MELANGES – SO WHAT? WHO CARES? – A GEOENGINEER'S PERSPECTIVE

Since 1919 when Greenly first identified “autoclastic mélange” in Anglesey, melanges have been of vital and controversial concern to geologists. Thousands of papers, books and abstracts have been written on various aspects of melanges and their aliases (friction carpets, wildflysch, broken formation, mega-breccias, sedimentary chaos, varicolored clays, etc.). Countless more professional and academic treatments have been published on other complex mixtures of hard blocks surrounded by weaker matrix, such as fault rocks, lahars, tillites, and weathered rocks.

Although there are considerable tunneling, excavations and landslides in melanges and similar chaotic rocks, there are few geotechnical engineering treatments. Chaotic rocks are of apparent disinterest to engineers, because when faced with characterizing geological complexity, many engineers simplify the chaos by assuming that the geomechanical properties of the weakest materials dominates. With such an approach, who cares how the rock formed? (Answer: Owners and Contractors care since they have to pay for the consequences of geotechnical over-simplification.)

However, regardless of their geological name or origin, geologically complex mixtures of rocks chaos can be simply characterized as bimrocks (block-in-matrix rocks), and the fundamental engineering properties estimated for design and construction. Bimrocks are “mixtures of rocks composed of geotechnically significant blocks within a bonded matrix of finer texture” The term “bimrock” is geologically neutral with no genetic connotations. The expression “geotechnically significant blocks” means that there is mechanical contrast between blocks and matrix, and that the geometry and proportion of blocks influence the rock mass properties at the scales of engineering interest. Of course, engineers must in recognize in the first place that they working with geologically complex rocks…