Stress-strain behaviour analysis of slate and intact and jointed granite lab samplesdeformational and strength parameters, scale effects and tentative models

Resumo

This thesis contributes with some innovations in the implementation of advanced tests in rock mechanics, their interpretation, and the development of tentative models. Rock mechanics is the discipline that analyses the stress-strain behavior of mineral aggregates forming extensive zones of the Earth's crust called rocks, providing basic ideas for the development of engineering and geological applications. Understanding how different types of rocks respond to natural and human-induced stress fields is crucial for designing safe and stable structures in civil engineering, mining, petroleum, and geological risk projects. In this thesis, several relevant aspects of the mechanical behavior of rocks and rock masses are explored, such as anisotropy, in terms of strength and deformability, and scale effects in both intact and fractured rocks, focusing on strength and deformability. Additionally, numerical simulations have been conducted using a particle code based on the Discrete Element Method (DEM) to advance the understanding of scale effects in intact rocks and their modeling. An important aspect of studying the mechanical behavior of rock masses is the inherent variability and complexity of geological formations. Rock masses may exhibit anisotropy, which will affect strength and deformability values as they can vary significantly depending on the direction in which they are measured. This variability presents challenges for engineers when designing structures to handle the diverse and often unpredictable conditions encountered in natural rock formations. Among the variety of rock masses that may exhibit this anisotropic behavior are those composed of slate. Slate, as a characteristic rock of regional metamorphism generated at high pressures and temperatures, exhibits complex mechanical behavior due to its inherent anisotropy. This means that its mechanical characteristics vary with direction, especially controlled by weakness planes associated with foliation, thus exhibiting a special type of anisotropy, known as transversely isotropic. This type of anisotropy essentially means that the mechanical properties of slate vary directionally with respect to foliation or schistosity planes. This structural peculiarity significantly impacts slate's response to applied forces, thereby influencing its strength, deformability, and failure mechanisms. For slate characterization, a rigorous procedure was followed, including various types of tests. Among the non-destructive tests, wave propagation velocity tests were performed. These tests provide dynamic elastic parameters that characterize the rock. Regarding destructive tests, two types were conducted in this study: uniaxial compression and triaxial compression tests. From these tests, besides the maximum strength, elastic parameters such as Young's modulus and Poisson's ratio were obtained. These parameters provide valuable information about slate's ability to withstand loads and deform under different stress states. Young's modulus indicates stiffness, i.e., the deformational response proportional to the load, while Poisson's ratio defines the response of a sample to deformation in a direction perpendicular to the applied load. These studies contribute to a better understanding of the transversely isotropic behavior of slate by obtaining accurate elastic and strength parameters of this intact material. Extrapolating the knowledge of this behavior to the scale of rock masses will contribute to developing strategies for more efficient and safe excavation designs in masses formed by these and other anisotropic rocks. Another key aspect to fully describe the behavior of a rock mass is the scale effect. While reviewing the literature, it is noted that extensive research has been conducted on the mechanical behavior of rocks under uniaxial loading conditions, relatively few studies have focused on triaxial tests regarding scale effects. Triaxial tests offer a more complete understanding of rock behavior under realistic stress states; however, their use has been limited. Traditionally, it was assumed that as the diameter of the tested rock sample increased, its strength decreased, following the Scale Effect Law (SEL). However, recent studies challenge this classical assumption. Observed trends exhibit an increase-decrease behavior as the size of the tested sample increases. These effects are captured in the Unified Scale Effect Law (USEL) and the Improved Unified Scale Effect Law (IUSEL) models. This discrepancy highlights the complexity of scale effects and underscores the importance of further research into this phenomenon. To enhance understanding in this field, close to a hundred triaxial tests were conducted. For these triaxial tests, the laboratory at the University of Vigo had Hoek cells for standard sizes of 30 and 54 mm. In addition, Hoek cells have been specifically designed and manufactured for the 38 and 84 mm tests. The conducted triaxial tests have improved the understanding of scale effects in hard rocks. By fitting the most recent models, USEL and IUSEL, significant improvements have been made in our ability to predict and understand the behavior of these materials in various situations and scales. Traditionally, there has been a lack of comprehensive research specifically focused on samples of jointed rock, and studying the scale effect on jointed rocks is even more challenging. This knowledge gap arises from the inherent difficulty in obtaining samples that faithfully replicate the fissures, joints, and natural fractures present in rock formations. However, recognizing the significant influence of joints on the mechanical behavior and stability of rock formations, recent efforts have been aimed at addressing this gap in research. For this study, triaxial tests were conducted on two types of samples following a specific methodology. To obtain these samples, molds and specific apparatus were designed and manufactured to obtain samples of smaller and larger sizes. Two types of samples were chosen: J1+2, which have one sub-vertical joint and two sub-horizontal joints, and J2+3, which have two sub-vertical joints and three sub-horizontal joints. Additionally, this document presents a study conducted using a numerical approach, employing a discrete element simulation framework (DEM) using the particle code PFC3D, to assess the scale effects on rocks. The study includes samples of different sizes to explore how scale influences the mechanical behavior of rock materials. By simulating the response of these samples to different loading conditions and stress regimes, the research aimed to elucidate the scale effects on factors such as strength. In summary, although studies of anisotropy, scale effects, and replica of rock masses in laboratory-scale specimens and numerical modeling are still evolving, they represent critical areas of research in rock mechanics and geotechnical engineering. By addressing the discrepancies between classical assumptions and empirical observations, advances can be made in the understanding of rock behavior, and more robust methodologies can be developed to evaluate the mechanical properties of rock masses.