The relationship between particle strength and particle breakage in loaded gravels

The particle breakage behaviour of coarse grains during shearing of a gravelly granular assemblage changes in accordance with the magnitude of the confining stress acting on the assemblage. The transition from particle rearrangement and sliding to particle breakage is studied for two gravelly materials subjected to a wide range of compressive and shear stresses (10kPa to 3.5MPa). The tested materials are a gravel fraction derived from crushed sedimentary sandstones and shales, and a gravel derived from a crushed rhyodacitic volcanic. At low confining stresses, breakage due to compression and shear are both small, with only some fine fragments produced. Confining stress increase results in a stage being reached where particle damage starts to increase greatly, under both compressive and shearing loads. The results indicate that, as stresses become very large, the particle breakage due to compression continues to increase, but the breakage due to shearing under the same vertical stress actually decreases. This is attributed to the attainment of a fabric which, through extreme damage during compression, allows subsequent shearing with reduced additional particle damage.