CHP 4B

• Rock mechanics is the theoretical and applied science of the mechanical behaviour of rock
• Rock mechanics is concerned with the response of rock to the force fields of its physical environment
• In the study of the mechanics of particles, the fundamental kinematical variable used is the position of the body, its velocity, and acceleration
• The interaction of a body with other bodies is quantified in terms of the forces exerted on the body
• Newton's law of motion states that the sum of the forces acting on a body is equal to the mass of the body times its acceleration
• For a body to be in equilibrium, the sum of the external forces and moments acting on it must vanish
• Basic mechanical concepts such as position, force, and Newton's law of motion apply to deformable bodies like rock masses
• Forces applied to a rock vary from point to point and are distributed over the body
• Different parts of a rock may undergo different amounts of displacement
• The relative displacement of neighboring particles, rather than the absolute displacement of a particular particle, can be equated to the applied tractions
• The displacement of nearby elements of the rock is quantified by an entity known as strain
• The stress tensor is a symmetric second-order tensor, and many important properties of stress follow directly from those of second-order tensors
• In the event that the relative displacements of all parts of the rock are small, the strain can be represented by a second-order tensor called the infinitesimal strain tensor
• Rheology is the science of the deformation and flow of materials
• The fundamental concepts of rheology are stress and strain
• Stress expresses the magnitude of forces acting within or at the surface of continuous media, while strain expresses deformation of the media
• Surface forces and body forces are different
• Traction is a vector that has the same direction as the force and is measured in Pascals (Pa)
• Traction can be decomposed into normal and tangential vector components
• Friction is the phenomenon where a tangential shearing force is required to displace two contacting surfaces along a direction parallel to their nominal contact plane
• Amontons' laws of friction state that the force of friction is directly proportional to the applied load, independent of the apparent area of contact, and kinetic friction is independent of the sliding velocity
• Rock mass classification schemes have been developed to assist in grouping rocks into common or similar groups
• Rock quality designation (RQD) is a rough measure of the degree of jointing or fracture in a rock mass, measured as a percentage of the drill core in lengths of 10 cm or more
• High-quality rock has an RQD of more than 75%, while low quality has less than 50%
• Rock Quality Designation (RQD) is the borehole core recovery percentage incorporating only pieces of solid core that are longer than 100 mm in length measured along the centerline of the core
• Pieces of core that are not hard and sound should not be counted even if they are 100 mm in length
• RQD was originally introduced for use with core diameters of 54.7 mm (NX-size core)
• RQD has considerable value in estimating support of rock tunnels
• RQD forms a basic element in some of the most used rock mass classification systems: Rock Mass Rating system (RMR) and Q-system
• Core recovery (CR) = (Total length of rock recovered / Total core run length) x 100
• Class I = Very good rock
• Class II = Good rock
• Class III = Fair rock
• Class IV = Poor rock
• Class V = Very poor rock
• Rock Mass Rating (RMR) method is started with two main conditions namely general conditions and discontinuity conditions
• General conditions include Intact Rock Strength, Rock Quality Designation (RQD), Discontinuity Spacing, Discontinuity Orientation, and Groundwater Condition
• Discontinuity conditions include Length, Separation, Roughness, Infill, and Weathering
• Rock Mass Classification: Class I = Very good rock, Class II = Good rock, Class III = Fair rock, Class IV = Poor rock, Class V = Very poor rock
• It expresses the quality of the rock mass in the Q-value, on which are based design and support recommendations for underground excavations