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- Genre: Doctoral Dissertation
- Member of: Theses and Dissertations
Perpetual Pavements, if properly designed and rehabilitated, it can last longer than 50 years without major structural rehabilitation. Fatigue endurance limit is a key parameter for designing perpetual pavements to mitigate bottom-up fatigue cracking. The endurance limit has not been implemented in the Mechanistic Empirical Pavement Design Guide software, currently known as DARWin-ME. This study was conducted as part of the National Cooperative Highway Research Program (NCHRP) Project 9-44A to develop a framework and mathematical methodology to determine the fatigue endurance limit using the uniaxial fatigue test. In this procedure, the endurance limit is defined as the allowable tensile strains at which a balance takes place between the fatigue damage during loading, and the healing during the rest periods between loading pulses. The viscoelastic continuum damage model was used to isolate time dependent damage and healing in hot mix asphalt from that due to fatigue. This study also included the development of a uniaxial fatigue test method and the associated data acquisition computer programs to conduct the test with and without rest period. Five factors that affect the fatigue and healing behavior of asphalt mixtures were evaluated: asphalt content, air voids, temperature, rest period and tensile strain. Based on the test results, two Pseudo Stiffness Ratio (PSR) regression models were developed. In the first model, the PSR was a function of the five factors and the number of loading cycles. In the second model, air voids, asphalt content, and temperature were replaced by the initial stiffness of the mix. In both models, the endurance limit was defined when PSR is equal to 1.0 (net damage is equal to zero). The results of the first model were compared to the results of a stiffness ratio model developed based on a parallel study using beam fatigue test (part of the same NCHRP 9-44A). The endurance limit values determined from uniaxial and beam fatigue tests showed very good correlation. A methodology was described on how to incorporate the second PSR model into fatigue analysis and damage using the DARWin-ME software. This would provide an effective and efficient methodology to design perpetual flexible pavements.
In this study, first, the water retention curve (WRC) and relative permeability in hydrate bearing sediments are explored to obtain fitting parameters for semi-empirical equations. Second, immiscible fluid invasion into porous media is investigated to identify fluid displacement pattern and displacement efficiency that are affected by pore size distribution and connectivity. Finally, fluid flow through granular media is studied to obtain fluid-particle interaction. This study utilizes the combined techniques of discrete element method simulation, micro-focus X-ray computed tomography (CT), pore-network model simulation algorithms for gas invasion, gas expansion, and relative permeability calculation, transparent micromodels, and water retention curve measurement equipment modified for hydrate-bearing sediments. In addition, a photoelastic disk set-up is fabricated and the image processing technique to correlate the force chain to the applied contact forces is developed.
The results show that the gas entry pressure and the capillary pressure increase with increasing hydrate saturation. Fitting parameters are suggested for different hydrate saturation conditions and morphologies. And, a new model for immiscible fluid invasion and displacement is suggested in which the boundaries of displacement patterns depend on the pore size distribution and connectivity. Finally, the fluid-particle interaction study shows that the fluid flow increases the contact forces between photoelastic disks in parallel direction with the fluid flow.
Over the past decade, research has commenced on biologically-mediated solutions like microbially induced carbonate precipitation (MICP) and biologically-inspired solutions like EICP for non-disruptive ground improvement. Both of these approaches rely upon hydrolysis of urea catalyzed by the enzyme urease. Under the right environmental conditions (e.g., pH), the hydrolysis of urea leads to calcium carbonate precipitation in the presence of Ca^(2+). The rate of carbonate precipitation via hydrolysis of urea can be up to 〖10〗^14 times faster than natural process.
The objective of this research was to ascertain the effectiveness of EICP for soil improvement via hydrolysis of urea (ureolysis) catalyzed by plant-extracted urease enzyme. Elements of this work include: 1) systematic experiments to identify an optimum EICP treatment solution; 2) evaluation of the mechanical properties of EICP-treated soil under different treatment conditions and with varying carbonate contents; 3) investigation of the potential for enhancing the EICP stabilization process by including xanthan gum, natural sisal fiber, and powdered of dried non-fat milk in the EICP treatment solution; and 4) bench-scale studies of the use of EICP to make sub-horizontal columns of cemented soil for soil nailing and vertical columns of cemented soil for foundation support. As part of this research, the effect of three preparation methods (mix-and-compact, percolation, and injection) was also examined as was the influence of the grain size of soil. The results of this study should help make the EICP technique an attractive option for geotechnical engineers for ground improvement and stimulate the development and use of other biogeotechnical techniques for civil engineering purposes.