<p> Fractures are often leakage pathways for fluids through low-permeability rocks that otherwise act as geologic barriers to flow. Flow of fluids that are in chemical disequilibrium with the host rock can lead to mineral precipitation, which reduces fracture permeability. When fracture surfaces contain a single mineral phase, mineral precipitation leads to fast permeability reduction and fracture sealing. However, the feedback between precipitation and permeability may be disrupted by mineral heterogeneities that localize precipitation reactions and provide paths of low-reactivity for fluids to persist over relatively long time-scales. In this dissertation, I explore the role of mineral heterogeneity on precipitation-induced permeability reduction in fractures. To do this, I use a combined experimental and numerical approach to test three hypotheses: (1) Mineral heterogeneity prolongs fracture sealing by focusing flow into paths with limited reactive surface area, (2) Precipitation-induced transport alterations at the fracture-scale are controlled by three-dimensional growth dynamics at the grain-scale, and (3) The effects of mineral heterogeneity become more pronounced as mineralogy and surface roughness become autocorrelated over similar length-scales. </p><p> Direct measurements of mineral precipitation using transmitted light methods in a transparent analog fracture show that mineral heterogeneity can lead to the progressive focusing of flow into paths with limited reactive surface area, which is in support of (1). In this experiment, flow focusing led to a 72% reduction in the max precipitation rate; measurements of the projected mineralogy show that this was due to focusing of large dissolved ion concentrations into regions that contained 82% less reactive surface area than the fracture-scale average. Results from a newly developed reactive transport model that simulates precipitation-induced fracture surface alterations as a three-dimensional process are in good qualitative agreement with these experimental observations. Comparison of these results with a reactive transport model that represents precipitation as a 1D alteration of the fracture surfaces show that this flow-focusing process is driven by lateral growth of reactive minerals across the fracture-plane, which supports (2). Lastly, results from simulations in fractures that contain varied degrees of heterogeneity show that precipitation leads to a competition between two feedbacks: (i) precipitation-induced reactive surface area enhancement, which increases precipitation rates, and (ii) precipitation-induced permeability reduction, which decreases precipitation rates. When surface roughness and mineral heterogeneity provide persistent paths of limited surface area, the reactive transport becomes very sensitive to local permeability reduction. Simulation results show that this prolongs the fracture-sealing process and can lead to a reduction in fracture-scale precipitation rate, which supports (3). Furthermore, the results presented in this dissertation demonstrate that predictions of fracture sealing by mineral precipitation can be easily misinformed by studies that ignore small-scale mineral heterogeneity and neglect the three-dimensional nature of precipitation-induced fracture surface alterations.</p><p>

<p> The characterization of silty soils is usually designated by the percentage of silt contained within the soil matrix, along with the soil&rsquo;s void ratio, which is used to describe the soil&rsquo;s current state. The use of these parameters to assess a soil&rsquo;s strength and undrained behavior is limited when finer material is contained within the soil. Therefore, additional parameters must be considered in order to correctly assess the strength and liquefaction potential of silty soils. These additional parameters include the skeleton void ratio, equivalent void ratio and granulometric factors. The current research investigates the influence of granulometric parameters, specifically the Median Grain Size Ratio (<i>D</i>₅₀/<i> d</i>₅₀), denoted as &mu;_DR (or MDR within graphs and charts), on the strength and liquefaction potential of loose silty sands. A series of undrained monotonic triaxial compression tests (&sigma;3&rsquo;= 69, 83, and 103 kPa) are performed on reconstituted soil samples, using three different base sand samples and a constant silt material. As a result, three distinct median grain size ratios (&mu;_DR = 4.2, 6.75, and 9) were tested with fines content ranging from 0-30% for each &mu;_DR. The undrained shear strength at all confining pressures tends to increase with in &mu;_DR; beyond 10% fines content there was no noticeable influence of &mu;_DR. At any &mu;_DR the excess PWP is higher than that of clean sand, when fines content is larger than 5% fines content. The slope of the instability line and phase transformation line are directly affected by the &mu;_DR and fines content, with an increase in the instability line and decrease in the phase transformation line with a growing &mu;_DR. The results indicate loose granular fills can be designed to be stronger and more resilient under extreme conditions by careful choice of materials in which the &mu;_DR>6.75 and the fines content does not exceed 10%.</p>

<p>The Sandia GeoModel is a continuum elastoplastic constitutive model which captures many features of the mechanical response for geological materials over a wide range of porosities and strain rates. Among the specific features incorporated into the formulation are a smooth compression cap, isotropic/kinematic hardening, nonlinear pressure dependence, strength differential effect, and rate sensitivity. This study attempts to provide enhancements regarding computational tractability, domain of applicability, and robustness of the model. A new functional form is presented for the yield and plastic potential functions. The model is also furnished with a smooth, elliptical tension cap to account for the tensile failure. This reformulation renders a more accurate, robust and efficient model as it eliminates spurious solutions attributed to the original form. In addition, this constitutive model is adopted in bifurcation analysis to track the inception of new localization and crack path propagation. For the post-localization regime, a cohesive-law fracture model, able to address mixed-model failure condition, is implemented to characterize the constitutive softening behavior on the surface of discontinuity. To capture propagating fracture, the Assumed Enhanced Strain (AES) method is invoked. Particular mathematical treatments are incorporated into the simulation concerning numerical efficiency and robustness issues. Finally, the aforementioned modified cap plasticity model is employed to investigate the nonlinear dynamic response of the earthen substructure of the rail. Studying the effects of high-speed trains on the track substructure.