Current design guidelines for reinforced soil walls and slopes recommend the use of granular soils such as gravels and sands as select fills. Cost savings could potentially be realized by using on-site clays and silts. Some clays are swelling and silts are frost susceptible. When considering the use of swelling clays and frost susceptible silts as fills, environmental loading due to swelling-shrinkage and freeze-thaw effects from environmental changes could become a design issue. To examine the hypothesis that consideration of environmental loading during design will produce improvements in the performance of geosynthetic-reinforced soil structures that use clays or silts as fill materials, experimental and numerical studies were undertaken. Geosynthetic-reinforced clay specimens were subjected to wetting and drying in a model test apparatus developed and commissioned for this study. In separate test set-up, reinforced silt specimens were subjected to freezing and thawing. Tests on unreinforced specimens were also performed in otherwise identical conditions for comparison purposes. Movements of the specimens, soil strains, reinforcement strains, soil suctions and soil temperatures were monitored during the application of environmental loading in addition to mechanical loading from external stresses. The results of the laboratory model tests showed that reinforcements reduced horizontal displacements of the clay specimens during wetting and drying. The same is true for the case of silt during freezing and thawing. The environmental loading induced strains, and therefore stresses in the reinforcements. The measured geogrid strain during the wetting-drying of reinforced clay specimen was up to 0.75%. Similarly, the measured geogrid strain in the reinforced silt specimen during freezing-thawing cycles was up to 0.57%. The strains were greater than the strains generated by mechanical loading for the range of applied stresses used in this study. Numerical models were developed to simulate wetting only induced swelling of reinforced clays and freezing only induced expansion of reinforced silts specimens. They were used to simulate the results of laboratory model tests. The performance of geosynthetic-reinforced soil slopes with swelling clay fills and frost susceptible silt fills was evaluated. Parametric studies were performed to determine important parameters affecting the performance of reinforced clay and silt slopes.

Current design guidelines for reinforced soil walls and slopes recommend the use of granular soils such as gravels and sands as select fills. Cost savings could potentially be realized by using on-site clays and silts. Some clays are swelling and silts are frost susceptible. When considering the use of swelling clays and frost susceptible silts as fills, environmental loading due to swelling-shrinkage and freeze-thaw effects from environmental changes could become a design issue. To examine the hypothesis that consideration of environmental loading during design will produce improvements in the performance of geosynthetic-reinforced soil structures that use clays or silts as fill materials, experimental and numerical studies were undertaken. Geosynthetic-reinforced clay specimens were subjected to wetting and drying in a model test apparatus developed and commissioned for this study. In separate test set-up, reinforced silt specimens were subjected to freezing and thawing. Tests on unreinforced specimens were also performed in otherwise identical conditions for comparison purposes. Movements of the specimens, soil strains, reinforcement strains, soil suctions and soil temperatures were monitored during the application of environmental loading in addition to mechanical loading from external stresses. The results of the laboratory model tests showed that reinforcements reduced horizontal displacements of the clay specimens during wetting and drying. The same is true for the case of silt during freezing and thawing. The environmental loading induced strains, and therefore stresses in the reinforcements. The measured geogrid strain during the wetting-drying of reinforced clay specimen was up to 0.75%. Similarly, the measured geogrid strain in the reinforced silt specimen during freezing-thawing cycles was up to 0.57%. The strains were greater than the strains generated by mechanical loading for the range of applied stresses used in this study. Numerical models were developed to simulate wetting only induced swelling of reinforced clays and freezing only induced expansion of reinforced silts specimens. They were used to simulate the results of laboratory model tests. The performance of geosynthetic-reinforced soil slopes with swelling clay fills and frost susceptible silt fills was evaluated. Parametric studies were performed to determine important parameters affecting the performance of reinforced clay and silt slopes.

Soft soil stabilization frequently uses cement, lime, fly ash, etc., but very limited studies were conducted on the long-term durability of stabilized soil. The present research work deals with the long-term durability of commercially available soil (i.e., EPK clay) stabilized with ordinary Portland cement and polypropylene fiber using a realistic approach, where the effect can be noticed in each weathering cycle. In the present study, two different tests (i.e., wetting-drying and freezing-thawing) were conducted to analyze the long-term durability of stabilized soil. Cycles of higher temperature followed by rainfall, which generally occurs in southern states of the US, were analyzed by the wetting-drying test; and on the other hand, cycles of freezing temperature followed by normal temperature, which generally occurs in northern states of the US and Canada, were analyzed by the freezing-thawing test. For the mid-continental region where freezing, normal, and higher temperature followed by rainfall are expected to occur, hence both the test method i.e., wetting-drying and freezing-thawing, were suggested. Laboratory experimental investigations were conducted to find the percentage loss of stabilized soil during wetting-drying and freezing-thawing tests, which were used as a durability indicator for cement and cement-fiber stabilized soil. Stabilized samples were subjected to harsh environmental conditions in a laboratory set up, and their deterioration was observed and studied after each wetting-drying and freezing-thawing cycle. In the real world, stabilized soil encounters seasonal cycles of monsoon and summer in long run of its service life which was simulated in rapid weathering cycles in laboratory setup. EPK clay samples were stabilized with different percentages of cement, and a mix of cement-fiber combination and were subjected to 12 cycles of wetting-drying and freezing-thawing cycles separately to determine the percentage loss of soil in accordance with the ASTM standards. Finally, based on percentage loss of soil of those stabilized samples which survived up to 12 cycles of weathering action, the optimum content of stabilizing agent was determined for wetting-drying and freezing-thawing tests. Results of wetting-drying tests indicate that EPK clay stabilized with ordinary Portland cement and fiber combination survived up to 12 cycles, but only 10% cement + 0.5% fiber was durable against wetting-drying based on percentage loss. For all the samples stabilized with 10% cement + 0.5% fiber combination, the percentage loss of soil when subjected to durability test was less than 7%, which satisfy the Portland Cement Association’s (PCAs) durability specification. The results of freezing-thawing tests indicate that the EPK clay stabilized with 10% cement, 5% cement + 0.5% fiber, and 10% cement + 0.5% fiber survived up to 12 cycles and were durable against freezing-thawing based on percentage loss of soil i.e., less than 7% which satisfy the Portland Cement Association’s durability specification.

abstract: In a laboratory setting, the soil volume change behavior is best represented by using various testing standards on undisturbed or remolded samples. Whenever possible, it is most precise to use undisturbed samples to assess the volume change behavior but in the absence of undisturbed specimens, remodeled samples can be used. If that is the case, the soil is compacted to in-situ density and water content (or matric suction), which should best represent the expansive profile in question. It is standard practice to subject the specimen to a wetting process at a particular net normal stress. Even though currently accepted laboratory testing standard procedures provide insight on how the profile conditions changes with time, these procedures do not assess the long term effects on the soil due to climatic changes. In this experimental study, an assessment and quantification of the effect of multiple wetting/drying cycles on the volume change behavior of two different naturally occurring soils was performed. The changes in wetting and drying cycles were extreme when comparing the swings in matric suction. During the drying cycle, the expansive soil was subjected to extreme conditions, which decreased the moisture content less than the shrinkage limit. Nevertheless, both soils were remolded at five different compacted conditions and loaded to five different net normal stresses. Each sample was subjected to six wetting and drying cycles. During the assessment, it was evident from the results that the swell/collapse strain is highly non-linear at low stress levels. The strain-net normal stress relationship cannot be defined by one single function without transforming the data. Therefore, the dataset needs to be fitted to a bi-modal logarithmic function or to a logarithmic transformation of net normal stress in order to use a third order polynomial fit. It was also determined that the moisture content changes with time are best fit by non-linear functions. For the drying cycle, the radial strain was determined to have a constant rate of change with respect to the axial strain. However, for the wetting cycle, there was not enough radial strain data to develop correlations and therefore, an assumption was made based on 55 different test measurements/observations, for the wetting cycles. In general, it was observed that after each subsequent cycle, higher swelling was exhibited for lower net normal stress values; while higher collapse potential was observed for higher net normal stress values, once the net normal stress was less than/greater than a threshold net normal stress value. Furthermore, the swelling pressure underwent a reduction in all cases. Particularly, the Anthem soil exhibited a reduction in swelling pressure by at least 20 percent after the first wetting/drying cycle; while Colorado soil exhibited a reduction of 50 percent. After about the fourth cycle, the swelling pressure seemed to stabilized to an equilibrium value at which a reduction of 46 percent was observed for the Anthem soil and 68 percent reduction for the Colorado soil. The impact of the initial compacted conditions on heave characteristics was studied. Results indicated that materials compacted at higher densities exhibited greater swell potential. When comparing specimens compacted at the same density but at different moisture content (matric suction), it was observed that specimens compacted at higher suction would exhibit higher swelling potential, when subjected to the same net normal stress. The least amount of swelling strain was observed on specimens compacted at the lowest dry density and the lowest matric suction (higher water content). The results from the laboratory testing were used to develop ultimate heave profiles for both soils. This analysis showed that even though the swell pressure for each soil decreased with cycles, the amount of heave would increase or decrease depending upon the initial compaction condition. When the specimen was compacted at 110% of optimum moisture content and 90% of maximum dry density, it resulted in an ultimate heave reduction of 92 percent for Anthem and 685 percent for Colorado soil. On the other hand, when the soils were compacted at 90% optimum moisture content and 100% of the maximum dry density, Anthem specimens heave 78% more and Colorado specimens heave was reduced by 69%. Based on the results obtained, it is evident that the current methods to estimate heave and swelling pressure do not consider the effect of wetting/drying cycles; and seem to fail capturing the free swell potential of the soil. Recommendations for improvement current methods of practice are provided. / Dissertation/Thesis / Ph.D. Civil and Environmental Engineering 2013

Le sol est un compartiment majeur de stockage du carbone (C) organique de l’écosystème terrestre. Il joue un rôle important dans la régulation du climat. Toute variation des flux de carbone entre l’atmosphère et l’écosystème terrestre pourrait avoir un impact important sur l’augmentation de CO2 dans l’atmosphère, mais aussi sur la diminution des teneurs en matière organique du sol et donc sur la fertilité des sols. Au Nord Cameroun, les sols sont exposés à de longues périodes sèches (5 à 6 mois par an) qui alternent avec une saison humide. La période de transition entre ces deux saisons, peut durer de mi-avril à fin juin et est caractérisée par des pluies très irrégulières. Ces cycles d’humectation-dessiccation pourraient selon la littérature accentuer la minéralisation du carbone organique du sol et le cycle des éléments nutritifs. L’objectif de cette étude est de quantifier l’impact des cycles humectation-dessiccation sur la minéralisation du carbone dans un contexte soudano-sahélien. Pour faire des mesures représentatives sur le terrain, il est nécessaire d’étudier la variation sur 24 heures de la respiration du sol après humectation suite à une période sèche. Cette mise au point méthodologique a montré que la respiration du sol présente une courbe quadratique au cours de la journée, devenant presque linéaire au cours de la nuit. La température et l’humidité du sol ont permis d’expliquer au moins 73% des variations sur 24 heures. Ces observations ont été utilisées pour proposer une méthode pour estimer la respiration moyenne diurne et nocturne après humectation des sols. La méthode proposée dans cette étude a l’avantage d’être basée sur un nombre réduit de mesures et est par conséquent plus facile à mettre en œuvre pour suivre la respiration du sol sur 24 heures après les premières pluies. Une première étude expérimentale de terrain a permis de montrer que la ré-humectation des sols et le mode de gestion des pailles ont augmenté la minéralisation du carbone de ces sols. En revanche, la fréquence des cycles humectation-dessiccation des sols sur une période de 50 jours n’a pas augmenté la minéralisation cumulée du carbone des sols. Au Nord Cameroun, la minéralisation rapide des pailles rend difficile l’augmentation des stocks de carbone du sol par conservation des pailles des cultures précédentes à la surface du sol. Dans une seconde expérimentation de laboratoire, en conditions contrôlées, les cycles humectation-dessiccation n’ont pas augmenté la minéralisation du carbone organique du sol et de l’azote (N) par rapport aux sols maintenus humides. Cependant, les émissions de CO2 ont augmenté avec l’addition de paille enrichie en carbone-13. Cette addition de la paille marquée a augmenté la minéralisation de la matière organique du sol (priming effect). La minéralisation de la paille a diminué avec les cycles humectation-dessiccation et la quantité de paille restante était de 102 µg Cg-1 sol sur les sols ré-humectés contre 48 µg Cg-1 sol sur les sols maintenus humides. L’absence de cette réponse de la minéralisation du carbone et d’azote du sol aux cycles humectation-dessiccation pourrait être liée à une baisse de l’activité microbienne durant les périodes de dessèchement et l’absence d’une augmentation soutenue des taux de minéralisation du carbone avec les cycles ultérieurs d’humectation-dessiccation. / Soil as a major storage component for terrestrial ecosystem’s organic carbon plays an important role in regulating climate and agricultural production. Any variation of carbon fluxes between the atmosphere and the terrestrial ecosystem can have a significant impact on the increase of carbon dioxide in the atmosphere but also the decrease in soil organic matter and thus accelarate soil fertility degradation. In northern Cameroon, the transition period between long dry periods with a wet season is characterized by very irregular rainfall that can last several weeks. These wetting-drying cycles can accentuate the mineralization of soil organic carbon and nutrient cycling. The objective of this study is to assess the impact of wet-dry cycles on carbon mineralization in a sudano-sahelian context. From methodological stand field measurements require to study the soil respiration variation over 24 hours after a wet period. This methodological test has shown that soil respiration has a quadratic curve during the day, becoming almost linear during the night. The temperature and soil moisture have explained together the variation over 24 hours (at least 73% ; p< 0.001). These observations have been used to propose a method for estimating the mean daytime and nighttime soil respiration after wetting the soil. Indeed the method proposed in this study has the advantage of being based on a small number of measurements and is, therefore, easier to implement to monitor 24-h soil respiration after the first rains following a long dry period. A first experiment has shown that the wetting of the soil and mulching increased soil carbon mineralization. However, wetting-drying cycles on soil did not increase the cumulative mineralization of soil carbon more than keeping the soil continuously moist. Indeed, in northern Cameroon, the rapid mineralization of crop residues makes it difficult to increase soil carbon stocks by mulching. In a second laboratory experiment, the wetting-drying cycles did not increase organic carbon and nitrogen mineralization from soils added with straw. However, carbon dioxide emissions increased on straw amended soils compared to soils without straw. This addition of the labeled straw increased mineralization of soil organic matter (priming effect). The mineralization of the straw also decreased with the wetting-drying cycles, thus the amount of straw remaining on soils was 102 µg C g-1 soil on re-wetted soils compared to 48 µg C g-1 soil for those with constant moisture. The lack of response for C and N mineralization during wetting-drying cycles may be linked to a decrease of microbial activity during dry periods and the lack of a steady increase in the carbon mineralization rate with subsequent wetting-drying cycles.

En la presente tesis se analizan los aspectos teóricos y prácticos necesarios para el desarrollo de un modelo numérico tridimensional válido para los estuarios típicos del Norte de España, cuyas características principales son la existencia de importantes gradientes de profundidad, , y de zonas que se inundan y se secan periódicamente debido al efecto de la marea astronómica.De la revisión del estado del conocimiento sobre modelado hidrodinámico tridimensional en zonas someras, se ha concluido que aunque existen varios modelos tridimensionales propuestos para estuarios, ninguno está preparado para su aplicación en los casos de estudio de esta tesis. La anterior aseveración está principalmente relacionada con tres temas fundamentales: sistema de coordenadas, representación de la turbulencia y simulación de la inundación - secado del dominio de cálculo. Se ha investigado con detalle estos aspectos y como resultado se ha desarrollado un modelo que incluye el resultado de la investigación realizada y que ha sido validado tanto con soluciones analíticas, como con datos de laboratorio y de campo.

This Ph.D. thesis aims to develop numerical models for two-dimensional and three-dimensional shallow water systems over mobile beds. In order to accomplish the goal of this dissertation, the following sub-projects are defined and completed. 1: The first sub-project consists in developing a robust two-dimensional coupled numerical model based on an unstructured mesh, which can simulate rapidly varying flows over an erodible bed involving wet–dry fronts that is a complex yet practically important problem. In this task, the central-upwind scheme is extended to simulation of bed erosion and sediment transport, a modified shallow water system is adopted to improve the model, a wetting and drying scheme is proposed for tracking wet-dry interfaces and stably predict the bed erosion near wet-dry area. The shallow water, sediment transport and bed evolution equations are coupled in the governing system. The proposed model can efficiently track wetting and drying interfaces while preserving stability in simulating the bed erosion near the wet-dry fronts. The additional terms in shallow water equations can improve the accuracy of the simulation when intense sediment-exchange exists; the central-upwind method adopted in the current study shows great accuracy and efficiency compared with other popular solvers; the developed model is robust, efficient and accurate in dealing with various challenging cases. 2: The second sub-project consists in developing a novel numerical scheme for a coupled two-dimensional hyperbolic system consisting of the shallow water equations with friction terms coupled with the equations modeling the sediment transport and bed evolution. The resulting 5*5 hyperbolic system of balance laws is numerically solved using a Godunov-type central-upwind scheme on a triangular grid. A spatially second-order and temporally third-order central-upwind scheme has been derived to discretize the conservative hyperbolic sub-system. However, such schemes need a correct evaluation of local wave speeds to avoid instabilities. To address such an issue, a mathematical result by the Lagrange theorem is used in the proposed scheme. Consequently, a computationally expensive process of finding all of the eigenvalues of the Jacobian matrices is avoided: The upper/lower bounds on the largest/smallest local speeds of propagation are estimated using the Lagrange theorem. In addition, a special discretization of the bed-slope term is proposed to guarantee the well-balanced property of the designed scheme. 3: The third sub-project consists in designing a novel scheme to estimate bed-load fluxes which can produce more accurate results than the previously reported coupled model. Using a pair of local wave speeds different from those used for the flow, a novel wave estimator in conjunction with the central upwind method is proposed and successfully applied to the coupled water-sediment system involving a rapid bed-erosion process. It was demonstrated that, in comparison with the decoupled model, applying the proposed novel scheme to approximate the bed-load fluxes can successfully avoid the numerical oscillations caused by simple and less stable schemes, e.g. simple upwind methods; in comparison with the coupled model using same flux-estimator for both hydrodynamic and morphological systems, the proposed numerical scheme successfully prevents excessive numerical diffusion for prediction of bed evolution. Consequently, the proposed scheme has advantages in terms of accuracy which are shown in several numerical tests. In addition, analytical expressions have been provided for calculating the eigenvalues of the coupled shallow-water-Exner system, which greatly enhances the efficiency of the proposed method. 4: The fourth sub-project consists in developing a three-dimensional numerical model for the simulation of unsteady non-hydrostatic shallow water flows on unstructured grids using the finite volume method. The free surface variations are modeled by a characteristics-based scheme which simulates sub- and super-critical flows. Three-dimensional velocity components are considered in a collocated arrangement with a sigma coordinate system. A special treatment of the pressure term is developed to avoid the water surface oscillations. Convective and diffusive terms are approximated explicitly, and an implicit discretization is used for the pressure term. The unstructured grid in the horizontal direction and the sigma coordinate in the vertical direction facilitate the use of the model in complicated geometries. 5: The fifth sub-project consists in developing a well-balanced three-dimensional shallow water model which is able to simulate shock waves over dry bed. Due to the hydrostatic simplification of the vertical momentum equation, the governing system of equations is not hyperbolic and can not be solved using standard hyperbolic solvers. That is, one can not use a high-order Godunov-type scheme to compute all fluxes through cell-interfaces. This may cause the model to fail in simulations of some unsteady-flows with discontinuities, e.g., dam-break flows and floods. To overcome this difficulty, a novel numerical scheme for the three-dimensional shallow water equations is proposed using a relaxation approach in order to convert the system to a hyperbolic one. Thus, a high-order Godunov-type central-upwind scheme based on the finite volume method can be applied to approximate the numerical fluxes. The proposed model can also preserve the ``lake at rest'' state and positivity of water depth over irregular bottom topographies based on special reconstruction of the corresponding parameters. 6: The sixth sub-project consists in extending the result of the fifth sub-project to development of a three-dimensional numerical model for shallow water flows over mobile beds, which is able to simulate morphological evolutions under shock waves, e.g. dam-break flows. The hydrodynamic model solves the three-dimensional shallow water equations using a finite volume method on prismatic cells in sigma coordinates based on the scheme prposed in sub-project 5. The morphodynamic model solves an Exner equation consisting of bed-load sediment transportation. The performance of the proposed model has been demonstrated by several laboratory experiments of dam-break flows over mobile beds.