Role of fluid pressure in jointing

Geologic evidence suggests that some joints are tension fractures developed perpendicular to the least principal stress direction in rock. In this study the mechanics of natural tension fracturing is examined in light of the fluid pressure theory developed by Hubbert and Rubey (1959). The Griffith theory of failure, expressed in terms of effective stresses, is taken as the failure criterion for rocks. In addition one of the total principal stress directions is assumed to be vertical and equal to the bulk weight of the overburden to the depth considered. Calculations based on these assumptions indicate that tension fractures can develop at increasingly greater depth in the Earth as the ratio of fluid pressure to overburden weight approaches one. . . . Previously formed fractures can be opened up by fluid pressure at great depth in the Earth9s crust. Such open fractures probably are important in the migration of ground water, hydrocarbons, and ore fluids.