This paper presents analysis of the behavior of roc k under induced compressive stress states. Three types of rock namely; Barre granite, Berea sandstone, and Indiana limestone were studied in the laboratory under various rates and modes of applied stresses, utilizing an electrohydraulic servo control testing machine. The dynamic responses a of these rock samples under the load were evaluated, Characteristic behavior of these rock materials were studied through analysis of specimens mode of failure, storing strain energy prior to failure, stress-strain characteristics, acoustic emission signature and total number of acoustic emissions (events) generated during loading and applied stress history, The mode of failure in each case was observed and photographed and an idealized failure mode and mechanism has been proposed. The results of the laboratory experiments indicated that the nature of applied stress and loading history has significant effect on strength and mode of failure in rock. Furthermore, the dynamic response of rock under the applied compressive stress can he alleviated by subjecting t he rock structure to both a low rate of loading and an inhomogeneous stress field.
The mechanical and behavioral characteristics of geologic material in the laboratory depends on the state of applied stress an d their chemical and physical properties (1,2,3). A static, stress, much lower than the strength of material could initiate creeping in the best, specimen and eventual failure would be in ductile mode. However, under dynamic rate of loading, the failure will in brittle mode. The degree of violence in failure of this material under doth static and dynamic conditions depends on t he level of its strength and strain energy stored within the specimens prior to their failure. Geologic materials respond differently under the same stress state because of variation in their mineralogical content, petrological structure and physical continuity of the material such as cracks, pore space and voids. Irregularities, inhomogeneity and discontinuities cause local stress concentration in the material when stressed and may raise stress level in the local areas higher than the strength of the material. Hence localized failure develops resulting in dissipation of strain energy in gradual manner (4). This heterogeneity of the stress field causes the material to become locally unstable before the mass fails as a whole, hence causing premature failure at lower stress level than if no local discontinuities existed, The level of violent failure in the specimens depends on the gradual development of local unstable conditions which is also related to the rate of increase in applied stresses. Failure of large scale structures in underground follows a pattern similar to the laboratory lest specimens (5). For example, failure of a mine pillar under static load would be a gradual deterioration of its integrity leading to squeeze condition, exhibiting ductile failure. However a dynamic load may cause a sudden release of strain energy in the pillar result.ing in rock burst and failure will be in brittle mode. In underground excavations, the stress state imposed on the supporting elements, pillars and sidewalls
