Non-linear shear strength for rock, rock joints, rockfill and interfaces

Although intact rock is frequently represented by linear Mohr–Coulomb shear strength envelopes, the actual true behaviour, if taken over a wide range of confining stress, is extremely non-linear. Why is this important? Probably because the real stress across points of contact in both rockfill and rock joints is approaching (or trying to exceed) the crushing strength, and if local confined stresses are equally high, strong non-linearity will be experienced. If we utilize the unconfined compression strength of the rockfill, or of the rock joint surfaces, suitably scaled-down due to size effects, we are part way towards a useful strength criterion. In fact, as we will see, we are “one-third” of the way, as the roughness (of particles and asperities) and a measure of the non-dilatant (residual) frictional strengths are also needed for what we will see are two very closely related strength criteria.