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Ice-driven mechanical weathering in mountainous environment is considered as an efficient process for slow but cyclical mechanical preconditioning of rockfall events. In this study, we simulate subcritical microfracture propagation under frost wedging conditions along pre-existing mechanical weaknesses of intact rock bridges with an innovative experimental approach. Two series of freeze-thaw experiments conducted in an environmental chamber were carried out to investigate and monitor the propagation of artificially induced fractures (AIF) in two twin gneiss samples. A displacement sensor recorded the sample's in situ deformation in an environmental chamber during the experiments. 3D X-ray CT scans, performed before and after the experiments, as well as thin sections showing the post-experiment state of the deformed samples allowed tracking and quantification of fracture propagation. Our results demonstrate that frost wedging propagated the AIFs 1.25 cm 2 and 3.5 cm 2 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 anisotropic rock, plays a key role in fracture widening and propagation. Based on these results, this study proposes that: (1) frost wedging exploits intrinsic pre-existing mechanical anisotropies of the rock; (2) the fracturing process is not continuous but alternates between stages of fast propagation and more quiet stages of stress accumulation; and (3) 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 and quantify the long-term efficiency of frost wedging in near-natural scenarios.

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