A2 Rock Strengths Flashcards
Stress
defined as the force per unit area acting on the rock. It can be compressive stress that tends to compress or shorten the material acting normal to the stressed area or shear stress that acts in a plane parallel to the stressed area or tensile stress that tends to stretch or lengthen the material; acting in the opposite sense to compressive stress.
Fabric
describes the spatial and geometric configuration of all the ‘components’ of a rock: the mineral crystals and their sizes, shapes and relative orientations or the mineral grains, the voids between and their relative sizes and dispositions. Close to the term ‘texture’ for sedimentary, igneous and metamorphic rocks.
Strain
the response of a system to an applied stress. When a material is loaded with a force, it produces a stress, which then causes a material to deform. Engineering strain is defined as the amount of deformation in the direction of the applied force divided by the initial length, volume or shape of the material (it has been normalised).
Confining pressure
the combined lithostatic and hydrostatic pressure. At depth all principal stresses are equal (σ1 =σ2 = σ3).
Ductile deformation
occurs when a rock suffers large strains without large-scale fracturing. It bends and flows.
Brittle deformation
causes the rock to fracture (possible after some elastic deformation has occurred).
Principal stresses
a useful way to describe what is going on. σ1 is the maximum, and σ3 being the minimum and σ2 is the intermediate principal stress direction. All three principal stresses are perpendicular to each other.
Lithostatic pressure
the vertical pressure due to the mass of the rock only. It is also referred to as the overburden pressure.
Asperity
the term mostly used to describe the roughness of the surface of a discontinuity.
Residual strength
the remaining resistance to movement after the rock has failed and been displaced.
Joint sets
are fractures across which there is little displacement. They are mostly to dissipate the residual stresses left after folding. As they are the result of regional stresses they tend to form sub-parallel sets.
Drift
the superficial deposits such as glacial and fluvioglacial material. The ‘solid’ maps show an overlay of drift. Alluvium refers to the recent deposits by rivers.
Seismic refraction surveys
can be made using a simple dropped mass or large hammer as the energy source. Waves are refracted by changes in velocity such as at the base of the weathered layer.
