ABSTRACT: 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.
Geological materials are generally neatly classified as either Soil or Rock. But engineers and geologists worldwide commonly encounter geologically complex mixtures of strong blocks of rock embedded in soil-like matrices. Nevertheless, many practitioners do not recognize geologically complex "soil/rock" mixtures, let alone adequately characterize them. The chaotic mixtures are created by several modes of genesis and are known by a myriad of geological names (melanges, olistostromes, cataclasites, fault rocks, breccias, lahar deposits, diamictites, etc.) which have firm and important connotations for geologists, but are confusing to most engineers. The vast geological nomenclature masks the similar physical and mechanical traits of complex mixtures. Therein lies the advantage of treating geologically complex mixtures as bimrocks (block-in-matrix rocks), an engineering material that can be systematically characterized for engineering purposes. This paper presents a broad overview on some fundamental observations on bimrocks, highlights themost significant problems encountered when characterizing them, and offers suggestions for future bimrock research.
2. FRANCISCAN MELANGES
This paper is appropriately published in the Proceedings of the 2008 American Rock Mechanics Association conference hosted in San Francisco, California. San Francisco, with its many picturesque hills - actually large resistant blocks surrounded by weaker sheared rock - is the geological type locality of the Franciscan Complex ("the Franciscan"), a regional scale jumble that covers a large proportion of Northern California. The Franciscan is world-famous amongst geologists who make pilgrimages to San Francisco and Northern California to investigate its complexity (Figure 1). Understanding of the Franciscan has evolved over nearly 150 years  during which time it has been variously known as the Franciscan Series, Franciscan Formation, Franciscan Assemblage, and currently, the Franciscan Complex [2, 3]. The name "Franciscan Complex" is geologically objectionable  but semantically appealing: the rocks, the structures, the history and the geological arguments are complicated. Although many engineers and geologists still use the incorrect and dated "Franciscan Formation" it is common and acceptable to refer to the Franciscan Complex as "the Franciscan" as a way to avoid the uncertainty of which name to use. The Franciscan has been identified in Baja California, the Catalina Islands, central California north of the Coast Ranges, and continuously from San Francisco north into Oregon.