Browsing by Subject "Experimental evaluation"

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Analytical, experimental, and field evaluations of soil-geosynthetic interaction under small displacements
(2016-08) Roodi, Gholam Hossein; Zornberg, Jorge G.; Stokoe, Kenneth H; Gilbert, Robert B; Prozzi, Jorge A; Taleff, Eric M
The increasing use of geosynthetics in stabilization of pavement systems under traffic loads and environmental changes requires proper understanding of the mechanisms that govern the soil-geosynthetic interaction. Significant research has already been conducted on the soil-geosynthetic interaction under ultimate conditions, which is relevant to reinforcement of retaining walls and steep slopes. However, little research has been undertaken to investigate the properties and mechanisms that govern the soil-geosynthetic interaction under small displacements, which is relevant to applications such as the geosynthetic stabilization of pavement layers. While characterization of the maximum geosynthetic strength (e.g., tensile strength or pullout resistance) is relevant for the design of soil-geosynthetic systems under ultimate conditions, proper design properties in systems where geosynthetics are used to control deformations should involve characterization of the stiffness of soil-geosynthetic composite. The objective of this research is to develop a better understanding of the soil-geosynthetic interaction under small displacements using analytical, experimental, and field evaluations. Three studies were conducted on different aspects of soil-geosynthetic interaction under small displacements: (1) Analytical and experimental evaluations of the soil-geosynthetic composite (SGC) model using large-scale soil-geosynthetic interaction tests, (2) analytical and experimental evaluations of soil-geosynthetic interaction using small-scale soil-geosynthetic interaction tests, and (3) field evaluation of soil-geosynthetic interaction under small displacements. Each study provides lessons and conclusions on specific aspects investigated in that study. Collectively, they suggest that the analytical model proposed in this study provides a good basis towards predicting the general performance of geosynthetic-stabilized pavements. The analytical formulation of the SGC model indicates that soil-geosynthetic interaction under small displacements can be characterized by the stiffness of soil-geosynthetic composite ( ), which is the slope of the linear relationship defined between the unit tension squared (T2) versus displacements (u) in each point along the active length of a geosynthetic. The linearity and uniqueness of the relationship between the unit tension squared (T2) and displacements (u) throughout the active length of specimens tested in a comparatively large soil-geosynthetic interaction device were experimentally confirmed. Overall, the experimental results from the large-scale soil-geosynthetic interaction tests were found to be in good agreement with the adopted constitutive relationships and with the analytical predictions of the SGC model. Evaluation of experimental results from tests conducted to assess repeatability indicated that the variability of the estimated values for the constitutive parameters ( and ) and the stiffness of soil-geosynthetic composite ( ) are well within the acceptable ranges when compared to variations of other soil and geosynthetic properties. Suitability of the assumptions and outcomes of the model was also confirmed for a variety of testing conditions and materials. Evaluation of the experimental data obtained from a subsequent experimental program involving small-scale soil-geosynthetic interaction tests indicates that although the assumptions of the analytical model do not fully conform to the conditions in a small-scale test, experimental results confirm the linearity and uniqueness of the relationship between the unit tension squared (T2) and the displacements (u) throughout the specimen. Evaluation of the results obtained from small- and large-scale interaction tests on five geosynthetics with a range of properties indicates that both large and small testing scales can be used for comparative evaluation of the stiffness of soil-geosynthetic composite among geosynthetics. However, since the stiffness values obtained from the two testing scales were found to be different, the stiffness values from the large-scale soil-geosynthetic interaction tests should be suitable for design purposes, while values from the small-scale interaction tests should be suitable for specification and comparison purposes. Evaluation of the long-term performance of full-scale paved test sections under both traffic and environmental loads indicates that stabilization with geosynthetics contributes to improving the road performance under both loading conditions. The benefits derived from using geosynthetics under traffic loads were realized by reducing the total length of rut or rutting depth. On the other hands, the benefits from using geosynthetics under environmental loads in roads founded on expansive subgrades were realized by mitigating the percentage of longitudinal cracks appears on the road surface. The latter benefits were found to be more pronounced towards the end of dry seasons, when longitudinal cracks tend to develop. Comparison among the performances of geosynthetic-stabilized test sections under environmental loads indicate that the benefit provided by geosynthetics correlates well with the stiffness of soil-geosynthetic composite ( ) characterized in the laboratory. Geosynthetic products with comparatively larger were found to lead to a comparatively better field performance.
Joint communication and radar at the millimeter-wave band
(2020-08-17) Kumari, Preeti; Heath, Robert W., Jr., 1973-; Vikalo, Haris; Ghosh, Joydeep; Qiu, Lili; Vorobyov, Sergiy A.
Millimeter-wave (mmWave) communication and radar are key technologies for many demanding applications, such as autonomous driving and smart connected devices. The combination of these two technologies into a single joint communication-radar (JCR) enables hardware reuse and a common signaling waveform. This leads to significant benefits in power consumption, spectrum efficiency, and market penetrability. Prior work has proposed a mmWave wireless local area network-based JCR, which provides a good baseline for designing a future mmWave JCR standard. These JCR systems, however, incurred limited radar estimation performance. In this dissertation, we present advanced adaptive JCR waveform and beamforming designs to achieve superior radar performance, at the cost of a small reduction in the communication rate. These contributions are foundational for the development of futuristic standard-based mmWave JCR systems in automotive applications. The summary of our contributions is as follows. First, we propose a virtual waveform design for an adaptive mmWave JCR to enhance velocity estimation accuracy without reducing the communication data rate much. We formulate three different minimum mean square error-based optimization problems for the adaptive JCR waveform design. We solve the JCR optimization problems for a uniform waveform as well as for nested and Wichmann virtual waveforms. Numerical results demonstrate that optimal virtual waveforms achieve significant performance improvements over a uniform waveform, especially at low SNR and high target density. Second, this dissertation presents a low-complexity fully-digital multiple-input-multiple-output measurement platform, named JCR70, to evaluate and demonstrate the JCR performance in the 71-76 GHz band. Additionally, an experimental proof-of-concept is also performed for low-resolution analog-to-digital converters by emulating quantization effect on the collected data. The JCR70 platform demonstrated higher resolution capability than the Radarbook, which is a leading automotive radar evaluation platform at 77 GHz. Finally, this dissertation develops an adaptive combined waveform-beamforming design for automotive mmWave JCR that uses a phased-array architecture. To estimate the radar channel in the Doppler-angle domain with a wide field-of-view, all the transmit antennas are used to generate a narrow coherent beam for communication and distribute the remaining energy uniformly along other radar sensing directions. By applying partial Fourier compressed sensing technique, the framework is shown to estimate radar channels with high accuracy, at the cost of a small reduction in the communication rate.
Randomized controlled trials to evaluate impact : their challenges and policy implications for medicine, education, and international development
(2012-12) Kahlert, Rahel C.; Ward, Peter M., 1951-; Treisman, Uri; Galbraith, James; Osborne, Cynthia; Roberts, Bryan
Policy makers in education and international development have lately gravitated toward the randomized controlled trial (RCT)—an evaluation design that randomly assigns a sample of people or households into an intervention group and a control group in order to measure the differential effect of the intervention—as a means to determine program impact. As part of federal regulations, the U.S. Department of Education and the U.S. Agency for International development explicitly declared a preference for the RCT. When advocating for adopting the RCT model as the preferred evaluation tool, policy makers point to the success story of medical trials and how they revolutionized medicine from Medieval charlatanry to a modern life-saving discipline. By presenting a more nuanced account of the role of the RCT in medical history, however, this study finds that landmark RCTs were accompanied with challenges, Evidence-Based Medicine had rightful critics, and opportunistic biases in drug trials apply equally to education policy and international development. This study also examines the recent privileged role of the RCT in education and international development, concluding that its initial promise was not entirely born out when put into practice, as the national Reading First Initiative exemplifies. From a comparative perspective, the RCT movements also encountered major RCT critics, whose voices were not initially heard. These voices, however, seem to have contributed to a swing of the pendulum away from RCT primacy back towards greater methodological pluralism. A major conclusion of this study is that policy makers should exercise great caution when using RCTs as a policy evaluation tool. This conclusion is arrived at via considering RCT biases, challenges, and limited generalizability; understanding its interpretive-qualitative components; and broadening the overall methodological repertoire to better enable evaluations of macro-policy interventions.