Study On The Anisotropic Mechanical Properties And Strength Criteria Of 3D Printed Soft Hard Interbedded Rock Mass Under Uniaxial Compression

Soft and hard interbedded rock masses are widely distributed and have different occurrences.Their mechanical properties differ from those of single rock masses,and some mechanical parameters are difficult to determine through conventional methods.Conducting longitudinal wave velocity testing and mechanical tests on rock masses can help establish the relationship between rock mass wave velocity and mechanical parameters.Under conditions such as rainfall and groundwater intrusion,the mechanical properties of soft rock within the soft hard interbedded rock mass will sharply decrease,which can easily cause excessive deformation or instability of the rock mass and trigger geological disasters.Therefore,it is of great significance to study the impact of the decline in mechanical properties of soft layers on the mechanical properties and failure modes of soft hard interbedded rock masses.It is difficult to conduct on-site sampling for soft and hard interbedded rock masses.3D printing technology has the advantages of no need for molds and high sample preparation efficiency.Its layered and stacked forming process facilitates the indoor production of soft and hard interbedded rock mass models,providing a new approach to indoor experimental sample preparation.Based on this,this article takes the soft and hard interbedded rock mass in Simianshan Town,the Three Gorges Reservoir Area as the research object.Using Cement-based 3D printing technology to prepare samples,the anisotropy of 3D printed rock materials is explored through indoor uniaxial compression tests,CT scans,and longitudinal wave velocity tests.The acoustic,mechanical properties,and failure mode anisotropy of soft and hard interbedded rock masses with different soft layer strengths are studied.Based on the Hoek-Brown strength criterion,a anisotropic strength criterion for soft hard interbedded rock masses considering longitudinal wave velocity is proposed.The main conclusions and achievements are as follows:(1)Based on the mechanical properties of the soft hard interbedded rock mass in Simian Mountain Town,the strength ratio of the soft hard rock is determined to be 1:4 and the elastic modulus ratio to be 1:5.The strength of the deteriorated soft rock decreases by about 30%.After repeated experiments,the ratio of rock mass simulation materials was obtained.Soft rock and deteriorated soft rock were simulated using cement-based materials containing solid waste.The solid waste utilization rate of soft rock simulation materials is 48%,and the solid waste utilization rate of deteriorated soft rock simulation materials is 53%.By using a 3D printer,models of complete hard rock,complete soft rock,layered hard rock,layered soft rock,and soft mixed hard interbedded rock masses are produced.There are three types of soft mixed hard interbedded rock masses: non deteriorated soft layer,only deteriorated upper soft layer,and deteriorated upper and lower soft layers.(2)The 3D printed rock mass models are all cuboids,with the vertical direction as the Z axis,the long side of the model cross section as the X axis,and the short side as the Y axis to establish a space Cartesian coordinate system system.The fluctuation amplitude of uniaxial compressive strength in the X,Y,and Z directions of intact hard rock is about 6%.The fluctuation amplitude of average CT value is within 3%,indicating that 3D printed rocks are nearly homogeneous.(3)The longitudinal wave velocities of the layered hard rock and layered soft rock,both increase with the increase of bedding angle.The longitudinal wave velocities of the three types of soft and hard interbedded rock masses show a trend of first decreasing and then increasing with the increase of bedding angle.The inhibitory effect of bedding planes on the longitudinal wave velocity of layered hard rocks and layered soft rocks gradually decreases as the bedding angle increases.And the inhibitory effect of heterogeneous bedding planes in soft and hard interbedded rock masses on wave velocity exhibits an "S" shaped change with the increase of bedding angle.(4)The uniaxial strength,elastic modulus,and deformation modulus of single rock layered rock mass and soft and hard interbedded rock mass show a "U" shaped trend of first decreasing and then increasing with the increase of bedding angle.The main failure characteristics of single rock layered rock mass and soft and hard interbedded rock mass are also basically the same.When the bedding angle is 0 °～30 °,the main occurrence of splitting tensile failure is;At 45 ° to 75 °,shear slip failure mainly occurs along the bedding plane;At90 °,splitting tensile failure along the bedding plane often occurs,and the matrix undergoes shear failure.The failure of the soft hard interbedded rock mass is mainly caused by the soft rock at one end,and the main failure end of the interbedded rock mass only after the deterioration of the upper soft layer is the deteriorated soft layer(5)After the mechanical properties of the soft layer in the soft and hard interbedded rock mass decrease,the decrease in mechanical properties of the rock mass is generally smaller than that of the soft layer.When both the upper and lower soft layers deteriorate,the mechanical properties of the interlayer rock mass significantly decrease,and the overall decrease in elastic modulus is the largest.When the dip angle of the bedding is between the internal friction angle and 90 °,the influence of soft layer degradation on the mechanical properties of soft and hard interbedded rock masses is relatively small.The strength anisotropy of the inferior soft hard soft type is highest in the soft hard interbedded rock mass.(6)Combining the Hoek-Brown and Jaeger strength criteria,introducing anisotropic parameters and matrix contribution,and proposing a method for calculating the uniaxial strength of complete samples of soft and hard interlayers through the proportion of rock volume,a Hoek-Brown uniaxial compressive anisotropic strength criterion suitable for soft and hard interlayered rock masses is finally obtained.The theoretical value of the new strength criterion is in good agreement with the measured value,with a maximum deviation rate of 14.06%.