Experimental microfracture propagation in gneiss through frost wedging

Ice-driven mechanical weathering in mountainous environment is considered an efficient process for slow preconditioning of rockfalls. In this study (Musso Piantelli et al., 2020), we simulate with an innovative experimental approach subcritical fracture-propagation under frost-wedging conditions through pre-existing weaknesses of intact rock bridges. Two series of freeze-thaw experiments in an environmental chamber have been designed to investigate and monitor the propagation of artificially-induced fractures (AIF) in two twin gneiss samples. By employing 3D X-Ray Computed Tomography and a displacement sensor, an accurate characterization and new insights into the fracture-propagation mechanism are provided. Our results demonstrate that frost wedging propagated the AIFs of 1.25 cm2 and 3.5 cm2 after 42 and 87 freeze-thaw cycles, respectively. The experiments show that volumetric expansion of water upon freezing, cooperating with volumetric thermal expansion and contraction of the rock, plays a key role in fracture widening and propagation. Based on these results, this study proposes that: (i) frost wedging exploits intrinsic pre-existing weaknesses of the rock; (ii) the fracturing process is not continuous but alternates propagation stages to phases of tensile stress accumulation; and (iii) downward migration of “wedging grains”, stuck between the walls of the fracture, increases the tensile stress at the tip, widening and propagating the fractures with each freeze-thaw cycle. The experimental design developed in this study offers the chance to visualize fracture-propagation in natural joints quantifying the long-term efficiency of this process in near-natural scenarios.

REFERENCES

Musso Piantelli, F., Herwegh, M., Anselmetti, F.S., Waldvogel, M., Gruner, U., (2020). Microfracture propagation in gneiss through frost wedging: insights from an experimental study. Natural Hazards, 1-18. https://doi.org/10.1007/s11069-019-03846-3