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ABSTRACT: This paper presents a micro-mechanical model of crack propagation in rock subject to mineral weathering. We focus on the example of biotite, which expands as it weathers and generates stresses on the surrounding rock matrix. Our approach is to model a rock Representative Elementary Volume as a composite made of a homogeneous matrix that contains ellipsoidal cracks of various orientations and ellipsoidal mineral inclusions that represent biotites. The mechanical properties of the bulk rock are obtained by two-step Mori-Tanaka homogenization. This model captures the propagation of anisotropic damage as a result of biotite weathering under various stress conditions, showing that weathering eigenstress dominates the propagation of micro-cracks over the deformation energy of the bedrock. Finite element simulations further show that biotite weathering changes the distribution and intensity of stress in the rock mass beyond the depth of the weathered zone. Simulations conducted for a sinusoidal topography with a sinusoidal weathering gradient out-of-phase with the topography show that the gradient of damage always follows the weathering gradient, regardless of biotite orientations. Simulations also show that the intensity of damage in each direction of space depends on the orientation of the biotite minerals, with higher damage for vertical bioites. 1 INTRODUCTION The expansion of biotite in granitic rock and regolith due to iron oxidation by meteoric waters is thought to generate enough stress to cause a breakdown in rock strength and initiate chemical weathering (Fletcher et al., 2006; Buss et al., 2008; Goodfellow et al., 2016). Goodfellow et al. (2016) showed evidence for distributed microcracking of the rock matrix surrounding biotite minerals upon oxidation and a coincident reduction in rock strength. How landscape-scale patterns of immobile regolith properties emerge from this microcracking is unknown. Since biotite only expands along the basal cleavage plane (Dong et al., 1998), rock with a dominant biotite orientation could exhibit different behavior and lead to immobile regolith with anisotropic properties (Leone et al., 2020). |
