Micromechanics-based Study On The Progressive And Creep Failure Properties Of Brittle Rocks

The rock mass is mainly consisted of the brittle rocks(e.g., granite or marble) in the deep earth curst. Under the high geostress in the deep earth, the strong time effect appears at the brittle rocks, i.e., the creep behavior. Due to the creep behavior, the local failure and even the collapse of the surrounding rock during construction excavation or during the long-term service could be generated, and the geological disaster could be caused during the long-term development process of earths curst. Furthermore, microcracks have great influence on the mechanical properties of the brittle rocks. So, the micromehcanic-based study plays an important role in understangding the mechanism of creep in brittle rocks. However, the micro-macro mechanical model is rarely studied based on the micromechanism, and it is always is an interesting and difficult question. Thus, based on the micromechanics, the macroscopic mechanical properties caused by microcracks will be analyzed in our study. The following conclusions are drawn from this study:(1)A micro-macro mechanical method is presented, the relation between the macroscopic mechanical properties and micromechanical crack growth is established in compression. Fistly, based on the relation of defintions between micromechanical and macroscopic damage, the correlation of microcrack growth and macroscopic strain is established(i.e., crack-strain relation). And then, coupling this crack-strain relation, microcrack model and crack growth law, the expressions of stress-strain relationship, and the intact three-level creep are derived. The effect of confining pressure on stress-strain relationship is studied. The crack initiation stress and peak stress(i.e., short-term strength) is also obtained. It provides a certain reference for the selection of the lower and upper limit of applied constant stress that could cause the creep behavior. Furthermore, the long-term strength also is analyzed based on this micro-macro mechanical model.(2)A stress path function is introduced into the proposed micro-macro mechanical model. This stress path function is defined by the relationship between stress and time. And then, the creep strain and strain rate under step loading of axial stress or step unloading of confining pressure are analyzed. The results show that the the step loading of axial stress and step unloading of confining pressure both accelerate the creep failure. The rationality of theretical creep results is verified by the experimental results of Jinping marble.(3)Coupling the Mohr-Coulomb failure certain and the proposed stress-crack growth relation, the relation between shear properties(e.g., shear strength, internal friction angle and cohesion) and micromechanical parameters are proposed. And then, combining this rlation and the law of the repeated laoding and unloading test results, the crack growth dependent shear strength is presented. Furthermore, introducing the crack-strain relation and crack growth law, the theoretical expression of strain or time dependent shear strength also are presented. The effects of initial crack size, friction coefficient and confining pressure on shear properties are studied. The rationality of these theoretical results is verified by the experimental results.(4)Based on the proposed micro-macro mechanical model, the effect of crack angle is considered. A new mehod is presented to deduce the theretical expression of stress-strain relation and three-level creep. The effects of crack angle on stress-crack growth relation, crack initiation stress, peak stress, creep failure time, creep failure strain and creep failure crack length are discussed. The rationality of this method also is verified by test.(5)Based on the presented micro-macro mechanical model affected by stress path, and combining the previous studies rockburst mechanism, the effect of unloading rate of confining pressure on creep failure is studied under the underground excavation. The coupling effects of step loading of axial stress and unloading of confining pressure or step loading of axial stress and cycle loading and unloading of confining pressure are also analyzed. And then, the effects of supporting time and size on creep failure are theoretical studied after the unloding of confining pressure. It provides certain help for understanding of time delayed rockburst mechanism and prevention for surrounding rock during the underground excavation.