Medium-high density soft micrites: Impact of microstructural features on state and compressibility

Three soft rock facies of the Middle Globigerina Limestone (MGL) from Malta, of mineralogical composition and index properties similar to some medium-high density Chalk facies, are disaggregated through prolonged agitation in water, to create reconstituted samples. The significant activity of their clay-sized calcite grains can impart a medium-high plasticity. SEM analyses of natural and reconstituted samples show the natural bonding as interlock, possibly induced by cohesive clay-sized calcite grains during the sediment compaction. Micro-analyses and compression test data also show that local calcite crystal over-growth under burial has reduced further the soft rock porosity, making it lower than that of the reconstituted material one-dimensionally compressed to the geological preconsolidation pressure. Nonetheless, the low stress sensitivity of natural MGL suggests that this interlock bonding does not strengthen much the material with respect to the highly compressed reconstituted soil. The natural MGL is of very low permeability, which reduces further with compression. Concurrently, the soft rock creep coefficient increases, reaching values far above those typical for clays. The microstructural features and the mechanical properties of the different MGL facies are shown to be sensitive to clay mineral content.