Meso-mechanical Study On Hydraulic Fracturing In High Core Rockfill Dam

Hydraulic fracturing is one of focuses in the design and construction of core dams. Large numbers of studies on the problem have been carried out using laboratory and field tests or numerical simulations based on continuum mechanics from macro perspectives by many domestic and foreign researchers, and plenty of research findings were obtained. However, because of the complication of the problem and the discontinuity of the core materials, there may be some limitations to research hydraulic fracturing in the core by methods based on continuum mechanic, the mechanism of hydraulic fracturing is still disputable. In this thesis, hydraulic fracturing is simulated using the particle flow code (PFC) and the preliminary studies on the mechanism are firstly completed from a mesoscopic perspective.Hydraulic fracturing in the core is essentially a fluid-solid coupling problem of geotechnical engineering. In order to simulate the interaction between fluid and solid in the core from a mesoscopic perspective, two fluid-solid coupling methods are studied and improved, which are polygon domain algorithm and triangle domain algorithm. The horizontal seepage flow model and effective stress model are then simulated using the two algorithms. Results show that the distribution of pressure along the flow direction is linear after reaching the seepage stability stage, which consistently corresponds with Darcy's flow law. The triangle domain algorithm is shown to be superior to the polygon domain algorithm in terms of accuracy and authenticity of applied boundary condition. Thus the osmotic consolidation test is simulated using the triangle fluid domain algorithm, and the result shows that the algorithm is able to simulate the coupling effect between the stress field and the seepage field.Presence of initial cracks or defects in the core is one of the requirements for hydraulic fracturing to occur. Therefore, the occurrence and development process of hydraulic fracturing can be studied from the perspective of fracture mechanics. In order to apply the continuum theory of linear elastic fracture mechanics (LEFM) to the study of the fracture property of the discontinuous core, linear elastic fracture mechanical behaviors of the core soil is reproduced using a set of self-similar tension specimens with a central crack and single-edge notched beam (SENB) tests based on the bonded particle model (BPM). Results indicate that with the same values of meso-parameters, the fracture toughness results of self-similar specimens tension tests are in good agreement with that of the single-edge notched beam tests. On this basis, several influence factors on fracture resistance of bonded particle model are studied, such as the particle size, contact stiffness, contact bond strength, friction coefficient and initial length of notch.In researches on the hydraulic fracturing in macroscopic aspect, no agreement has yet been made on the reson of its occurrence. In this study, a particle flow model of hydraulic fracturing in the core is established and a criterion that could be used to judge hydraulic fracturing occurrence in the model is proposed. The initiation and development process of hydraulic fracturing is simulated from a mesoscopic perspective and the results are compared to those from the fracture mechanics theory, which verifies that the proposed model is reasonable and reliable. Test results show that the fracture resistance of the model can be improved by increasing the vertical stress. In order to analyze the mesoscopic evolvement of the model in hydraulic fracturing process, evolving processes of the average coordination number, porosity and the number of failure bond are examined. In addition, the influences of particle size ratio, impounding speed and initial crack length of model on fracture initiation pressure are studied with particle flow model of hydraulic fracturing.Based on the numerical simulation results on hydraulic fracturing, the stress states and the strain energy of the particles and the region in front of the initial crack tip are analyzed from a mesoscopic perspective. Results indicate that the normal tension failure is the major cause of the transverse crack formed in the model, i.e. hydraulic fracturing in the core is a tension failure. The evolving process of strain energy at the crack tip verifies that the appearance of cracks is always accompanied by the release of strain energy at the meso-level. At last, according to the research findings above in meso-mechanism on hydraulic fracturing in the core, several practical engineering problems that may exist are analyzed, which could serve as references and basis for engineering measures.