ABSTRACT
The physical foundations and justification of plasticity theory for application to rock and soil, geo-materials, is necessarily based on experimental evidence. Plasticity theory was for ductile materials that showed large post-yield strain. The amount of plastic strain that occurs after reaching the elastic limit is not a requirement of plasticity theory as such. Joints in rock formations are almost always spaced on a meter scale, and they have major effects on the response of rock to excavation-induced loads. Certainly, the response of intact rock between joints is readily determined in laboratory testing at a centimeter scale. Although the few data available describing jointed rock response to load indicate an elastic-plastic formulation is reasonable, a reliable model based on first principles is needed in the general case. From the mechanical viewpoint, the difference between soil and intact rock is more one of degree than of kind. Both are aggregates of solid particles enclosing voids of varying shape and size.
