Study On Anti Floating Of Subway Station In Special Red Sandstone Formation In Western China

With the rapid development of China's economy and the acceleration of urbanization,the available space on urban land is gradually saturated,and land resources are increasingly scarce.Urban traffic construction is gradually shifting from the ground to the underground,but the complex underground soil conditions and the distribution of underground water have an inestimable impact on the safe and reliable operation of various underground projects,and bring about a series of new underground engineering safety problems,among which the anti-floating problem of the underground structure basement has always been a concern.Whether the anti-floating waterproof level of underground structure is reasonable and whether the calculation of buoyancy load on underground structure is correct are not only related to the safety and reliability of the whole structure,but also have a great influence on the design scheme,cost and duration of the project.In view of the above research background,the research of this paper relies on the deep foundation pit project of Dongfanghong Metro Station of Lanzhou Urban Rail Transit Line 1.In order to know the permeability characteristics of red sandstone in Lanzhou area and the reduction coefficient of groundwater buoyancy of foundation pit floor in this sandstone layer are comprehensively studied by means of laboratory test,field test,FLAC^3D Finite difference simulation software and theoretical calculation.The main work and research results in this paper include:1.The permeability and permeability coefficient of red sandstone are solved and validated by the combination of laboratory test and field test.Laboratory test includes constant head permeability test and variable head permeability test;field test includes field seepage test and field pumping test.The results show that the permeability coefficient of the specia red sandstone layer unique to the station is 3.13E-03cm/s,which belongs to medium and weak permeability layer.2.The reduction coefficient of groundwater buoyancy is 0.304 when the floor of underground structure is in the special red sandstone layer,given by indoor model box simulation test.The bottom angle of the groundwater buoyancy reduction test model box is 45.Four-sided iron box is used to simulate the building,which can effectively eliminate the friction between the side wall and the soil of the building structure,make buoyancy equal to the weight of the structure and float on the building immediately,and make the test results closer to the true value.3.Combining with the example of anti-floating engineering of deep foundation pit of Dongfanghong Metro Station on Line 1 of Lanzhou Urban Rail Transit,the actual working condition of the project is simulated by FLAC^3D finite difference software,and the pore water pressure at the bottom of foundation pit under different working conditions is obtained.The calculated results are compared with those obtained by theoretical calculation method,and the bottom of foundation pit under different working conditions is calculated.The buoyancy reduction coefficient of the slab in the special red sandstone layer is between 0.18 and 0.44.Finally,the buoyancy reduction factor is determined to be 0.30 by comparing the simulation results with the test results.4.The ultimate buoyancy reduction factor is applied to specific engineering examples and appropriate anti-floating measures are selected to meet the requirement of anti-floating stability.In addition,the seepage effect in the pit due to the permeability of red sandstone is calculated by the equivalent beam method,which results in the maximum bending moment of the supporting structure increasing by 1.25times,the length of the occluded pile increasing by 6%,and the axial force of the support in the pit greatly reducing.