrounding rock pressure tends to be stable.The internal force of the main reinforcement of the primary support of the buckle arch surged during the construction of the secondary lining,because the removal of the primary support of the pilot hole caused a part of the external load have been borne by it to be transferred to the buckle arch.Through the in-depth analysis of the data,it is found that in the construction of the hole pile method in the loess area,there is a big difference between the theoretical earth pressure value calculated by the whole soil column method and the field monitoring value,indicating that the earth pressure value calculated by the whole soil column method is used for supporting structures.The design is biased towards safety.