Mineral Weathering and Bedrock Weakening: Modeling Microscale Bedrock Damage Under Biotite Weathering

Abstract

Bedrock weakening is of wide interest because it influences landscape evolution, chemical weathering, and subsurface hydrology. A longstanding hypothesis states that bedrock weakening is driven by chemical weathering of minerals like biotite, which expand as they weather and create stresses sufficient to fracture rock. We build on recent advances in rock damage mechanics to develop a model for the influence of multimineral chemical weathering on bedrock damage, which is defined as the reduction in bedrock stiffness. We use biotite chemical weathering as an example application of this model to explore how the abundance, aspect ratio, and orientation affect the time‐dependent evolution of bedrock damage during biotite chemical weathering. Our simulations suggest that biotite abundance and aspect ratio have a profound effect on the evolution of bedrock damage during biotite chemical weathering. These characteristics exert particularly strong influences on the timing of the onset of damage, which occurs earlier under higher biotite abundances and smaller biotite aspect ratios. Biotite orientation, by contrast, exerts a relatively weak influence on damage. Our simulations further show that damage development is strongly influenced by the boundary conditions, with damage initiating earlier under laterally confined boundaries than under unconfined boundaries. These simulations suggest that relatively minor differences in biotite populations can drive significant differences in the progression of rock weakening. This highlights the need for observations of biotite abundance, aspect ratio, and orientation at the mineral and field scales and motivates efforts to upscale this microscale model to investigate the evolution of the macroscale fracture network.

Micromechanical modeling for rate‐dependent behavior of salt rock under cyclic loading

“…By analogy with damage mechanics, we introduce two parameters C K and C G to account for the stiffness degradation of the crack inclusions as a consequence of sliding (similar to the evolution of microscopic cracks). C K and C G range from 0 to 1 48 . The bulk modulus K i and the shear modulus G i of the crack inclusions of family i are calculated as follows: where K m and G m are bulk and shear moduli of the matrix, respectively.…”

Micro-macro mechanics of damage and healing in rocks

“…In the reference case, the biotite volume fraction is 15%, the biotite aspect ratio is 3, the biotite orientation is 30 o and the boundary conditions are proportional loading with a vertical stress of 10 MPa and a horizontal stress equal to half of the vertical stress. Modified and reprinted from [Shen et al, 2019] with permission from Wiley. attractive because fabric is measurable by image analysis and provides a way to predict microstructure evolution through a macroscopic damage variable.…”

“…The dilation of biotite minerals is thought to be the cause of crack propagation in granite. Here, we summarize the Mori-Tanaka homogenization scheme proposed by the author's group in [Shen et al, 2019] to understand how weathering at the biotite inclusion scale can trigger damage at the REV scale. Following previous studies [Buss et al, 2008, Goodfellow et al, 2016, we focus on the mechanism that induces the largest expansion: the replacement of interlayer of potassium cations by hydrated magnesium cations, which transforms biotite to vermiculite and smectite.…”

“…Figure 20 shows the predicted time evolution of damage in the matrix in various scenarios. A more extensive set of parametric studies is presented in [Shen et al, 2019].…”

Characterization of glacial silt and clay using automated mineralogy

“…Despite similarities in the distributions, the sedimentary, metamorphic and igneous processes that give rise to the mineral composition, shape, size, extent of cementing and microfracture spacing within rocks impart clear differences on sediment and rock fragments of both lithologies (e.g. Bandini and others, 2014; Shen and others, 2019). For example, plutonic quartz grains with fresh microfractures (e.g.…”