Study On The Spalling Failure Of Hard Rock And The Mechanism Of Strainburst Under High In-Situ Stresses

Surface-parallel spalling is a failure mode often observed in highly stressed hard rocks in underground excavation. When the opening is at great depth or the in-situ stresses are very high, violent engineering disasters such as rock burst may occur in the surrounding rock masses. Based on this, the author has done the research titled with "The brittle spalling failure of hard rock and the mechanism of strainburst under high in-situ stress", sponsored by the National Natural Science Fund, the China Scholarship Council and so on. Integrated the use of laboratory testing, theoretical analysis, numerical simulation and engineering application, the studies on the spalling failure of hard rock under high in-situ stress and the mechanism of strainburst have been systematically carried out.First, uniaxial compression tests and direct tensile tests of hard and soft rock specimens have been designed. The mechanical properties of two rocks have been compared under different load conditions. The change of strength, deformation, elastic modulus, Poisson's ratio, acoustic emission and other physical parameters were carefully analyzed with applied load. The stress-strain curves and acoustic emission characteristics of the two rocks have been obtained under direct tension and compression. The tensile fractures of typical rock specimens were measured by a 3D surface measurement machine. The micro-structure and the roughness profile of the tensile fractures were obtained.Second, the Iddefjord granite from a quarry in the south Norway was selected to carry out the laboratory tests on spalling failure. The main mechanical properties of the granite were got from the uniaxial compression tests on the standard cylindrical specimens and the splitting tests on the Brazilian disc specimens. Then uniaxial compression tests on the plate specimens with different ratio of height to width were carried out. It is observed that the failure mode will be transformed from shear to slabbing when the height/width ratio is reduced to 0.5 in the plate specimens. Microσ1-parallel fractures initiate when the lateral strain departs from its linearity. Slabbing fractures are approximately parallel to the loading direction. The lab tests show that the slabbing strength (σsl) of hard rock is about 60% of its uniaxial compression strength (UCS). In the numerical modelling part, two kinds of constitutive models, elastic model and Mohr-Coulomb strain-softening model, are adopted to simulate the mechanical behavior of the plate specimens. The end effect and the mesh size were taken into consideration during the numerical simulation.The analysis on the micro fracture mechanics of hard rock was also carried out for studying the spalling failure of hard rock. It is found that there is obvious defects and micro-heterogeneity in the Iddefjord granite under the microscope photos. By using the linear elastic fracture mechanics method, the crack initiation and propagation along the wedge-shaped sliding fracture under uniaxial compression has been deducted. The relationship between the crack initiation stresses with the initial crack length, crack dip angle, friction angle and fracture toughness of rock has been obtained. The relationship between the compression load and the crack propagation length has also been calculated. The stress intensity factor of the crack tip has been analyzed when the wing crack was propagated under biaxial compression.In addition, buckling instability of hard rock slabs and the mechanical response of rock pillars under dynamic disturbance in high in-situ stresses at depth have been analyzed from the macro point of view. On the one hand, the buckling failure of highly-stressed rock slabs surrounding underground openings is analyzed by using Euler's formula. The eccentric loading is taking into consideration. It shows that the slenderness, the eccentricity, the elastic modulus of the slabs and the loading stress play important roles on the buckling failure of rock slabs. The confining pressure provided by fill can prove adequate to control buckling failure under certain loading conditions. According to the buckling theory, an example analysis was applied at the Maluping Mine in Guizhou province. On the other hand, the mechanical response of the highly static stressed rock pillars under dynamic disturbance has been discussed by using the numerical simulation method. It is found that the stability of highly stressed rock mass is more distinctly influenced by the outside dynamic disturbance with the original static stress increasing. When the pillar is endured very high static stress, even a small dynamic disturbance may lead to its plastic destroying and result in a domino effect in the deep mine.Finally, to simulate the mechanism of strainburst in the excavation of tunnels, the Iddefjord granite specimens with prefabricated holes have been used in the uniaxial compression tests. It is found that with the increasing of the stress in the specimen, splitting cracks occurred and gradually link up in the cross section perpendicular to the direction parallel to the hole. The rock block ejections, slabbing failure and some physical phenomena like strainburst occurred near the hole in the specimens. And then, numerical modeling has been constructed to simulate the failure process of the physical specimens. It is found that in the initial damage process the splitting failure surface usually can be observed near the hole. It is very similar to the site investigation like spalling failure or slabbing faiure at depth.The 3D stress analysis has been done in a deeply located circular tunnel by FLAC3D. The stress distribution and the failure zone of the tunnel versus the advance direction have been obtained. It is seen that the major principal stress contours are curved near the tunnel face. A case study of stress analysis in Qinling Zhongnanshan highway tunnel has been carried out by numerical modeling. The rockburst frequency has been reported not only related to the time interval of blasting rounds but also to the distances to the tunnel face. The delaying effect of strainburst exited in tunnel excavation like the delaying effect of maximum principal stress. From the 3D numerical stress analysis, the mechanism of strainburst can be better understood.