3.6 Further Theories and Readings
Sections 2 and 3 describe basic concepts that focus on explaining fracture orientation and patterns; those concepts provide information on the magnitude of stress and other criteria required for generation of different fracture types and propagation modes. However, other fracture characteristics are important in relation to their role in hydrogeology. These characteristics include, for example, spacing and persistence, also referred to as intensity and size, respectively. These characteristics are relevant as they directly influence the fracture network connectivity and, therefore, the groundwater flow. To improve the understanding of such characteristics, it is important to consider the mechanical description of processes leading to initiation and propagation of fractures in rocks. Theories have been proposed on this topic such as linear elastic fracture mechanics (LEFM), initially developed to explain processes leading to fracture propagation in metals. More recently, LEFM has been applied to understand fracture propagation processes in rocks. Basic knowledge and applications of LEFM to structural geology problems are currently described in structural geology textbooks and articles such as Gudmundsson (2011), Fossen (2016) and Pollard and Martel (2020). Although the theory is perhaps too advanced for beginners, it is relevant and informative and readers may find the mentioned references useful.
Understanding mechanisms that explain the generation of sheeting joints, or relief joints, is also relevant for hydrogeology as these fractures may affect groundwater flow near the ground surface. They parallel the ground surface which implies that the minimum principal stress (σ3) is generally vertical. The scientific community generally accepts that the most probable mechanism causing the formation of sheeting joints is axial splitting associated with in-situ compressive horizontal stresses and unloading due to erosion, (e.g., Holzhausen, 1989; Hencher, 2011). Higher horizontal stresses, in relation to the vertical stresses, may appear during erosion processes, and this condition near the ground surface approaches an unconfined state of stress. Using a uniaxial laboratory test as an analogy, Griffith´s theory (Jaeger & Cook, 1979) predicts the creation of sheeting joints as the result of axial (parallel to σ1) propagation of critically oriented microcracks. These fissures are normally seen in thin sections of hard rocks and their overall propagation results in sheeting joints that are parallel to σ1. As confining stresses increase with depth microcracks tend to close, thereby preventing axial splitting from taking place. As proposed by Martel (2017), the large extensions that are often observed for sheeting joints could be explained by gentle, large-amplitude curvatures of the ground surface.