Browsing by Subject "Soil"

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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.
Characterization of soil unsaturated flow properties using steady state centrifuge methods
(2014-12) Plaisted, Michael David; Zornberg, Jorge G.
Three testing procedures were developed in this research to allow expeditious characterization of soil unsaturated flow properties using steady state centrifuge methods. The first testing procedure, referred to as the “instrumented” procedure, focuses on using in-flight measurement of the suction and volumetric water content of soil samples under centrifugation. The measurements are used to calculate the soil water retention curve and hydraulic conductivity function (K-function) of soil samples. A good agreement was found between results determined using the “instrumented” procedure and standard testing methods. Several possible sources of inaccuracy were determined with the “instrumented” procedure. The void ratio, the changes of which were not measured, was found to decrease during centrifugation and the lower boundary condition, which was not accounted for in the evaluation, was found to affect a large portion of the sample. In order to improve the accuracy of results, two additional testing procedures were developed that accounted for these issues and incorporated the void ratio of the soil as an additional variable. The first additional procedure was used to measure the soil water retention surface (SWRS) of soil samples while the second was used to measure the unsaturated hydraulic conductivity surface (K-surface) of soil samples. Both new procedures, referred to as the “hydrostatic” and “imposed flow” procedures, were used to characterize the unsaturated flow properties of a low plasticity clay (“RMA” soil). The unsaturated flow characteristics of the RMA soil were evaluated for a wide range of void ratio and three compaction moisture conditions. As a result, the effects of void ratio and compaction moisture content on the unsaturated flow characteristics could be determined for the RMA soil. The compaction water content was shown to have significant effects on both the retention behavior and the unsaturated hydraulic conductivity of the RMA soil. In general, increases in compaction water content resulted in a decrease of large pore sizes in the soil, resulting in higher water retention and lower unsaturated hydraulic conductivity. The void ratio was found to have comparatively lesser, but still significant, effects on both retention and conductivity characteristics. Specifically, decreases in void ratio were shown to reduce the unsaturated hydraulic conductivity. In addition, decreases in void ratio were shown to result in either increases or decreases on the soil water retention, depending on the level of suction in the soil. A good agreement was found between results obtained using standard methods and those from the hydrostatic and imposed flow procedures. Accordingly, steady state centrifuge methods were ultimately found to provide a both expeditious and accurate method for characterizing the unsaturated flow properties of soil.
Conservation engineering and agricultural terracing in Tlaxcala, Mexico
(2014-05) LaFevor, Matthew Cole; Doolittle, William Emery
This research examines the practice of hillslope terracing in the state of Tlaxcala, Mexico. It explores how one popular terrace form, zanja-bordo (ditch-and-border) terraces, is employed in two distinct, though sometimes related contexts: (1) producing crops (agriculture) and (2) protecting natural resources (conservation). It first traces the use of zanja-bordo terraces in traditional agriculture in the region, highlighting some of the major elements of their form and function, issues surrounding their antiquity, and their possible role in the landscape degradation so prevalent in the region today. Moving from this agricultural context, the dissertation next examines the role of zanja-bordo terraces in landscape restoration efforts in Tlaxcala. It demonstrates the key role that active and prolonged maintenance plays in long-term efforts to restore previously degraded farmland back to productive capacity. The dissertation then examines more broadly how government programs to promote zanja-bordo terracing in the region impact farmers, whose ancestors have been building zanja-bordo terraces for centuries. Findings from the collection, synthesis, analysis, and groundtruthing of written data on governmental terracing projects in the state reveal that while perhaps well intentioned, these programs did little to promote sustainable agricultural development or environmental conservation in the region. Finally, the dissertation moves above the 3,000-meter mark to examine the relatively recent phenomenon of high-elevation terracing in Mexico's national parks. Conceived as a means of erosion mitigation, water conservation, reforestation, and even fire suppression, government agencies now construct zanja-bordo terraces throughout the understory of many of Mexico's subalpine forests. A case study of the la Malinche (Malintzi or Matlalcueyatl) National Protected Area illustrates some of the difficulties in examining each of the claimed benefits of terracing in these environments. Whether for agriculture or restoration, as a techno-developmental strategy, or as a tool for soil and water conservation, zanja-bordo terraces are shown to be an adaptable and effective hillslope management technology. This dissertation demonstrates, however, that successful adaptation and implementation of zanja-bordo technologies into different contexts largely depends on the effective planning, monitoring, and maintenance of terrace structures and processes. Ultimately, the sustainability of zanja-bordo terracing relates more to issues of contextualization and human motivation than to questions of technological innovation.
Dynamic properties of fine liquefiable sand and calcareous sand from resonant column testing
(2015-05) Wang, Yaning, Ph. D.; Stokoe, Kenneth H.; Cox, Brady R.
The study of the dynamic properties of two specific kinds of granular soils is performed using torsional resonant column testing. The sandy soils are: (1) liquefiable sand from Christchurch, New Zealand, and (2) calcareous sand from Puerto Rico. The effects of isotropic effective confining pressure, shear strain amplitude, void ratio, and total unit weight on the small-strain and nonlinear dynamic properties of both types of sand are presented and discussed. Empirical models from previous studies are examined to determine how well the models fit the test results.
Ecological mechanisms underlying soil microbial responses to climate change
(2013-12) Waring, Bonnie Grace; Hawkes, Christine V.
Soil microbes influence the global carbon cycle via their role in the decomposition and formation of soil organic matter. Thus, rates of ecosystem processes such as primary production, soil respiration, and pedogenesis are sensitive to changes in the aggregate functional traits of the entire microbial community. To predict the magnitude and direction of microbial feedbacks on climate change, it is necessary to identify the physiological, ecological, and evolutionary mechanisms that underlie microbes’ responses to altered temperature and rainfall. Therefore, I examined microbial community composition and function in relation to manipulations of resource availability and precipitation in two contrasting ecosystems: a tropical rainforest at La Selva Biological Station, Costa Rica, and a semi-arid grassland in central Texas. I conducted a leaf litter decomposition experiment at La Selva to identify the physiological constraints on microbial allocation to extracellular enzymes, which degrade organic matter. I found strong evidence that microbial enzyme production is decoupled from foliar stoichiometry, consistent with weak links between leaf litter nutrients and decomposition rates at the pan-tropical scale. Next, to examine whether ecological trade-offs within microbial communities may drive shifts in carbon cycling at local spatial scales, I quantified changes in soil fungal and bacterial community composition in response to an in situ precipitation exclusion experiment I established at La Selva. Although drought-induced shifts in community structure were small, large increases in biomass-specific respiration rates were observed under dry conditions. These findings suggest that physiological adjustments to drought may constitute an important feedback on climate change in wet tropical forests. Finally, I focused on microbial community responses to climate change within a meta-community framework, using a reciprocal transplant experiment to investigate how dispersal shapes bacterial community structure along a natural rainfall gradient in central Texas. I found that soils from the wet end of the precipitation gradient exhibited more plastic functional responses to altered water availability. However, soil bacterial community composition was resistant to changes in rainfall and dispersal, preventing functional acclimatization to precipitation regime. Together, the results of these experiments emphasize the potential for physiological plasticity or microevolutionary shifts within microbial populations to drive ecosystem carbon cycling under climate change.
Evaluating the direct and indirect effects of wildfires on soil biological activity and ecosystem regeneration
(2017-06-28) Cooperdock, Sol; Breecker, Dan O.
There is substantial uncertainty regarding the effects of wildfire on forest ecosystem regeneration. Some studies suggest that fires can prime ecosystems for regeneration whereas others suggest that the loss of organic matter as a result of wildfires limit regeneration. Which reaction occurs likely depends on the severity of the fire and, as we suggest here, the climate of the region. We studied a forest in central Texas, where two recent fires have occurred (2011 and 2015) and the hot, dry summers represent an analog for the future climate of many forest regions. Due to the role of the soil biological community in nutrient breakdown and cycling with an ecosystem, soil biological activity is an important indicator of recovery after wildfires. Two methods of measuring soil respiration were used as proxies for biological activity: field-based CO₂ flux measurements and lab-based microcosm incubations. Soil temperature, water content, total C, N, δ¹³C, and pH were measured to determine the impact of wildfires on these variables and the impact of changes in these variables on biological activity. All samples for lab analysis were taken from the top 5 cm of the soil. Decreases in total soil organic matter (SOM) were observed in burned soils, however, lab-based respiration measurements (which controlled for temperature and water content) suggest that decomposition rates in soils burned in the 2011 fire are similar to unburned soils and decomposition rates in soils burned in the 2015 fire are only slightly reduced. On the other hand, field measurements indicate respiration rates in burned soils were much lower than they were in unburned soils during hot and dry months due to differences in soil temperature and water content between burned and unburned soils. Increased temperatures in burned soils as a result of the removal of canopy cover, the removal of organic matter insulation and the deposition of black ash on the soil surface cause more severe water limitation in burned soils. We conclude that, although the composition of the soils was not impacted enough to reduced microbial activity where the burns occurred, the surrounding environment was disturbed enough to have severe indirect effects on the soil, most importantly increased heat absorption which led to lower water contents and ultimately lower respiration rates. Because this forest represents an analog for future climates, the data presented here suggest that soil ecosystem regeneration will be slowed after wildfires due to extreme temperature and water limitation.
Prehistoric techniques for enhancing soil fertility in aridisols at the Creekside Village Archaeological Site, New Mexico, and its implications for dryland soil and water management policy
(2023-04-24) Olivier, Gwen; Beach, Timothy Paul; Eaton, David J.; Krause, Samantha
Studying a prehistoric culture’s soil management can provide information to modern societies and their relationships with the biophysical environment. The archaeological record illuminates humans' long-term connection with soil, especially the soil taxonomic class of aridisols (dry soils). Prehistoric Indigenous people have practiced dry agriculture in the Southwest United States region for hundreds of years, and by researching past management of soils, it may be possible to understand how to better conserve and protect today’s fragile soil resources. This research examines the prehistoric techniques at the Jornada Mogollon Creekside Village Archaeological Site (CSV) near Tularosa, New Mexico, used to manage soil moisture and fertility in aridisols by integrating water reservoirs. This thesis uses micromorphological and geoarchaeological proxies to analyze how past humans altered CSV’s reservoir starting in the Mid-Mesilla Phase (650–700 A.D.) and its implications for soil health during the occupation and after abandonment. Results trend toward enhanced soil fertility and indicate that the water reservoir was part of the site’s runoff agricultural system. This is the first micromorphological analysis of the site’s reservoir, and more research is needed to complete the story. Many indigenous communities, such as the Jornada Mogollon, who once practiced niche soil management, have moved away from their ancestral sites, resulting in modern communities that lack an understanding of their landscapes. Studying the connection between humans and ecosystems as a symbiotic system is essential to implementing ecocentric resource management, especially in dryland agriculture. The innate challenges of farming in aridisols paired with current poor land-use practices motivate land-use policy research of prehistoric indigenous soil and water management practices, such as those at CSV. Geoarchaeology analyses highlight the unique identities of Southwestern United States prehistoric sites and the importance of niche soil management specific to each soil ecosystem. This ongoing research contributes new micromorphological analyses, stable isotopic analysis, and literature for Jornada Mogollon sites in proximity to CSV.
Simulation of ice wedge polygon geomorphic transition, Prudhoe Bay, Alaska
(2015-08) Abolt, Charles Joseph; Young, Michael H.; Johnson, Joel P; Sharp, John M
A numerical model is presented to simulate the changes in topography associated with ice wedge polygon transition from low-centered to high-centered form. The model applies a hillslope diffusion equation to an eroding polygon using a finite-difference approach. It is calibrated using a LiDAR dataset from a site where low-centered polygons exist within meters of high-centered polygons, whose formation appears to have been triggered by construction of the Dalton Highway. The loss of hydrologic storage and the transport of soil from the polygon center into polygon troughs during transition are estimated from model simulations. Optimized values of the hillslope diffusion coefficient suggest that multiple physical processes, including frost heave and continuous soil creep, may drive lateral soil transport at the site. The optimized parameters, furthermore, capture the decreasing influence of anthropogenic disturbances (in this case, the Dalton Highway) on polygon form at distances greater than 35 meters. Overall, a match between the topography of simulated and observed high-centered polygons confirms that the hillslope diffusion paradigm approximates much of the complexity of polygon transition. Future refinements to the model should include more process-based treatment of the mechanisms that drive soil transport and control rates of polygon erosion.
Wetland agroecoystems in the Maya Lowlands of Belize : LiDAR and multi-proxy environmental change
(2018-09-14) Krause, Samantha Marie; Beach, Timothy Paul; Luzzadder-Beach, Sheryl; Doolittle, William; Rosen, Arlene; Valdez, Frez
Globally, scholars have identified wetlands as critical environments for global carbon storage, water filtration, biodiversity, and many other ecosystem functions. Despite their importance, too few studies have focused on wetlands in the tropical Americas. This dissertation provides new and innovative knowledge on the extent and character of prehistoric indigenous wetland agricultural systems in the Maya Lowlands of Belize by exploring wetland development and paleoenvironmental change over the course of the late Holocene. The overall results of this project are as follows: provide new information and understanding of never before researched wetland agroecosystems; 2. determine the extent of human and environmental interaction and regional climate history in this crucial zone of the Neotropics, 3: answer questions concerning the scale and types of Maya to modern wetland management over time; 4: offer greater awareness concerning contemporary landscape degradation within critical wetland systems in the face of future climate change scenarios, and 5. provide future resources for policy and management of American wetlands as a critical geographic area.