Experimental Study On Tensile And Compressive Properties And Failure Law Of Gently Inclined Layered Rock

The bottom deformation of high-speed railway tunnel with gently inclined layered soft surrounding rock widely exists in engineering practice.At present,the industry is not clear about the causes and mechanism of tunnel bottom deformation.Therefore,it is necessary to systematically study the relevant mechanical properties of gently inclined layered rock,explore the deformation and failure law of gently inclined layered rock,and provide corresponding guidance for engineering practice at the meso-scale.In order to reveal the tensile and compressive mechanical properties and failure mechanism of gently inclined layered rocks,Brazilian split test,uniaxial compression test and biaxial compression test were carried out on INSTRON and TRW-3000 devices for 0°,5°,10° and 15° layered sandstones and self-made interbedded soft and hard rocks.With the help of high-speed camera and digital image correlation technology(DIC),the deformation and failure process of the sample is recorded and analyzed.Based on the experimental data and strain analysis,the tensile and compressive mechanical properties of gently inclined layered rock are studied.The main contents and concluding results of this paper are as follows:(1)In the Brazilian split test of gently inclined layered rock,the tensile mechanical properties of layered sandstone and interbedded soft and hard rock are obtained: the tensile strength of gently inclined layered sandstone increases slowly with the inclination of bedding.The tensile strength of interbedded soft and hard rocks decreases significantly with the increase of bedding angle.The deformation and failure process of the sample is monitored in real time by DIC.It is found that the failure mode of the layered sandstone sample can be summarized into the straight line at 0°bedding angle and the fold line at gentle angle.The failure mode of soft and hard interbedded rocks is splitting tensile failure along the bedding plane in 0° bedding angle and shear failure along the bedding plane in gently inclined bedding angle.Comparing the strength characteristics of the two types of rocks,it is found that the bedding angle has a weak influence on the tensile strength of sandstone,but has a significant influence on the tensile strength of soft and hard interbedded rocks.(2)In the uniaxial compression test of gently inclined layered rock,the uniaxial strength characteristics of layered sandstone and interbedded soft and hard rock are obtained: with the increase of bedding angle,the uniaxial compressive strength and elastic modulus of gently inclined layered sandstone decrease first and then increase slowly,while the uniaxial compressive strength and elastic modulus of interbedded soft and hard rock decrease significantly.After analyzing the full field strain by DIC,it is concluded that the failure mode of layered sandstone changes from the mixed failure of through layer shear and splitting tension at 0° bedding angle to the composite shear failure of through layer and along layer at a gentle angle.Soft and hard interbedded rocks gradually change from splitting tensile failure along bedding plane and tensile shear mixed failure in rock stratum to shear slip failure along bedding plane.After comparing the strength and deformation characteristics of the two types of rocks,it is found that the inclined angle of bedding plane is the key factor affecting the uniaxial strength of soft and hard interbedded rocks,but has no significant effect on the uniaxial strength of layered sandstone.The transverse deformability of interbedded soft and hard rocks is stronger than that of layered sandstone in the limit state.(3)In the biaxial compression test of gently inclined layered rock,the biaxial strength characteristics of layered sandstone and interbedded soft and hard rock are obtained: under the same lateral pressure level,the biaxial strength of layered sandstone and interbedded soft and hard rock decreases with the increase of bedding angle.Under the same bedding angle,the biaxial strength of layered sandstone and interbedded soft and hard rock increases with the increase of intermediate principal stress.Using three-dimensional DIC technology,the uplift deformation value of the free face before failure is measured.It is found that the uplift deformation of interbedded soft and hard rocks is greater than that of layered sandstone,and the uplift phenomenon is similar to that of the floor at the tunnel site.After comparing the biaxial strength characteristics of the two types of rocks,it is concluded that the biaxial strength of interbedded soft and hard rocks is more sensitive to the bedding angle,and the increase of lateral pressure can reduce the strength anisotropy caused by the bedding plane of layered rocks to a certain extent.