Introduction to rock mechanics

In geology, ‘rock’ may be defined as a natural aggregate of minerals (Whitten and Brooks, 1972), which is a very broad definition that encompasses all naturally-formed soil and rock as otherwise distinguished in engineering practice. In geotechnical engineering, the term ‘rock mechanics’ is usually confined to the stronger and lithified end of the spectrum – the state when the mineral grains have become bonded and cemented together over geological time. In sedimentary rocks, this is partly due to the secondary effect of minerals such as calcite, silica and iron oxides that are transported and then deposited in the pore space. These secondary minerals cement the sedimented mineral grains to one another. Strong electrochemical bonds also exist directly between individual mineral grains in all rocks. In sands, grains in contact deform under high pressure to form strong bonds (‘pressure solution’ and ‘interpenetration’); in igneous and metamorphic rocks strong bonds are formed on cooling and the transition from liquid to solid. This bonding, together with cementation, results in intact rocks having tensile and cohesive strength, which is their defining characteristic (see Section 1.6 for definitions).