LABORATORY DEVELOPMENT AND MOLECULAR-SCALE SIMULATION OF SENSOR-ENABLED GEOGRIDS
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Structural health monitoring (SHM) and performance assessment are increasingly integrated to modern civil engineering projects in order to prevent and mitigate their catastrophic premature failures. Significant advancements in sensor and communication technologies during the last decades have boosted research on SHM and revolutionized its traditional and low-tech techniques. Inherent variability and uncertainties in soils arising from different sources (e.g. data insufficiency) pose significant challenges to the design of geosystems (e.g. geosynthetically-modified structures), but the increasing trend in using SHM and performance evaluation techniques could offer substantial help in counterbalancing the design uncertainties and to identify the impending failure of high-risk geosystems. Strain gauges, optical fibers and extensometers are current technologies to measure strains in geosynthetics where the sensing is achieved by attaching these devices to a geosynthetic layer in desirable positions. However, these devices require complex and expensive data acquisition systems to collect information. Also, strain gauges attached to a reinforcement material need to be calibrated against global strains from crosshead displacements in in-isolation tensile tests. However, the resulting calibration factors are typically not accurate when the reinforcement layer is embedded in soil due to the local stiffening effect of the bonding assembly, difference in the in-soil mechanical properties and other complications such as soil arching. During the last few years, a novel technique has been under development at the University of Oklahoma based on the strain sensitivity of polymer nanocomposites to measure the tensile strain in modified geosynthetics without the need for conventional instrumentation. In this technique, electrically-conductive fillers are used to induce conductivity in geosynthetics in order to produce sensor-enabled geosynthetics (SEG). The electrical conductivity of a SEG product with a prescribed concentration of a conductive filler is highly sensitive to the applied strain, affording the product the self-sensing function. As part of this long-term study to develop SEG materials, an interdisciplinary study was carried out as described in this dissertation to develop sensor-enabled geogrids (SEGG) through laboratory experiments and molecular-scale simulations. The study yielded several formulations and production processes in the laboratory to fabricate nanocomposites that would exhibit adequate mechanical and strain-sensitive electrical properties for SEGG applications. Molecular dynamics and Monte Carlo simulations were used to gain insight into the laboratory results on a more fundamental level. The molecular dynamics simulations were carried out to study the mechanical properties of the composites whereas Monte Carlo simulations were used to examine their electrical conductivity (i.e. percolation) behavior. Results showed that, contingent upon further development and addressing practical issues such as durability and protective measures for field installation, the SEGG technology holds promise to offer a practical and cost-effective alternative to the existing technologies for performance-monitoring of a wide range of geotechnical structures during and after construction.
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