Academic literature on the topic 'Wet and Dry cycles'

Transport and reactivity of carbon in the critical zone are highly controlled by reactions of dissolved organic matter (DOM) with subsurface soils, including adsorption, transformation and exchange. These reactions are dependent on frequent wet–dry cycles common to the unsaturated zone, particularly in semi-arid regions. To test for an effect of wet–dry cycles on DOM interaction and stabilization in subsoils, samples were collected from subsurface (Bw) horizons of an Entisol and an Alfisol from the Catalina-Jemez Critical Zone Observatory and sequentially reacted (four batch steps) with DOM extracted from the corresponding soil litter layers. Between each reaction step, soils either were allowed to air dry (<q>wet–dry</q> treatment) before introduction of the following DOM solution or were maintained under constant wetness (<q>continually wet</q> treatment). Microbial degradation was the dominant mechanism of DOM loss from solution for the Entisol subsoil, which had higher initial organic C content, whereas sorptive retention predominated in the lower C Alfisol subsoil. For a given soil, bulk dissolved organic C losses from solution were similar across treatments. However, a combination of Fourier transform infrared (FTIR) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopic analyses revealed that wet–dry treatments enhanced the interactions between carboxyl functional groups and soil particle surfaces. Scanning transmission X-ray microscopy (STXM) data suggested that cation bridging by Ca²⁺ was the primary mechanism for carboxyl association with soil surfaces. STXM data also showed that spatial fractionation of adsorbed OM on soil organo-mineral surfaces was diminished relative to what might be inferred from previously published observations pertaining to DOM fractionation on reaction with specimen mineral phases. This study provides direct evidence of the role of wet–dry cycles in affecting sorption reactions of DOM to a complex soil matrix. In the soil environment, where wet–dry cycles occur at different frequencies from site to site and along the soil profile, different interactions between DOM and soil surfaces are expected and need to be considered for the overall assessment of carbon dynamics.

Climate change has led to more extreme weather during the last decades. Seasonally hot weather regions have experienced harsh winters. Similarly, the global warming has contributed to raising the mean summer temperature in the cold regions. Geomaterials in various engineering applications are expected to experience such changes in the weather patterns and may undergo freezing, thawing, wetting and drying processes. This thesis presents an experimental study of the one-dimensional volume change behaviour of some geomaterials under the combined influence of freeze-thaw and wet-dry cycles. Laboratory tests were carried out on three selected materials (Speswhite kaolin, Pegwell Bay soil and cement kiln dust). A custom-made test set up was used to carry out the laboratory tests that enabled performing the required tests involving freezing, thawing, drying and wetting processes. Initially saturated slurried materials and compacted materials were subjected to freezing and thawing processes to study frost heave and thaw settlement. Compacted materials were taken through several wet-dry cycles and then exposed to freezing and thawing processes to study the impact of intermittent freeze-thaw cycles on the volume change behaviour of the materials. Similarly, compacted materials were subjected to an increasing number of freeze-thaw-wet-dry cycles to study the combined influence of various processes on the volume change behaviour of the materials. Compacted materials exhibited higher magnitudes of frost heave than initially saturated slurried materials. The swelling and shrinkage strains of compacted materials decreased with an increasing number of wet-dry cycles and attained an equilibrium at which the strains remained very similar. Intermittent freeze-thaw cycles were found to destabilize the equilibrium strain that was achieved during the previous wet-dry cycles; however, a new equilibrium in terms of the vertical strain was attained by the materials with an increasing number of wet-dry cycles. The materials having a history of being exposed to several wet-dry cycles exhibited a higher frost heave, a higher segregation potential and a greater magnitude of thaw settlement as compared to the same materials but without any cyclic wet-dry history. Upon subjected to an increasing number of wet-freeze-thaw-dry cycles, the materials exhibited uprising movement accompanied by strain accumulation prior to attaining an equilibrium at which the vertical strains associated with wetting, freezing, thawing and drying processes were found to be dissimilar; however, the sum of swelling deformation and frost heave was found to be equal to the sum of thaw settlement and shrinkage deformation.

Van den Akker et al. (2003) stated that “European subsoils are more threatened than ever in history” in an editorial referring to the results of the European Union concerted action “Experiences with the impact of subsoil compaction on soil crop growth and environment and ways to prevent subsoil compaction”. Compaction of agricultural soils not only reduces yields but can lead to pollution of surface waters and the release of greenhouse gases. Understanding, therefore, how, and to what extent, subsoil compaction may be reversed seems of vital importance. The literature suggests that soils with a range of textures could recover structural properties such as total and macro-porosity, infiltration rate and stability, although the extent and depth of these changes varies greatly. Factors affecting the extent and depth of changes are the rate of wetting and drying and the matric potentials achieved during drying (influenced by crop cover and irrigation practices as well as climate), organic matter content and tillage history. There is, however, a limited amount of information regarding changes to the pore-size distribution and unsaturated hydraulic conductivity of soils with wet/dry cycles. Although there are models able to predict the compaction that would occur for a given set of conditions, as well as the compactibility of soils and surface crack generation, there are no predictive models that describe structural changes due to wet/dry cycles. The aims of this study were to assess the changes in bulk density and hydrological parameters of a range of soils, of varying texture and other physical and chemical properties, with wet/dry cycles and to explore the relationships between any measured changes and the measured soil properties. A series of laboratory experiments were designed to measure changes to the total porosity and pore size distribution and hydraulic functions of soil samples. Changes to bulk density and moisture content of undisturbed soil cores at field capacity with wet/dry cycles were monitored. Changes to the moisture release curve and hydraulic conductivity curve (as well as bulk density) of remoulded clayey subsoil samples, with wet/dry cycles, were estimated using the multi-step outflow method. The surface cracking of the samples during the wetting and drying process was also explored. Changes to these parameters were statistically compared between soils and between initial bulk densities, and related to soil properties such as texture, organic matter content and Atterberg limits with a general linear model for repeated measures. Changes to the bulk density of the soils after three wet/dry cycles were found to be significantly different (at the 95 % probability level) between soils and between initial bulk densities. It was found that 90 % of the changes in bulk density could be predicted from the initial bulk density and the liquid limit of soils alone. The porosity gained or lost during wet/dry cycles was not confined to the macro-pore region, but included changes to pores that were undrained at pressures up to and including 100 m of water. Changes in the pore size distribution, as described by the parameters of the water release curve, modelled with the van Genuchten-Mualem equation, were found to be statistically related to soil texture and Atterberg limits. Changes to the hydraulic conductivity function of the soils were found to be affected by changes to the connectivity of the pore network, not just to changes in the volume of conducting pores. This was particularly true of the saturated hydraulic conductivity which was found to increase the most in soil samples that experienced a concomitant increase in bulk density and decrease in macro-porosity. Changes to the area of surface cracks were found to be significantly related to changes in the saturated hydraulic conductivity and to changes in the parameters of the van Genuchten-Mualem equation. It was also found that cracks did not close up entirely when the samples were saturated and there were many instances where wetting increased the area of surface cracks, relative to the preceding dry event, implying that the wetting phase was as important in structural formation as the drying phase. There is a strong suggestion that it would be possible to construct a relatively simple model, based on easily measurable parameters, which could be used to predict the changes to soil structure achievable with wet/dry cycles. Further research is needed, therefore, that extends the data set upon which the predictive model designed in this study was constructed. Refinement of the model to include information on the rates of wetting and drying would also contribute greatly to its applicability to the field situation. However, important steps have been made by this study towards a more comprehensive understanding of soil structural dynamics.

Moisture or water damage in asphalt mixtures can be defined as the loss of strength and durability, and an increase in stiffness leading to a more brittle behavior of materials and cause the adhesive failure at the binder-aggregate surface and/or cohesive failure within the binder or mastics. The performance and behaviour of asphalt mixtures are strictly dependent on the response of bituminous binders and mastics. The scope of this research is to acquire information on how moisture may affect rheological and chemical properties of different mastics made by different fillers. A neat bitumen and four mastics were prepared and tested, each of which combined with a specific filler. The conditioning procedure consisted in repeating wetting and drying cycles for three times. After each conditioning phase, couple of samples were tested by means of Dynamic Shear Rheometer (DSR) and Fourier Transfer Infrared spectrometer (FTIR). DSR was employed to determine the rheological properties of materials, such as the complex shear modulus (G*) and the phase angle (δ) at different temperatures and frequencies. FTIR has been used to investigate the chemical composition of bitumen and mastics after its reaction with water, in particular a specific range of wavelength was investigated to determine the absorption of the water and to combine data coming from rheological analysis. At high frequency and/or low temperature all materials behave as a solid; both rheological parameters (complex shear modulus and phase angle) tend to an asymptotic value throughout all the conditioning processes, whereas at low frequency and/or high temperatures is evident an increase of the complex shear modulus, due to water intrusion. Differences between mastics and neat bitumen are present in the intermediate frequency region. FTIR spectra for all mastics confirmed their rheological behavior.

The overarching goal of the doctoral thesis was thus the development of a systematic procedure capable to examine and enhance the role of geomechanical and climatic processes in rockfall susceptibility, performed with statistically based and Machine Learning techniques. To achieve this purpose, two case studies were analysed in the Italian Alps (Valchiavenna, Lombardy Region; Mountain Communities of Mont Cervin and Mont Emilius, Aosta Valley Region). For both case studies, Generalized Additive Models (GAM) were used for rockfall susceptibility assessment; for the Valchiavenna case study, a Random Forest (RF) model was tested too. All models were validated through k-fold cross validation routines and their performance evaluated in terms of area under the receiver operating characteristic curve (AUROC). Predictors’ behaviour physical plausibility was verified through the analysis of the mathematical functions describing the predictors-susceptibility modelled relationships. Specific objectives of the two case studies differed. The Valchiavenna case study was dedicated to testing the role of the outcrop-scale geomechanical properties in a rockfall susceptibility model. Specific objectives were: (i) the optimal selection of sampling points for the execution of geomechanical surveys to be integrated within an already available dataset; (ii) the regionalization over the study area of three geomechanical properties, namely Joint Volumetric Count (Jv), rock-mass weathering index (Wi) and rock-mass equivalent permeability (Keq); (iii) the implementation of the regionalized properties as predictors in a rockfall susceptibility model, along with the traditional morphometric variables; (iv) the investigation of prediction limitations related to inventory incompleteness; (v) the implementation of a methodology for the interpretation of predictors’ behaviour in the RF model, usually considered a black box algorithm; (vi) the integration of the RF and GAM outputs to furnish a spatially distributed measure of uncertainty; (vii) the exploitation of satellite-derived ground deformation data to verify susceptibility outputs and interpret them in an environmental management perspective. The additional geomechanical sampling points were selected by means of the Spatial Simulated Annealing technique. Once collected the necessary geomechanical data, regionalization of the geomechanical target properties was carried out by comparing different deterministic, regressive and geostatistical techniques. The most suitable technique for each property was selected and geomechanical predictors were implemented in the susceptibility models. To verify rockfall inventory completeness related effects, the GAM model was performed both on rockfall data from the official landslide Italian inventory (IFFI) and on its updating with a field-mapped rockfall dataset. Regarding the RF model, the Shapely Additive exPlanations (SHAP) were employed for the interpretation of the predictors’ behaviour. A comparison between GAM and RF related outputs was carried out to verify their coherency, as well as a quantitative integration of the resulting susceptibility maps to reduce uncertainties. Finally, the rockfall susceptibility maps were coupled with Synthetic Aperture Radar (SAR) data from 2014 to 2021: a qualitative geomorphological verification of the outputs was performed, and composite maps were produced. The key results were: (i) geomechanical predictor maps were obtained applying an ordinary kriging for Jv and Wi (NRMSE equal to 13.7% and 14.5%, respectively) and by means of Thin Plate Splines for Keq (NRMSE= 18.5%). (ii) Jv was the most important geomechanical predictor both in the GAM (witha deviance explained of 7.5%) and in the RF model, with a rockfall susceptibility increase in correspondence of the most fractured rock masses. (iii) Wi and Keq were penalized (i.e., they had low influence on rockfall susceptibility) in the GAM model, whereas Keq showed an importance comparable to Jv in the RF model. (iv) In a complex Machine Learning model (RF), the SHAPs allowed the interpretation of predictors’ behaviour, which demonstrated to be coherent with that shown in the GAM model. (v) The models including the geomechanical predictors resulted in acceptable rockfall discrimination capabilities (AUROC&gt;0.7). (vi) The introduction of the geomechanical predictors led to a redistribution of the high-susceptibility areas in plausible geomorphological contexts, such as in correspondence of active slope deformations and structural lineaments, otherwise not revealed by the topographic predictors alone. (vii) Models built with solely the IFFI inventory, resulted in physically implausible susceptibility maps and predictor behaviour, highlighting a bias in the official inventory. (viii) The discordance in predicting rockfall susceptibility between the GAM and the RF models varied from 13% to 8% of the total study area. (ix) From the integration of InSAR data and susceptibility maps, a “SAR Integrated Susceptibility Map”, and an “Intervention Priority Map” were developed as operational products potentially exploitable in environmental planning activities. The Aosta Valley case study was dedicated to challenge the concept of “susceptibility stationarity” by including the climate component in the rockfall susceptibility model. The availability of a large historical rockfall inventory and an extensive, multi-variable meteorological dataset for the period 1990-2020 were crucial input for the analysis. Specific objectives were: (i) the identification of climate conditions related to rockfall occurrence (ii) the summary of the identified relationships in variables to be used in a susceptibility model; (iii) the optimization of a rockfall susceptibility model, including both topographic, climatic and additional snow-related predictors (from a SWE weekly gridded dataset). Starting from an hourly meteorological dataset, climate conditions were summarized in indices related to short-term rainfall (STR), effective water inputs (EWI, including rainfall and snow melting), wet-dry cycles (WD) and freeze-thaw cycles (FT). Climate indices and rockfall occurrence time series were paired. Critical thresholds relating rockfall occurrence to climate indices not-ordinary values (&gt;75th percentile) were derived through a statistical analysis. As summary variables for the susceptibility analysis, the mean annual threshold exceedance frequency for each index was calculated. Model optimization consisted in stepwise modifications of the model settings in order to handle issues related to inventory bias, physical significance of climatic predictors and concurvity (i.e., predictors collinearity in GAMs). The starting point was a “blind model”, i.e., a susceptibility model created without awareness of the rockfall inventory characteristics and of the physical processes potentially influencing susceptibility. To reduce the inventory bias, “visibility” masks were produced so to limit the modelling domain according to the rockfall collection procedures adopted by administrations. Thirdly, models were optimized according to the physical plausibility of climatic predictors, analysed through the smooth functions relating them to susceptibility. Finally, to reduce concurvity, a Principal Component Analysis (PCA) including climatic and snow-related predictors was carried out. Subsequently, the obtained principal components were used to replace the climatic predictors in the susceptibility model. The key results were: (i) the 95% of the rockfalls occurred in severe (or not ordinary) conditions for at least one among the EWI, WD and FT indices; (ii) ignoring inventory bias led to excellent model performance (0.80≤AUROC ≤0.90) but physically implausible outputs; (iii) the selection of non-rockfall points inside the “visibility mask” was a valuable approach to manage the inventory bias influence on outputs; (iv) the inclusion of climate predictors resulted in an improvement of the susceptibility model performance (AUROC up to 3%) in comparison to a topographic-based model; (v) the most important physically plausible climate predictors were EWI, WD, with a deviance explained varying from 5% to 10% each, followed by the maximum cumulated snow melting with a deviance explained varying from 3% to 5%. The effect of FT was masked by elevation. (vi) When the climate and snow related predictors were inserted in the susceptibility model as principal components, concurvity was efficiently reduced. The inclusion of climate processes as non-stationary predictors (i.e., considering climate change) could be a valuable approach both to derive long-term rockfall susceptibility future scenarios and in combination with short-term weather forecasts to adapt susceptibility models to an early warning system for Civil Protection purpose.

La destruction par brûlage de munitions chimiques de la Première Guerre Mondiale a provoqué une contamination importante de la partie supérieure du sol du site de la Place-à-Gaz par l’arsenic, le zinc, le cuivre et le plomb. Le traitement thermique a eu pour effet de minéraliser l’As des agents de guerre organoarséniés, et de former un assemblage minéral inattendu composé d’arséniates de Zn, Cu et Fe, et d’une phase amorphe riche en Fe, As, Zn, Cu et Pb. Ce matériel amorphe est la principale phase porteuse de l’As et des métaux dans la zone la plus polluée. Le site est sujet à des changements environnementaux pouvant affecter la stabilité des contaminants inorganiques. Afin d’évaluer l’impact d’épisodes de saturation en eau et de l’apport de matière organique sur les cycles biogéochimiques des métaux et de l’As, une étude en mésocosme a été menée. Les résultats montrent que la phase amorphe est instable en conditions saturées, et libère des contaminants dans l’eau interstitielle du sol. Comme sur le site, les contaminants les plus mobiles sont le Zn et l’As. L’addition de matière organique a induit une immobilisation de l’As, par piégeage de l’As V sur les oxyhydroxydes de fer, dans la partie saturée du sol. La caractérisation du compartiment microbien a été effectuée via des dénombrements, une analyse de la diversité bactérienne et des tests d’activités d’oxydation de l’As III et de respiration et. Les résultats montrent que les microorganismes ont contribué activement au métabolisme du C et de l’As. L’apport de matière organique a promu la croissance des microorganismes As III-oxydants et As Vréducteurs et modifié la structure des communautés bactériennes. Cependant, un effet négatif de la matière organique sur la vitesse d’oxydation de l’As III a été observé, entrainant une augmentation des concentrations d’As III en solution. Cette étude en mésocosme a montré que le dépôt naturel de litière organique a des conséquences antagonistes sur le transfert des contaminants inorganiques. Ces résultats fournissent de plus amples informations sur l’impact environnemental de la Grande Guerre et, de façon plus générale, sur les processus biogéochimiques contrôlant le comportement des métaux/métalloïdes sur les sites pollués<br>The thermal destruction of chemical munitions from World War I, on the site of “Place-à-Gaz”, induced intense local top soil contamination by arsenic and heavy metals. The heat treatment mineralized As from organoarsenic warfare agents, resulting in a singular mineral assemblage, composed of Zn, Cu and Fe arsenates and of an amorphous phase rich in Fe, As, Zn, Cu and Pb. The amorphous material was the principal carrier of As and metals in the central part of the site. The site undergoes environmental changes which may alter the stability of inorganic contaminants. To assess the impact of water saturation episodes and input of bioavailable organic matter on the biogeochemical cycles of metal(loid)s, a mesocosm study was conducted. Results showed that amorphous phase was instable in saturated conditions, and released contaminants in soil water. As previously observed on site, the most mobile contaminants were Zn and As. The addition of organic matter induced the immobilization of As by trapping of As V onto hydrous ferric oxides in the saturated soil. Microbial characterizations including counting, bacterial community structure, respiration, and determination of As IIIoxidizing activities were performed. Results showed that microorganisms actively contribute to the metabolisms of C and As.The addition of organic matter induced the increase of As III-oxidizing and As V-reducing microorganisms concentrations and modified the bacterial diversity. However, a negative effect of organic matter on the activity of As III oxidation was observed resulting in higher As III concentration in soil water. This study showed that the natural deposition of forest organic litter on the site, induced antagonist effects on the transfer of inorganic pollutants did not immobilize all the Zn and As and even contributed to As III transport to the surrounding environment. These results provide more information about the environmental impact of the Great War and more generally about the processes driving the behavior of metals/metalloids on polluted sites

Thesis (M.S.N.)--University of Florida, 2004.<br>Typescript. Title from title page of source document. Document formatted into pages; contains 73 pages. Includes Vita. Includes bibliographical references.

This thesis deals with the problems of wounding of low potential transferred from steam turbine condenser. First, in the theoretical part variations of steam condenser design are described. Then there is a description of variations of cooling cycles and possibilities of their operation range. In second part of the thesis there are two common cooler options chosen. Those are wet cooling tower with natural draft and dry chiller with forced draft. Two types of cooling liquid are chosen to be used for dry cooling. These are water and the other one is 50 % mixture of water and propylene glycol. Based on the calculation results of both cooling cycle variations appropriate pumps are chosen, fan for forced convection respectively. Parts of the thesis are also projection drawings for both calculated variations.