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Abstract Seismic and magnetotelluric studies suggest hydrous silicate melts atop the 410 km discontinuity form 30–100 km thick layers. Importantly, in some regions, two layers are observed. These stagnant layers are related to their comparable density to the surrounding mantle, but their formation mechanisms and detailed structures remain unclear. Here we report a large decrease of silicate melt viscosity at ~14 GPa, from 96(5) to 11.7(6) mPa⋅s, as water content increases from 15.5 to 31.8 mol% H₂O. Such low viscosities facilitate rapid segregation of melt, which would typically prevent thick layer accumulation. Our 1D finite element simulations show that continuous dehydration melting of upwelling mantle material produces a primary melt layer above 410 km and a secondary layer at the depth of equal mantle-melt densities. These layers can merge into a single thick layer under low density contrasts or high upwelling rates, explaining both melt doublets and thick single layers. 

Single-objective optimization algorithms search for the single highest quality solution with respect to an objective. Quality diversity (QD) optimization algorithms, such as Covariance Matrix Adaptation MAP-Elites (CMA-ME), search for a collection of solutions that are both high quality with respect to an objective and diverse with respect to specified measure functions. However, CMA-ME suffers from three major limitations highlighted by the QD community: prematurely abandoning the objective in favor of exploration, struggling to explore flat objectives, and having poor performance for low-resolution archives. We propose a new QD algorithm, CMA MAP-Annealing (CMA-MAE), and its differentiable QD variant, CMA-MAE via a Gradient Arborescence (CMA-MAEGA), that address all three limitations. We provide theoretical justifications for the new algorithm with respect to each limitation. Our theory informs our experiments, which support the theory and show that CMA-MAE achieves state-of-the-art performance and robustness on standard QD benchmark and reinforcement learning domains. 

Geosynthetic clay liners (GCLs) used in waste and chemical containment applications exhibit high swell and low hydraulic conductivity to water (e.g., k < 10−10 m/s), such that diffusion dominates contaminant transport occurring through the barrier. Thus, knowledge of expected diffusion coefficients for GCLs under relevant environmental conditions is required for performance-based design and accurate assessment of the barrier system. Unfortunately, diffusion testing for bentonites can be challenging and time consuming, limiting data availability for GCL diffusion coefficients. The dialysis leaching test (DLT) method has been utilized in recent studies for simple, time-efficient diffusion measurements for bulk sodium bentonites (NaB), enhanced bentonites, and NaB pastes. This study used a new, modified version of the DLT method for measuring diffusion in GCLs comprising NaB. Diffusion tests were performed using dilute (20 mM) and aggressive (100 mM) calcium chloride (CaCl2) solutions to measure apparent diffusion coefficients (Da) for chloride for the GCLs. Values of Da were in the range of 10−10 m2/s, consistent with expectations from the literature for longer-term traditional testing. Diffusion coefficients increased as CaCl2 concentration increased, as expected due to cation exchange and suppression of the diffuse double layer in the bentonite in the GCL. The results of the study demonstrate the potential use of a new, time-efficient test method for assessing diffusion properties of GCLs, further improving our ability to predict contaminant transport through barrier systems. 

Abstract Measuring the diffusion coefficient of clay-based liner materials is important in estimating and predicting long-term barrier performance in waste containment facilities. Various theoretical models, including the finite cylindrical model, have been commonly used to determine the diffusion properties of clay-based liner materials in leaching tests. However, the assumption of zero-concentration boundary conditions of the traditional finite cylindrical model contradicts the measured variation of concentration in real leaching tests, likely resulting in (1) underestimated and unconservative diffusion coefficient, or (2) requirement of a relatively large liquid-to-soil ratio and frequent leachate replacement in the experiment to maintain the zero-concentration boundary condition. In this study, a theoretical model was developed to evaluate the solute diffusion process within a soil specimen under arbitrary, time-dependent concentration boundary conditions. The proposed model, incorporating the time-dependent boundary conditions, provides efficient calculations of the concentration distribution and the cumulative fraction leached of solute across the soil specimen. The example application of the proposed model to experimental data demonstrates the capability of the proposed model to determine apparent diffusion coefficients of clay-based liner materials without introducing errors associated with the assumption of a zero concentration boundary condition. The proposed model provides a comprehensive method to investigate the dynamic transport behaviors of solutes through clay-based liner materials in future studies. 

This work-in-progress research paper describes the development and pilot administration of a survey to assess students’ perceptions about sociotechnical issues in engineering. After refining the survey through iterative rounds of review, we piloted it in an “Introduction to Circuits” course at a large, public university in the Midwestern USA in which we deployed a short module addressing technical and social issues. In this paper we document our instrument development process and present descriptive statistics and results of paired t-tests used to analyze the pilot data. We also describe ways our instrument can be implemented by instructors and researchers in multiple contexts.