Small-scale measurement of the transition in fracture behavior of marine sediments

Bubbles grow and burrows extend through cohesive, muddy marine sediments by fracture. In contrast, sands are non-cohesive, granular materials. Natural sediments comprised of heterogeneous mixtures of muds and sands are common in coastal areas and provide important habitat for infaunal animals. To explore the transition from cohesive to non-cohesive mechanical behavior of natural sediments, we modified a probe designed for measuring fracture toughness (KIc). The helical probe is rotated and translated into sediment to grip a plug of sediment, then translated upward to break off the plug while force is measured. Fracture toughness is calculated from the peak net force. The probe shows clearly distinct results in muddier sediments, in which fracture occurs, and in sandier sediments, in which no fracture occurs. The modified probe is limited to near-surface sediments, but it provides a novel method for distinguishing cohesive sediments with tensile strength from non-cohesive sediments on scales relevant for burrowing animals or bubble growth. This measurement allows for comparison of surface and subsurface cohesion and for assessing how tensile strength depends on other properties of sediments.