It is necessary, mainly in areas under risk, to understand the mechanisms governing rock instability in order to adopt suitable measures and avoid accidents. In many cases, events like detachment of rocks may happen in periods without rain. Recent studies have raised the possibility that the seasonal and daily thermal oscillations are responsible to generate thermal stresses which are capable to propagate cracks. In order to understand these processes, thermomecanical analyses were carried out of slopes using the Finite Element software Abaqus 6.14. In the simulations, simplified geometries were analyzed, representing intact and fractured rock plates. The rock mass was considered as homogeneous and isotropic. The daily thermal oscillations and the rock parameters, representative of rocks from Rio de Janeiro, were obtained from a previous study (Vargas et al, 2012). Various scenarios were considered in which the effects of the gravity and non-uniform temperature variations could be analyzed separately. The obtained values of stress intensity factors and tensile stresses were compared with values of fracture toughness and tensile strength typical for granites and gneissic rocks.
Rocks, like any other material, is susceptible to the time effects which can lead to future instability events. There is a tendency to associate such events with periods of significant rainfall. However, several studies have shown that in many cases these occurrences are not concomitant with heavy rains and are not even preceded by them. Studies such as those developed by Vargas et al. (2012), Collins & Stock (2016), Yan & Zheng (2016), Yu-Yong et al (2015), for example, indicate that the effects of thermal cycling on rock have a great influence in triggering these events. The thermal stresses generated by cycles of contraction and expansion could be sufficient to propagate pre existent fractures or open new fractures.
Within this context, it is relevant to link the concepts from fracture mechanics to thermally induced stresses. Temperature changes are capable of imposing deformations and modifying the state of stresses of a body. Noda et al. (2003) explores this matter by using simplified bar models, whose principles can be extrapolated to other geometries. In this sense, the theory of fracture mechanics (Griffith, 1920) complements the study of thermomechanical processes that occur in slope stability problems, since continuum mechanics does not consider the existence of discontinuities and, therefore, does not admit the possibility of collapse by propagation of fractures. Broek (1982) and Atkinson (1987) describe in detail these processes. The present work aims to study the influence of daily temperature variations on the generation of thermal stresses, through the results of numerical analyzes performed using the Finite Element Method (FEM) for simplified geometries representing rock slope conditions. The magnitude of the maximum tensile stresses and the stress intensity factors for mode I were evaluated in order to determine whether there is a possibility of failure in the rock mass by tensile stresses or by propagation of fractures.
