Effects Of Unloading Damage On The Mechanical Properties Of Sandstone

In nature,rock mass has undergone various geological and structural movements,so most rock mass is damaged nowadays.During the excavation and unloading process of various projects,further unloading damage has been caused to the rock mass.Unloading damage affects the mechanical properties of rock mass and is a key factor affecting the properties of engineering rock mass.Therefore,based on some of the sandstone of the three gorges reservoir through indoor experiment was carried out by unloading damage rock peak strength,residual strength on unloading damage specimens and different initial axial stress level influence on sandstone unloading strength.The main research contents and conclusions are as follows:1)Triaxial compression test was carried out for sandstone with different initial unloading damage degree,obtaining the stress-strain curve,the variation of stress-strain curve,peak strength,strain confining pressure flexibility,deformation modulus and axial strain energy density of sandstone were also obtained.Comparing with the results of conventional triaxial compression test,we analyze and summarize the influence of unloading damage degree of strength parameters,deformation parameters,axial strain energy density and establish corresponding relation curves.Observing the fracture maps of damaged rock samples with different initial unloading damage,the relationship between failure mode and load shedding is determined.At the same time,the corresponding yield criterion is established according to the obtained test data and conclusion.2)Repeated loading tests were carried out on the sandstone with different initial unloading damage degree after failure,obtaining the stress-strain curve of repeated loading.The effects of unloading damage on residual compressive strength,stress-strain curve and failure mode were studied and analyzed.At the same time,a new single specimen repeated loading method is proposed to determine the compressive strength parameters of the residual sandstone in the residual stage,and then the shear strength parameters of the residual sandstone in different initial unloading damage stages are measured.The measured results are compared with the shear strength of the residual stage measured by the conventional multi-specimen method,the results show that the test results obtained by repeated loading method of single specimen are less discrete,which further proves that the method is reasonable and feasible.3)The unloading test of different initial axial stress horizontal sandstone was carried out,obtaining the total stress-strain curve during the test.The variation laws of unloading strength,deformation modulus,strain confining pressure flexibility,axial strain energy density and uniform confining pressure drop of sandstone were studied,analysising and summarizing the effects of axial stress on strength parameters,deformation parameters and axial strain energy density.According to the fracture expansion map of rock sample after failure,the influence of axial stress level on the unloading failure mode of sandstone is summarized.At the same time,the corresponding yield criterion is established according to the obtained test data and conclusion.4)According to different initial axial stress horizontal sandstone unloading test results,establishing the corresponding unloading constitutive model.The test curve is compared with the theoretical curve,which found that the established theoretical model can better reflect the stress-strain curve change rule.