The geologically neutral term bimrocks (block-in-matrix rocks) includes melanges, fault rocks, lahars, weathered rocks and other geologically complex mixtures of strong rock blocks embedded within weaker matrix rocks. Bimrocks have mechanical contrast between blocks and matrix, and the geometry and proportion of blocks influence the rock mass properties. The most intractable bimrocks are melanges, exemplified by those of the Franciscan Complex (the Franciscan) of Northern California.
The rich geological literature on geologically complex mixtures provides little guidance to practitioners, despite the considerable tunneling, excavation and landslide remediation projects in chaotic rocks worldwide. To effectively engineer with bimrocks, geological chaos must first be recognized and then disciplined characterization performed. The strength of melange bimrocks is simply related to the volumetric proportion of blocks, the evaluation of which must accommodate uncertainties. Although progress is being made in the engineering of geological chaos, much research must yet be performed – some suggestions are offered.