مطالعه و تحلیل فرآیند شکست سنگ توف بر اساس آزمون‌های آزمایشگاهی و مدلسازی عددی سه‌بعدی

Summary
Failure mechanism is different due to stress conditions and rock types, and it is very important to stability of structures. One of the investigations of failure mechanism’s techniques is using triaxial compression test. The aim of the research is determination of failure plane and failure mechanism of Rhyolite Tuff experimentally in different confining pressures and then investigating failure mechanism numerically using three-dimensional finite difference method in details. From the experimental results it was found that with increasing confining pressure, failure angle changes from vertical to oblique. Then numerical model was calibrated with laboratory data and analyzed the details of failure process, which could not be investigated experimentally. Investigations showed that failure process, started with plastic strains at the center of the sample and continued to achieve shear failure plane. Moreover, the results showed that the numerical model is in good agreement with the laboratory tests.
 
Introduction
Given the increasing production of minerals, the need to expand and deepen mines is inevitable. With deeper mining, sustainability issues have become more important.  All of underground structures have been constructed in rock and the recognition of the rock mechanics issues in relation to the stability of these structures is very important. One of the main topics of rock mechanics, which plays a significant role in the stability of structures, is the rock failure mechanism. Since these structures are located deep in the ground and the behavior of rock failure in different conditions of stress is different, recognizing the rock deformation process under stress conditions can provide a better picture of rock behavior in real conditions .When the distribution and turbulence of the stress creates an imbalance in the rock mass, rock failure occurs by the distribution and interconnection of the micro cracks. Still, there are ambiguities about how rocks are really broken and how the cracks begin to spread. Therefore, prediction of models for fracturing of rocks is very complicated, and it is impossible to predict the failure of rocks with simple models. Therefore, fundamental studies are needed to evaluate the failure process and damage of rocks in different stages of stress.
 
Methodology and Approaches
By using uniaxial and triaxial tests, a stress-strain graph was drawn for a rhyolite rock sample. Analyzing the experimental results, it was observed that, with increasing lateral stress, rock resistance increased, and the direction of the fracture plane was maximally illuminated and more angular than the direction of the main stress. It was also found that the behavior of rock with the Mohr Columb's failure criterion is good. For numerical modeling of laboratory experiments, cohesion parameters, internal friction angle, shear modulus, volume modulus and specific gravity were used. To calibrate the model, the meshes and the rate of application applied to the top of the specimen were changed to see the fracture and ultimate strength similar to the laboratory sample in the numerical model. After calibrating the model, the details of the failure stages were carefully examined.
 
Results and Conclusions
In numerical models with increasing lateral stress, the angle of deflection compared to the original stress was increased. Tensile plastic strains were formed at the beginning of the loading at the center of the sample. As the loading steps progressed, shear plastic strains started from the corners of the sample and proceeded to the center of the sample. This condition continued until the complete failure of the model and the creation of the plane shear failure. Also, with increasing lateral stress, the initiation of plastic strain occurred in higher stresses and later formed in the sample. Plastic tensile strains appeared at the center of the sample at the beginning of the loading. In the following, the strain began from the two corners of the sample and extended towards the center of the sample. It then extended its path until a plane was formed. Shear and tensile strains were formed in the form of shear-n and (tensile-n) in the sample. These strains may create a shear failure sheet. In fact, joining these strains continuously created a failure plane in the model. The final shear plane was the result of shear-n joining of the shear strains.