Mechanical and transport properties of crustal rocks: from single cracks to crack statistics

Abstract Cracks control the physical and mechanical properties of rocks. Using fracture mechanics, it is possible to analyse the behaviour of a single crack. From standard tests (double torsion, double cantilever beam), experimental data, obtained on various minerals and rocks in the sub-critical crack growth regime, lead to the conclusion that fracture strength is weakly rate dependent. Real rocks contain a population of cracks. Physical and mechanical properties of rocks must therefore be interpreted by combining the behaviour of a single crack with statistical methods. Two properties are considered here: permeability and fracture strength. Statistics and percolation theory show that there are two important thresholds for crack concentration: the first corresponds to the onset of permeability and the second to the development of macroscopic fracture.