Application of the material force concept to formation of discontinuities in soft sedimentary rocks

The possibility of rock joint formation from discontinuities which originate in poorly lithified sediments is presented. The mechanism for the development of these discontinuities along the maximum compressive stress T _max is discussed and quantitatively investigated. Micro-defects (holes, cavities) may exist in sediment due to non-ideal packing. It is shown that in certain situations, a physically warranted tendency to reduce the total energy of the system splits into two separate holes which move away relative to material particles (grains) along the T _max axis. The trace left by the moving hole is interpreted as a macro-discontinuity evolving with time into the closed mode I macro-crack. Some justifications for the mechanism are presented.