Typical curves for rock compressibility have been reviewed. Certain characteristic pressures have been distinguished and the volumetric modulus of elasticity has been expressed in a function of porosity. The characteristic pressures are responsible for the occurrence of the characteristic points on the curves representing the dependence between the confining pressure and strength, strain at failure, elasticity limit and Young's modulus. The influence of the pore pressure, of the interaction of water and of the permanent deformation of the rock due to pressure on the behaviour of these curves and on the characteristic pressures has been discussed.
The thermodynamic state of a rock is determined by pressure, temperature and its specific volume. Its mechanical properties depend on the state and the way the rock has been loaded. In particular, already at room temperature, the loading of the rock with hydrostatic pressure including merely the elastic strain, determines, according to the value of the pressure, the behaviour of the rock under deviatoric stress. A sufficiently high pressure produces permanent deformations of the rock, both of the brittle and ductile type. It becomes, eventually, besides temperature, one of the factors leading to phase transitions. Another factor determining the rock properties is the presence of the pore fluid under pressure. The fluid may act by way of pressure on the pore surfaces and by way of sorption which becomes, more intensive as the pore pressure increases. The present paper deals with the mechanical properties of rocks as depending on the confining pressure, on the hydrostatic pressure which leads to permanent deformations of the rock, by the pore pressure induced by an inert gas and by water present in the pores as an active fluid. Deformation of rocks under hydrostatic pressure The rock taken in the scale of a laboratory sample is made up of a solid phase, isometric pores and cracks. The solid phase of many rocks is characterized by a proportional dependence between the volumetric strain and the hydrostatic pressure (Fig. 1a). In a rock containing exclusively isometric pores the proportional dependence between the pressure and the volumetric pressure is usually retained and, according to the theoretical considerations of Walsh (1965 a), its coefficient of compressibility when compared with that of its solid phase (Fig. 1b for limestone) is on the increase. An example of a rock containing exclusively isometric pores and characterized by a non-linear pressure-volumetric strain relation is the sapropelic coal (Fig. 1b). The inflection of a part of the curve is probably due to the closing of the micropores between the crystallites and the easily deformable (in comparison with the solid phase of the above mentioned rocks) hydra.. carbon chains connecting them. According to Walsh (1965 a) the presence of cracks in the rock enhances its compressibility much more than the presence of isometric pores of the same porosity as that of the cracks. This is decidely due to the fact that the cracks are easily closed under growing pressure.
