Experimental Study On Mechanical Properties And Constitutive Model Of Soft And Hard Interbedded Rocks

Layered rocks exist widely in nature,among which layered composite rocks in the form of soft and hard interbedding are the most common.Soft and hard interbedding rocks are composed of two types of rocks with different mechanical properties,and their strength characteristics,deformation characteristics,failure mechanisms,and failure modes have obvious anisotropic characteristics.The bedding angle has a significant impact on the mechanical properties of soft and hard interbedding rocks,The mechanical properties of soft and hard interbedded rocks vary with the dip angle of the rock stratum.Studying the mechanical properties and damage mechanism of soft and hard interbedded rocks as they change with the dip angle of the rock stratum will have important engineering significance for the development of underground space and the exploitation of resources such as oil and gas.Therefore,this paper studies the strength,deformation characteristics,and damage mechanism of soft and hard interbedded rocks through laboratory tests,theoretical analysis,and numerical simulation.The main research contents and achievements include:(1)Hard rock(sandstone-like)was made by mixing cement,sand,silica powder,water,defoaming agent and water reducing agent according to the ratio of1:0:0:0:0.003.Soft rock(limestone-like)was made by mixing cement,sand,water and defoaming agent according to the ratio of 1:0:0:0:0.003.Two different ratios were used to make soft and hard interbedded rock materials according to the layer-by-layer pouring method,and finally four different rock inclination angles of 0°,30°,60°and90°were drilled out for the soft and hard interbedded rock specimens.(2)The uniaxial compression tests on soft and hard interbedded rocks were conducted by combining various means,such as acoustic emission technology and Digital Image Correlation Technology(hereinafter referred to as DIC),to obtain the evolution of displacement and strain fields,acoustic emission ringing counts and cumulative ringing counts,and damage patterns of the specimens were obtained.The uniaxial compressive strength of soft and hard interbedded rocks decreases sequentially when the dip angle of the rock stratum is 0°,30°,and 60°.The strength reaches the maximum at 90°,and the rock sample is slightly higher than 0°.This is because when the dip angle of the rock stratum is 0°,the soft rock stratum is directly affected by the maximum principal stress,which first produces cracks.When the dip angle of the rock stratum is 90°,the maximum principal stress is borne jointly by the soft rock and the hard rock,so the rock with the dip angle of 90°has higher strength.The change rule of elastic modulus is basically consistent with the compressive strength,showing a phenomenon of first decreasing and then increasing with the increase of angle.(3)Triaxial compression tests were conducted on soft and hard interbedded rocks to obtain the mechanical properties and strength characteristics of soft and hard interbedded rocks under triaxial compression conditions.The strength of rocks with a dip angle of 0°and 90°is higher and the strength difference between the two is not significant.The strength of rocks with a dip angle of 90°is slightly higher than that of rocks with a dip angle of 0°,while the strength of rocks with a dip angle of 30°and 60°decreases in turn.The strength of rocks with a dip angle of 60°is the lowest,and the elastic modulus and triaxial compressive strength laws are basically consistent,Poisson’s ratio decreases with increasing confining pressure.(4)The damage constitutive model of soft and hard interbedded rocks is established using the transverse isotropic theory.According to the change rule of elastic modulus of soft and hard interbedded rocks with different rock dip angles,the damage variable Dθis established,Reflects the rock damage caused by rock inclination.According to the characteristics of random distribution of rock damage,a damage variable D_W based on Weibull distribution is established to reflect the rock damage caused by load.The total damage variable D of the rock is obtained through the coupling of the two damage variables,and the Drucker Prager yield criterion is introduced as a criterion for determining the failure of rock microelements.The rationality of the constitutive model is verified by comparing the triaxial test curve with the transversely isotropic damage constitutive theory curve.(5)The COMSOL Multiphysics numerical software is used to simulate the stress state,displacement field distribution,and failure characteristics of soft and hard interbedded rocks.The Mohr Coulomb failure criterion under three-dimensional conditions is used to determine rock failure.The rock failure condition Fa is defined,where Fa>0 indicates that the rock is in a stable state,Fa=0 indicates that the rock is in a critical state of starting to fracture,and Fa<0 indicates that the rock is unstable and damaged,The numerical simulation results well reflect the failure mode of soft and hard interbedded rocks.