Study on frost heaving characteristics of sandstone under different freezing conditions北大核心CSCD

At present，artificial freezing method has become one of the effective methods for coal mine shaft to pass through water-rich soft rock strata，which can stop the movement of groundwater and limit the deformation of surrounding rock. In order to study the frost heaving characteristics of sandstone under different freezing conditions，frost heaving tests of saturated and dry Cretaceous red sandstone samples under different freezing rates （10 ℃·h-1，5 ℃·h-1，2 ℃·h-1，1 ℃·h-1）and different confining pressures（5 MPa，10 MPa，15 MPa，20 MPa，25 MPa）were carried out by using GCTS（Geotechnical Consulting & Testing Systems）servo-controlled low temperature and high pressure triaxial rock testing system. In this paper，based on the existing theory of physical and mechanical properties of frozen soil，we studied the frost heaving law of sandstone under different freezing conditions and explored the frost heaving mechanism. The result shows that in the process of cooling，the dry rock sample always produce cold shrinkage deformation，while the saturated rock sample first produce cold shrinkage deformation，then produce frost deformation，and finally the deformation tends to be stable. The deformation of saturated rock samples is much larger than that of dry rock samples. The larger the stress level of rock samples at the same temperature is，the smaller the frost deformation is，which shows a linear negative correlation，mainly because the high confining pressure limits the volume expansion of the water phase in the pore inside the rock samples when it becomes ice. The frost deformation of rock samples is mainly affected by confining pressure and water content，while the frost heaving rate is mainly affected by cooling rate. Under this test condition，the higher the cooling rate of sandstone is，the higher the frost heaving rate is，and the relationship between them is approximately linear. For saturated rock samples，the confining pressure reduces the rock frost heaving by limiting the expansion during the phase transformation of ice water，and the temperature affects the rock frost heaving by affecting the freezing rate of pore water and the thermal expansion and cold contraction of rock skeleton. For dry rock samples，the deformation is mainly due to the volume contraction of rock mineral particles caused by thermal expansion and cold contraction effect，and the greater the temperature change，the greater the deformation. Based on the experimental results and theoretical analysis method，a calculation formula of rock frost heaving considering the influence of confining pressure was established. By calculating the frost heave of sandstone samples under different confining pressures，it is found that the calculated values are in good agreement with the experimental results. Moreover，according to the calculation formula of frost heaving，the influence factors of rock frost heaving during freezing can be divided into two categories：internal cause and external cause. The internal cause includes porosity，saturation，volume modulus of ice and rock skeleton，and the external cause includes temperature and confining pressure. For saturated rock，the frost heaving is mainly affected by factors such as confining pressure，temperature and porosity. When the saturation，porosity and freezing rate are low，the rock may only produce shrinkage deformation，because these indicators determine whether the rock produces frost heave or freeze shrinkage. The mechanism of rock frost heaving is very complicated due to the interaction and restriction between the internal and external factors and the dynamic changes of rock micro-structure and mechanical properties during the process of frost heaving. The research results can provide theoretical reference for freezing construction scheme design of deep coal seam mine construction，and also provide a theoretical basis for the study of physical and mechanical properties and engineering application of soft rock in frozen soil area. © 2022 Science Press (China).

Study on frost heaving characteristics of sandstone under different freezing conditions

At present， artificial freezing method has become one of the effective methods for coal mine shaft to pass through water-rich soft rock strata， which can stop the movement of groundwater and limit the deformation of surrounding rock. In order to study the frost heaving characteristics of sandstone under different freezing conditions， frost heaving tests of saturated and dry Cretaceous red sandstone samples under different freezing rates （10 ℃·h^-1， 5 ℃·h^-1， 2 ℃·h^-1， 1 ℃·h^-1） and different confining pressures （5 MPa， 10 MPa， 15 MPa， 20 MPa， 25 MPa） were carried out by using GCTS （Geotechnical Consulting & Testing Systems） servo-controlled low temperature and high pressure triaxial rock testing system. In this paper， based on the existing theory of physical and mechanical properties of frozen soil， we studied the frost heaving law of sandstone under different freezing conditions and explored the frost heaving mechanism. The result shows that in the process of cooling， the dry rock sample always produce cold shrinkage deformation， while the saturated rock sample first produce cold shrinkage deformation， then produce frost deformation， and finally the deformation tends to be stable. The deformation of saturated rock samples is much larger than that of dry rock samples. The larger the stress level of rock samples at the same temperature is， the smaller the frost deformation is， which shows a linear negative correlation， mainly because the high confining pressure limits the volume expansion of the water phase in the pore inside the rock samples when it becomes ice. The frost deformation of rock samples is mainly affected by confining pressure and water content， while the frost heaving rate is mainly affected by cooling rate. Under this test condition， the higher the cooling rate of sandstone is， the higher the frost heaving rate is， and the relationship between them is approximately linear. For saturated rock samples， the confining pressure reduces the rock frost heaving by limiting the expansion during the phase transformation of ice water， and the temperature affects the rock frost heaving by affecting the freezing rate of pore water and the thermal expansion and cold contraction of rock skeleton. For dry rock samples， the deformation is mainly due to the volume contraction of rock mineral particles caused by thermal expansion and cold contraction effect， and the greater the temperature change， the greater the deformation. Based on the experimental results and theoretical analysis method， a calculation formula of rock frost heaving considering the influence of confining pressure was established. By calculating the frost heave of sandstone samples under different confining pressures， it is found that the calculated values are in good agreement with the experimental results. Moreover， according to the calculation formula of frost heaving， the influence factors of rock frost heaving during freezing can be divided into two categories： internal cause and external cause. The internal cause includes porosity， saturation， volume modulus of ice and rock skeleton， and the external cause includes temperature and confining pressure. For saturated rock， the frost heaving is mainly affected by factors such as confining pressure， temperature and porosity. When the saturation， porosity and freezing rate are low， the rock may only produce shrinkage deformation， because these indicators determine whether the rock produces frost heave or freeze shrinkage. The mechanism of rock frost heaving is very complicated due to the interaction and restriction between the internal and external factors and the dynamic changes of rock microstructure and mechanical properties during the process of frost heaving. The research results can provide theoretical reference for freezing construction scheme design of deep coal seam mine construction， and also provide a theoretical basis for the study of physical and mechanical properties and engineering application of soft rock in frozen soil area.