Experimental Study And Analysis On Collapse Characteristics Of Red Sandstone In Deep Foundation Pit Of Lanzhou Metro

Red sandstone is widely distributed in Lanzhou area.It is a rock stratum with special engineering properties encountered in Lanzhou subway project.Because of its complex composition and special structure,it generally has the characteristics of softening and disintegration when encountering water,which brings more support and drainage problems to the deep and large foundation pit of subway and high-rise buildings in Lanzhou area.Lanzhou Metro Line 1 to line 4 pass through the red sandstone stratum,and the groundwater level is high.It is very important to study the collapse characteristics and mechanism of red sandstone for foundation pit support engineering.In this paper,taking the weak permeable red sandstone in the foundation pit of Lanzhou Metro Line 2 and line 3 as the research object,the disintegration characteristics and mechanism of red sandstone in Lanzhou area are analyzed and studied by means of laboratory test,field test,theoretical derivation and numerical simulation.The main research contents and results are as follows:(1)The red sandstone in Lanzhou area is preliminarily divided into three categories by the test of immersion disintegration.The type A disintegration speed is fast,the engineering characteristics are poor,and the en gineering problems caused are relatively serious.The type B red sandstone disintegration speed is second.The engineering characteristics are general,the engineering problems caused are small,the type C red sandstone almost does not disintegrate and the engineering characteristics are good.The physical indexes such as dry density,porosity,permeability coefficient and shear strength of three kinds of red sandstone are obtained by routine geotechnical test and field test,and the further classification characteristics of red sandstone are studied.(2)Through the self-made red sandstone disintegration rate measurement device and the self-designed red sandstone disintegration rate measurement test.The disintegration rate measurement test is carried out o n 20 red sandstone samples with different physical indexes in the foundation pit of six stations.The disintegration rate measurement and comparative analysis are carried out on red sandstone with different physical indexes in the same station and the same physical indexes in different stations,and the disintegration rate measurement and comparative analysis of differe nt physical indexes are studied.The relationship between the disintegration rate and the disintegration mechanism of different kinds of red sandstone is analyzed,so as to improve the disintegration mechanism of red sandstone.(3)Through the electron microscope scanning test,XRD diffraction test,rock and mineral analysis,nuclear magnetic resonance test of red sandstone,combined with the factors affecting the disintegration rate.The disintegration mechanism of red sandstone in Lanzhou area is analyzed,and the disintegration process of red sandstone and the disintegration reason and difference of three kinds of red sandstone are obtained.(4)Using the linear anti rotation contact model embedded in the three-dimensional particle flow numerical simulation program PFC3 D to simulate the disintegration process of red sandstone.Using different test parameters of three types of red sandstone to establish different particle models of three types of red sandstone and servo control the established particle model to achieve the theoretical test state,and then use the ideal state model to carry out the numerical calculation of the whole process of r ed sandstone disintegration.The numerical simulation results of the whole process of red sandstone disintegration are obtained and the rationality of disintegration test is verified.