Introduction
The lithosphere of the Earth is composed of the crust and the mantle and both are rigid. Inside the lithosphere, however, is still dynamic with plates in constant movement and their continuous interaction with each other. As a consequence of the pressure and friction generated by this interaction, rocks behave in varied ways according to the type of stress applied and the characteristics of the material itself. Pore fluid pressure, mineral composition and temperature are all factors that influence the behaviour of the rock. In this context, rock stress is the study of the effect of these forces on the resulting behaviour of the rocks.
Principal stresses
When a force is applied to a material body, the body reacts in various ways producing strain and deformations. From the physical point of view, all material bodies are composed of cells and each of those cells is subject to a stress, the principal stress. This principal stress is the result of the interaction of the bodys internal properties and the external forces being applied to it. In most cases, these principal stresses are composed of three components, the horizontal components s_x, s_y and the vertical component s_z, with which the behaviour of the material body can be studied and modelled in order to predict how it will react when subjected to a certain force.
Shear stress
When two forces are applied to two opposite sides of a material body, with the same magnitude but in opposite directions, a shear stress is produced. This shear stress produces the body to deform in a plastic manner, meaning that it will permanently change its shape if the shear stress is large enough. Shear stress is also used in engineering and can be helpful to understand floor formation in underground mines and tunnels as well common problems in pipes and mechanical components.
Stress gradients
The stress field of a material body is the distribution of the principal stresses throughout it. Usually, this stress field is not uniform and often anisotropic, meaning that each direction shows a different type or magnitude of principal stress. When there is a change in the direction of principal stress from one point to another, there is an associated stress gradient. This gradient is produced by the interaction of the forces applied to the material body, their motion and distribution, and the mechanical properties of the body itself. Stress gradients can have a profound effect on the behaviour of the body and can even cause deformation or failure in some cases.
Stress concentration
Wherever there is a change in stress and strain, there is a point of maximum stress concentration. This point is usually located at a crack or other opening in the body and results in a displacement or displacement gradient of the material. The areas of stress concentration and/or strain gradient are usually weaker than the surrounding material, meaning they can be damaged more easily. Engineers take great effort to dissipate or reduce the stress concentration near any crack or opening, so that the system remains safe and operational.
Rock Bolt Systems
A rock bolt system is a reinforcement method used in underground mining and civil engineering projects. This system consists of steel bolts that are installed within the rock mass and connected to a structural panel or wall. When the bolts are in place, they transfer the load from the panel or wall to the rock mass, thus preventing it from collapsing. Rock bolts can be further strengthened when grouted with a concrete mixture due to increased adhesion between the two materials and improved confinement.
Conclusion
Rock stress is a complex phenomenon that affects materials such as rocks found in the Earth’s lithosphere. The principal stresses, shear stress, stress gradients and stress concentration are all important factors when studying the behaviour and stability of rocks. Additionally, rock bolt systems can be used to strengthen soft rock structures and prevent collapses. Understanding rock stress is important in a wide variety of applications and industries, especially when designing structures required to withstand a lot of pressure and strain.
