| In order to further explore the internal relationship between strength theory and failure law in the rock mechanics, the theoretical model and the experimental phenomena of the rock under the action of dynamic and static loads are unified to make the theoretical calculation and the indoor experiment better applied to the engineering practice. In this paper, the study of rock strength and damage characteristics is carried out from three perspectives: static load, inpact load and explosive load. Through theoretical analysis and laboratory experiment, the following conclusions are drawn:Based on the theory of limit analysis, the theoretical relationship between rock cohesion C and internal friction angle φ with uniaxial compression failure is deduced during conventional uniaxial compression loading. In this paper, three kinds of typical failure modes, which are dominated by shear stress in the uniaxial compression experiment, are summarized as the failure modes of the main shear plane at both ends of the specimen, the conical surface tensile failure and the axial stretch splitting form. The upper bound solution of limit analysis theory of rock uniaxial compressive strength under static load is obtained by calculating the area of shear failure surface of rock. The calculation formula of the static compressive strength solution can be expressed as:σc=CS cosφ/A cos (φ+βThe meaning of the uniaxial compressive upper bound solution of the rock is that when a specific failure is occurred in the specimen there is a minimum intensity which would cause that happen. This formula illustrates a one-to-one correspondence between the macroscopic failure mode and the compressive strength of the rock. The direct cause of the different failure modes of the specimen is the different loading speed of the rock uniaxial compression. While the rock cohesion C and the internal friction angle φ are the key factors influencing the uniaxial compressive strength ofrock.The UCS and TCS tests of rock in Zhangji coal mine in Anhui Province were completed in the laboratory. It was found that for each kind of rock specimens, the upper bound solutions calculated by three possible uniaxial shear failure modes are about 10% deviated compared with the UCS of the rock directly obtained by the experiment. For a certain group of rock specimens, there were failure modes always corresponded to the UCS test results. Put the corresponding macro-destruction area into calculation, the error between the two stable to about 6%.Based on theoretical analysis and laboratory test, an inverse analysis method for cohesive force C and internal friction angle φ in Mohr-Coulomb strength theory is established. Since there is a stable correlation between rock strength and failure modes under static loading, binary equations with rock cohesion C and internal friction angle φcan be listed by the classification calculation of uniaxial compression results, the corresponding unknown quantity can be obtained by solving the binary equations.Further, more tests with kinds rock obtained from Jiaguo coal mine in Shanxi Province and a quarry in Sichuan Province are done. The experimental results showed that the physical and mechanical properties of the rock samples in the same core drilling were very close to each other. The uniaxial compressive strength of the rock is also in good agreement with the macroscopic failure mode. Taking the siliceous sandstone as an example, the cohesive force C is 38.34MPa and internal friction angle φ is 40°which are obtained by TCS tests. and limit anslysis solution are 36.58MPa and 30.83 °.The results show that the cohesive force is nearly the same, and the internal friction angle calculated by the inversion formula is about 10° less than the experimental solution. The reason is that the rock cohesion represents the strength of the mutual bond between the rock particles, and the results obtained by the two methods are approximately equal. The internal friction angle of the rock reflects the friction characteristics of the rock, including both the surface friction of the rock when the friction occurs, and the bite force between the particles inside the rock. When designing the TCS test, rock mechanics parameters are obtained mainly withconfining pressure less than 10MPa, it is known that with the confining pressure increases, the rock failure envelope is no longer along the straight line, but gradually become gentle, The internal friction angle gradually decreases. On the other hand, the internal friction angle of the siliceous sandstone obtained by the calculation formula represents the size of the dilatation angle between the rock failure surface and the potential velocity direction when the shear deformation occurs under uniaxial compression. The meaning of the two is slightly different, there should be no right or wrong points.Based on the basic principle of limit analysis, the upper bound solution of dynamic compressive strength of rock under one-dimensional impact load is deduced, and the formula is validated and optimized by SHPB experimental system. Under the action of one-dimensional impact load, the rock specimen exhibits a failuremode that can be described in a certain strain rate range. As the process of dynamic failure and the stress wave propagation of the rock specimen is simplified, set the peak compressive strength on stress curve as a calculation point, virtual work principle is used under the assumption of small deformation and ideal plastic. Then the upper bound solution of dynamic compressive strength of rock in a specific mode of damage is obtained:Comparing with the experimental results of granite, frozen red sandstone and deep sandstone in literature, it is confirmed that the accuracy of equation (4.40) to calculate the dynamic compressive strength of rock. At the same time, the experimental study on the dynamic mechanical properties of siliceous sandstone under impact load is carried out based on φ37mm SHPB experimental system. The Stress wave take off point and stress balance analysis is completed with the absolute time method. The results show that the dynamic compressive strength and the failure strain of the siliceous sandstone are increasing with the impact velocity of the SHPB experiment system. The damage strain and the dynamic compressive strength of the siliceous sandstone fluctuate within the interval at similar impact velocity. With the increase of the impact velocity, the failure modes appear from simple to complex and single to diverse, the change in the mode of failure reflects the ability of siliceous sandstone to resist shock damage.Further analysis of the dynamic failure modes of siliceous sandstones found that the dynamic mechanical properties of siliceous sandstones are also closely related to the failure modes. By comparing the upper limit solution calculated by the equation (6.3)and the dynamic compressive strength measured by the test, it can be found that the results are very close, the difference is within 10%, considering the irregularity of the crack itself and other unavoidable of error, the result meets the requirements in the end.The impact of the bullet is external cause of rock damage, the adhesion of small particles inside the rock and friction is internal factors of rock failure, rock failure mode is macro external manifestation byparticle fracture of rock. Combined with the conclusion of the third chapter, the comprehensive analysis of compressive strength under the action of dynamic and static load of siliceous sandstone shows that the small diameter SHPB test system amplifies the influence of rock minor joint on the dynamic mechanical properties. As a complex and diverse mixture of materials, the strength with 35.0mm diameter, 21.5mm height rock sample greatly impact by the structural plane and other physical conditions as well as the drilling and grinding process. Secondly,after careful analysis of the stress waveform, it is considered that the presence of"abnormal" crests in the reflected wavehead may shorten the time of stress balance in the sample, accelerate the failure of the siliceous sandstone specimen.Based on the basic principle of limit analysis, the importance of shear failure zone on experimental results is analyzed in dynamic Brazilian disc splitting experiment. And the elastic solution formula of dynamic tensile test of rock is optimized. In the dynamic BD experiment based on SHPB experimental system, the similarities and differences of the destruction form of the siliceous sandstone are mainly manifested in the failure of the specimen with the loading end, and the deformation and failure of the specimen are gradually increased, The middle of the specimen always has obvious tensile crack along the loading direction, and the rock on both sides of the tensile crack increases with the impact velocity. These crush failure zones and tensile crack zones share the typical failure phenomena of dynamic rock experiments in rock and are significantly different from those of Brazilian discs under static loading. A model is built and calculated based on the final failure mode of the rock specimen and the relationship between tensile and pressure of rock is simplified. It is considered that the failure of the rock specimen conforms to the Mohr-Coulomb failure criterion and the virtual work equation is established.Thus the ultimate tensile failure strength upper bound solution is obtained.In the formula (4.20), ft, which meansthe tensile strength limit analysis solution under different impact load, is the optimization and complementarity of the elastic solution still in use.Q= ftlRtam(α+φ)+fclR(1-sinφ)/24sin α cos(α+φ)Based on the basic principle of limit analysis, a blasting damage model of single-hole column-like kraft in shallow strata was established. Under the guidance of Mohr-Coulomb strength theory, from the angle of rock subjected to shear failure under the action of explosive load, most of the energy of rock particles is absorbed by compressive stress, and the physical and mechanical properties of rock are differentlyweakened, but has not yet lose the carrying capacity, the shear deformation area is divided into explosive damage area. The rock mass with cohesive force and internal friction angle in the damaged area is Coulomb material, and the explosive load is a short-time strong-load with high strain rate. The pseudo-static method is used to simplify the blasting load to the full length of the damage radius Effective load. The explosive damage zone is divided into a circular surface with a central angle of 90° in two-dimensional model, and consists of several rigid triangular regions with vertex angle. Under the explosive load of radiation, the rigid triangle has a trend of rotation to the ground depending on the center of the hole. Adjacent triangles occur relatively dislocationwith energy consumption. There are two forms of energy loss in the damage area caused by the explosion: the friction energy dissipation of the rock in the damage area and the potential shear sliding energy dissipation of the damage outside the area.The stress wave generated by the explosive explosion creates a virtual work equation for the energy of the rock equal to the internal energy loss rate of the rock damage area,and the single hole blasting damage model is obtained as follows:The results of the theoretical calculation can be verified from the engineering practice. In the shallow strata, the blasting load has a potential throwing effect on the surrounding rock. The closer to the ground, the stronger pressure on the rock and overburden is. the drug bag blasting gradually approaching the blasting of the free-faced rock mass, The effect to the rock at the bottom of the crest is more obvious; in the deep hole column-like package blasting, the surrounding rock in three-dimensional space generate a cylindrical damage area concentric with the borehole. In the process of increasing the depth of the rock, the damage range of the rock has changed from"blasting funnel"～"arc-shaped damage area "～"columnar damage area". The blasting damage model was validated from the mechanical properties of the surrounding rock,the explosive performance of the explosive and the parameters of the pillars, and the results were compared with the numerical results in literature which show that the rock blasting damage calculated by the formula (6.4) conform with the general explosion of explosives, consistent with the actual engineering experience.The experimental study on the rock blasting damage theory was carried out by using blasting form with small dose medicine package and high precision damage test system. Through the establishment of a complete set of acoustic emission and damage test system, a two-dimensional damage profile of three sets of test planes was obtained when rock sample was obtained. The experimental results shows that during the process of stress wave propagation, the rock damage is geometrically attenuated with distance,and when the radial displacement increases to a certain extent, the damage attenuation tends to be gentle and negligible. The damage degree of rock is affected by the parameters such as charge density, uncoupled coefficient and charge length. The damage regulation of rock under different blasting parameters is basically the same. The creep effect in cube specimen is not obvious, and the damage decay trend of the rock specimen is basically the same as that ofcylindrical specimen.Comparison of theoretical results and laboratory results shows: To a single group of experiments, the theoretical calculation of the rock blasting damage radius with the increase of the amount of explosive, the growth trend is stable. The results obtained by the experiment are unstable, mainly caused by punching or other factors which are normal test error; from the development trend, the two results have approximate slope,as verification to each other. |
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