THE ROLE OF SLABS AND WEAK LAYERS IN FRACTURE ARREST

ABSTRACT: Though recent work on fracture has provided us with new stability tests and improved our knowledge of avalanche release, our understanding of fracture arrest is still limited. We studied fracture arrest by making modifications to weak layers and slabs in a series of propagation saw tests (PSTs). We conducted more than 100 tests at a single study plot over eight weeks during which cracks along a surface hoar weak layer consistently propagated. Slab characteristics changed dramatically over time, with increasing slab depth, density, and hardness. High speed videos of more than 70 tests allow the analysis of fracture arrest dynamics. Our results show that removing the weak layer had no effect on propagation. Fracture arrest only occurred when we replaced a 30 cm long section of the weak layer with a non-collapsible structure. Modifying the slab by introducing slope normal cracks (either from the surface down or from the weak layer up) showed that sometimes small changes to soft (F hardness) parts of a slab were sufficient to arrest fractures through slab fracture, while other times only a strong thin portion of a thick slab was capable of communicating fracture farther down the beam. Our results suggest that the tensile strength of the upper layers of the slab is a key component for crack propagation. This work demonstrates the importance of the slab in slope stability, and it also suggests avenues for developing tests capable of assessing slab characteristics conducive to fracture propagation