Research On Scale Effects Of Physical And Mechanical Properties Of Intact Rock

The term scale effect refers to the influence of specimen size on the measured physical or mechanical characteristic of a material,and is the most common and important phenomenon in any mechanical testing of brittle and quasi-brittle materials.As a quasi-brittle material that is homogeneous,isotropic and without macroscopic cracks,intact rock often exhibits significant scale effects in its physical and mechanical parameters.Due to the occurrence of the scale effect,the mechanical parameters at the laboratory scale cannot directly reflect the true characteristics of rock at the actual engineering scale.This undoubtedly poses significant safety hazards to engineering projects and has long been a critical issue troubling the engineering geology community.Therefore,it is of great significance for deformation calculation and stability analysis of intact rock to study the scale effects of physical and mechanical characteristics of intact rock,so as to extend the test results at laboratory scale to engineering scale.In this paper,by combining experimental research,numerical simulation and theoretical analysis,the law,mechanism and scale effect model of the intact rock are systematically studied.The main research contents and achievements are as follows:（1）The scale effect law of intact rock physical properties and mechanical behavior was analyzedThe variation law of physical characteristic parameters with size was studied.A series of quantitative indexes representing geometric heterogeneity,mineral heterogeneity and micro-crack heterogeneity were proposed.Combined with image processing technology and acoustic measurement technology,the scale effects of these physical characteristic parameters were systematically analyzed.The results show that:the dispersion of the geometric heterogeneity index decreases with the increase of the sample size.When the sample diameter D is greater than 100mm,the dispersion degree of the geometric heterogeneity index becomes smaller and remains unchanged with the increase of the sample size.The statistical mean and span of mineral heterogeneity index also decrease gradually with the increase of sample size,and when the sample diameter D is greater than 100mm,the change of both is no longer obvious.The P wave velocity decreases with the increase of the size,suggesting the content of micro-fissure increases with the increase of the size.The stress-strain evolution of intact rock under uniaxial loading is analyzed based on DIC.Scale effect of intact rock mechanical experimental study was carried out.Meanwhile,the evolution law of radial tensile strain and shear strain field were quantitatively analyzed.The results show that:under uniaxial compression,uniaxial compressive strength and damage stress show an obvious scale effect,which gradually decreases with the increase of sample size and presents an obvious stage.Based on the analysis of the crack growth process under uniaxial compression by DIC,the failure mode of the samples with a diameter less than 100mm is mainly tension-shear failure,while that of the samples with a diameter greater than 100mm is mainly axial tensile failure.The scale effect of real time process of rock damage behavior under load is investigated.A study on the physical and mechanical samples of intact rock with real-time ultrasonic characteristic scale effect was carried out,and the variation rules of P-wave（radial direction）,S1 wave（axial direction）and S2 wave（radial direction）of samples with different sizes were quantitatively analyzed during loading.The results show that:the wave velocities of P-wave,S1-wave and S2-wave show an obvious phase change rule with the increase of axial stress.At different stages of wave velocity,the change rates of different sizes show obvious differences.In addition,based on the amplitude of the first wave,a damage factor was established to quantitatively characterize the damage degree of rock.The relationship between the damage degree and the size of the sample during loading was systematically analyzed.The results show that:the damage factor also increases with the increase of the size,which means that the large-size sample is more prone to fracture damage.（2）The key control mechanisms of geometric heterogeneity,mineral mechanics heterogeneity and micro-crack heterogeneity on the formation of scale effect were revealedThe influence mechanism of geometric heterogeneity on scale effect was analyzed.A discrete element numerical model considering only geometric heterogeneity was established,and the influence of geometric heterogeneity on the scale effect of intact rock was deeply analyzed.The results show that:due to the effect of geometric heterogeneity,the concentration of local tensile stress and shear stress is more likely to occur in large size samples,which leads to the gradual decrease of mechanical parameters with the increase of size.According to the proposed mechanical model of crack initiation induced by geometric heterogeneity,it is shown that the"weak link"formed by particle clusters with high geometric heterogeneity index is the internal mechanism of scale effect.The influence mechanism of mineral heterogeneity on scale effect was discussed.A discrete element numerical model considering mineral heterogeneity was established to explore the influence of mineral heterogeneity on the scale effect of intact rocks.The results show that mineral heterogeneity is more likely to produce local tensile stress and shear stress concentration than geometric heterogeneity,which further enhances the scale effect of mechanical parameters.According to the proposed mechanical model of crack initiation induced by mineral heterogeneity,it is shown that the"weak link"formed by particle clusters with higher mineral heterogeneity index further enhances the scale effect of intact rock.The effect of micro-crack heterogeneity on the scale effect mechanism was analyzed.A numerical model considering the heterogeneity of micro-fissure is established to study the effect of micro-fissure heterogeneity on the size effect of intact rock.The results show that:the scale effect obtained by the micro-fissure heterogeneity model has good heterogeneity with the mechanical test results.As a"weak link"in rock,micro-fissure directly leads to the scale effect of intact rock.The effect of microfracture heterogeneity on the mechanism of size effect was analyzed.A numerical model considering microfracture heterogeneity is established to study the effect of microfracture heterogeneity on the size effect of intact rock.The results show that the scale effect obtained by the microfracture heterogeneity model is in good agreement with the mechanical test results.As the"weak link"in rock,micro-cracks directly leads to the scale effect of intact rock.（3）A universal scale effect model reflecting the control effect of heterogeneity was constructedBased on test data and collected the existing literature data,considering the shape effect,a new the scale effect of a new universal model is set up,the uniaxial compressive strength of the scale effect model for:UCSN=0.72+0.76e^{（-D/0.97）}.According to this model,the constant strength of intact rock is 72%of the uniaxial compressive strength of standard samples.