| With the rapid development of economy and society,the shortage of land resources greatly restricts the progress of urbanization and thus brings about a series of economic and social problems.To address the shortage of urban land resources,developing and utilizing un derground space can effectively solve ground traffic congestion.Subway construction becomes an upsurge in large and middle cities in China.In the process of building the subway,cross passages must be built to connect the upper and lower tunnel.T hey can provide the passengers for safe evacuation and meet the drainage requirements between two stations in the subway operation.Although the construction of the cross passage acts as a minor project,negligence of its importance is likely to lead to the safety risk.Specifically,the stability of the main tunnel is greatly reduced after the opening of the steel sheet.Once soil instability occurs,it is likely to cause engineering disasters.Therefore,ground freezing is applied widely in the construction of cross passages due to its high soil stabilization,good water sealing performance and non-ground occupation.Ground freezing was carried out in the construction of the cross passage between Minzudaxue Station and Qingchuan Station in the first phase of Nanning Rail Transit Line 1 where strata were rich in water gravel.Because the thermal physical parameters and mechanical indexes of the strata are less studied at home and abroad,a systematic study on the water-rich gravel strata can provide accurate parameters and indicators for design an d construction companies.We studied the gravel samples taken from the site through tests.The thermophysical tests included initial freezing temperature test,specific heat capacity test and thermal conductivity test.The mechanical tests included uniaxial compressive strength and triaxial compressive strength.Based on experimental results and on-site data,the ground freezing method in cross passage construction was designed.The experimental results are as follows:(1)The study obtained thermophysical properties of water-rich gravel formations.We studied the effect of salt content on the initial freezing temperature of gravel samples and the interactions among freezing temperature,specific heat capacity and thermal conductivity of the samples based on their initial parameters measured by a self-made test device.The experimental results show that the increasing salt content leads to continuous decrease of the vapor pressure of water in the samples,which requires lower temperature and releases more energy for the samples to freeze.Specially,with the increase of salt content,the initial freezing temperature of sample 1,2 and 3 decreased from-0.46 C to-1.15℃.With the decrease of freezing temperature,the ice content in the samples increased while the unfrozen water content decreased.Because the specific heat capacity of ice is half of that of water,the specific heat capacity of the samples decreased continuously from 1.60J/(g·℃)to 1.06J/(g·℃).Because the thermal conductivity of ice is much larger than that of water,the thermal conductivity of the samples increased from 1.71W/(m·℃)to 2.13W/(m·℃).As the ice content and unfrozen water content in the samples changed with temperature,the liquid-solid phase transformation of the water caused the variation in physical properties of the samples.(2)The study obtained the mechanical properties of the water-rich gravel strata.Based on the GCTS test platform,the special-purpose large-size press head and large-size mould were used to investigate:a)the relationship between the uniaxial compressive strength and the freezing temperature of the samples;b)the relationship between the triaxial compressive strength and freezing temperature of the samples and their confining pressure.Decreasing the freezing temperature increased the ice strength,the ice-brown joint strength,and reduced the unfrozen water content.With the decrease in freezing temperature,the uniaxial compressive strength of the gravel sample increased from 2.18 MPa to 3.10 MPa.At the same time,the increase in confining pressure made the extrusion between the sample particles more obvious.The triaxial compressive strength of the samples increased from the initial 2.7MPa to 5.2.MPa accordingly.Results from the uniaxial and triaxial strength tests of the samples under different grades showed that the sample coarse aggregate content increased the strength of the samples.(3)The study obtained the design parameters of the freezing method of the water-rich gravel strata.The freezing method was designed for the cross passages between Minzudaxue Station and Qingchuan Station(formerly Chencun Station).In order to obtain design parameters of the freezing method,it mainly included the design of the frozen wall thickness,the calculation of the frozen wall deformation,the calculation of the freezing wall closure time,and the calculation of the active freezing time.The minimum frozen wall thickness meeting the strength requirement was 2.0 meters.When the average spacing of the freezing pipes was 1.3 meters,in order to reach 2-meter thickness of frozen walls and-10℃ average temperature of the frozen wall,the required time of the closure was 26 days,and the active freezing time was 50 days. |
