MICROSTRUCTURAL CONTROLS ON THE GEOMECHANICAL BEHAVIOR OF IGNIMBRITE

''Ignimbrite'' is a genetic name relating to the pyroclastic origin of the material. Ignimbrite materials have a wide range of geomechanical characteristics; the materials range from soft, non-jointed soils wit h dry compressive strengths < 1 MN m-2, to hard rocks with extensive s ystems of cooling joints and dry compressive strengths > 50 MN m-2. Te nsile strengths of oven-dry rocks range from < 1 to 7.1 MN m-2, cohesi on from 0.14 to 13 MN m-2, and friction angles from 27-degrees to 35-d egrees. Porosities of 17 to 51 % lead to a considerable loss of streng th on saturation, and second-cycle slake durabilities range from 30 to 99%. The groundmass microstructure exerts the primary control over th e geomechanical behaviour of ignimbrite. Compressive strength and slak e durability are controlled by the closeness of packing of the groundm ass shards, and the nature of the welding between individual shards at their points of contact. These factors control the ease with which mi crofractures can propagate through the groundmass. Crystal and clast s ize exerts a secondary control on compressive strength by influencing the initiation of microfractures through stress concentration around i nhomogeneities; the proportion of such inclusions is insignificant, so long as a small number are present to initiate microfractures. Tensil e strength is primarily influenced by the degree of shard alignment, w ith only a small anisotropy existing in terms of the direction of shar d alignment with respect to the principal stress direction. Shear stre ngth is not clearly related to microstructure.