Sediment heterogeneity affects navigation by burrowers: a modelling study

Worms extend burrows through muddy sediments by fracture, and the mechanics of crack propagation through heterogeneous sediments affects both navigation by burrowers and the release of particulate material, which is mixed through bioturbation. Crack propagation follows the path of least resistance or the lowest fracture toughness. Previous work showed that applying asymmetrical stress to burrow walls to simulate steering had minimal effect on crack propagation direction, suggesting that crack branching or the fusing of microcracks near the crack tip with the main burrow allows for burrowers to navigate by choosing between two directions. Here we use the lattice element method for modelling of fracture in heterogeneous materials to examine how fracture toughness, variability in fracture toughness and worm behaviours affect crack branching and microcracking. Experimental observations of worms burrowing in custom-built ant farm tanks support the modelling results that burrowing activities create microcracks both within the vicinity of the crack tip and in the surrounding sediment. In addition, hydraulic fracture driven by burrow irrigation reduces microcracking outside of the fracture process zone, potentially increasing the efficiency of burrowing. These results highlight the potential feedback between burrowing activities and sediment heterogeneity that characterize ecosystem engineering of sediment habitats by infaunal burrowers.