Deformation and failure of brittle rocks under compression

Detta är en avhandling från Luleå : Luleå tekniska universitet

Sammanfattning: This thesis describes the deformation and failure of rocks under compression through a micromechanics model and laboratory studies of the Kaiser effect. It comprises an overview and seven appended papers. The deformation model is developed based on the analysis of the behaviour of microcracks. The macroscopic deformation of a rock is decomposed into three components in the pre-peak stage: response of the rock matrix, closure of open cracks and microcracking-induced deformation. Microcracking is the mechanism of the nonlinear deformation behaviour of brittle rocks in this stage. The nonlinear behaviour is simulated by considering the axial microcracking. The basic model units are sliding cracks. Wing cracks are initiated at tips of sliding cracks and propagated in the direction of the major principal stress. The relationships between the compressive stresses, the growth of microcracks, and the microcracking-induced deformation are analytically established. A deformation solution to an elliptic crack is employed to describe the closure effect of an open crack under compression. The total deformation caused by the closure and fracture of cracks is obtained by summing up the components of individual cracks. In this model, the nonlinear behaviour of deformation results either from the closure of open cracks at low stress levels, or from the fracture of microcracks at high stress levels. In the post-peak stage, the shear failure dominates the deformation process. The shear stiffness of the faulting plane decreases with the development of the shear failure. A faulting-induced deformation component is added into the macroscopic deformation. This process is simulated by a damage mechanics model. This micromechanics model is helpful in understanding the failure process in rocks. The model can be used to simulate the complete stress-strain behaviour. Hysteresis is captured under cyclic loading. The model simulations are consistent with experimental results. The laboratory acoustic emission (AE) tests reveals that liquids are better than metal foils as acoustic couplants for P wave transmission. It is recommended, therefore, that viscous liquids should be used as acoustic couplants in conventional AE tests. The Kaiser effect is a measure of damage developed in rock. It was investigated using 61 core specimens of eight types of rocks under uniaxial cyclic loading. The effects of the delay 6= between subsequent loading cycles and the holding time on the Kaiser effect were examined using granite specimens. The characteristics of AE occurring during unloading and holding were also studied. The experimental results showed that most of the rocks, with the exception of some iron ores, showed an obvious Kaiser effect before the load was very near the level of the strength. The delay and holding times did not strongly influence the Kaiser effect. Breakdown of the Kaiser effect is expressed by the felicity ratio, which may be taken as a measure of the quality of rocks. The onset of the continuously increasing AE in the first loading cycle can be taken as a measure of the damage in rocks. The mechanism of the Kaiser effect was studied with the aid of the miromechanics model. An expression for the damage surface in the stress space was derived from the model. The theoretical expression was compared with the results of the Kaiser effect tests, which showed a satisfactory consistence. The Kaiser effect has been attempted to be used to assess the in-situ damage of rocks. Cores were drilled from a hard rock tunnel. The specimens prepared from the cores were uniaxially compressed in the laboratory and the AE emitted from the specimens was recorded. The onset of the increasing AE was taken as an indication of the degree of blasting-induced damage in the rock. The results showed that the AE-onset stress decreased with the distance from the tunnel contour. The range of disturbance caused by blasting could be estimated using the curve of the AE-onset stress.

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