Experimental Study On Mechanical Characteristics Of Cretaceous Frozen Sandstone Under Lateral Unloading

The deep coal seam in Northwest China is covered with a large number of Cretaceous sandstone.Due to the low strength and rich water of Cretaceous sandstone,manual freezing method is often used to excavate the shaft.In the process of excavation,the original stress in the horizontal direction of the rock mass is gradually unloaded,which is actually the process of reducing the confining pressure of frozen sandstone and increasing the partial stress.The study of the mechanical properties of the frozen sandstone under lateral unloading can provide a basis for the design of the frozen shaft.In this paper,the Cretaceous water rich sandstone in Binchang mining area of Shaanxi Province is selected,and the unloading mechanical tests and synchronous acoustic emission tests are carried out under different freezing temperature,initial confining pressure and lateral unloading rate by using the low temperature and high pressure rock triaxial test system.(1)The mechanical test results show that the peak stress and elastic modulus of triaxial compression of saturated sandstone increase about three times from 20 ? to-5?.Under the condition of lateral unloading,the peak stress of frozen sandstone is about 2?3 times of that of 20?,and the peak strain of 20?sandstone is smaller than that of frozen sandstone.When the initial confining pressure increases,the axial and radial strain growth rates increase,and the radial strain dominates the failure process.At high unloading rate,the deformation is small and the failure is sudden.At low unloading rate,the total deformation is large,including some creep deformation.According to the stress-strain curve,the variation of deformation modulus with confining pressure unloading amount is calculated,and the variation law of deformation modulus with confining pressure unloading amount is obtained under different freezing temperature,different initial confining pressure and different lateral unloading rate.(1)The acoustic emission equipment is used to monitor the lateral unloading process of frozen rock samples.According to the intensive triangle area and energy cluster area of acoustic emission amplitude,the precursory point of lateral unloading failure is determined.With the decrease of freezing temperature,the number of AE amplitude decreased and the peak energy increased.When the initial confining pressure of rock sample increases at-10?,the energy value of cluster area decreases and the peak value of energy increases.At-10?,the unloading rate of rock sample increases,the number of amplitude points increases obviously,the energy value of cluster area increases,the peak value of energy increases,the cumulative energy at the moment of failure increases,and the damage is greater.The three-dimensional location points of acoustic emission before precursory can show the initiation and development track of internal cracks in frozen rock samples,and can be used to strengthen the rock mass according to the location of acoustic emission before precursory.(3)Before the peak stress,the theoretical stress-strain curve is close to the experimental data,and the parameters are calculated by the formula,which is more convenient and accurate.