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Uplift capacity of helical piles in thawing permafrost
Get PDFThis thesis focuses on the investigation of the effect of subsurface temperatures on the axial capacity of helical piles embedded in silty permafrost soils. The overall of goals of this thesis are (1) characterization of the impact of different environmental and loading conditions on the axial capacity of helical piles and (2) development of meaningful relationships on the physical processes in the subsurface, especially the interdependency of thermo-hydraulic behavior and the load-displacement behavior of piles. A balanced numerical and experimental campaign is carried out to understand the mechanisms that affect pile capacity with warming temperatures in a silt layer. A series of laboratory-scale pile experiments are performed, and the results are compared to predictions from theoretical methods. A numerical model is developed and validated against experimental results from the literature and is used to estimate load-displacement curves and uplift capacity of helical piles. The predictive numerical model is further used to probe more information about critical soil parameters that affect the capacity of piles in frozen soils.
The laboratory-scale model consists of a double-helix helical pile embedded in an unsaturated Bonny silt layer inside a tank instrumented with an array of dielectric sensors to measure temperature, volumetric water content, and suction. The pile-soil system is frozen then thawed to different temperatures. Then, the pile is subjected to pull-out loading at constant displacement rates. The experimental program included seven tests performed at soil temperatures of -8˚C, -6˚C, -2˚C, -0.2˚C, and 17˚C. The experiment performed at -6˚C was repeated to investigate the impact of strain rate, and the experiment at 17˚C was repeated to understand the effect of thaw rate on pile axial capacity. The helical pile experiences brittle failures at displacements of about 0.5D for soil temperatures below -2˚C, where D is the helix diameter. Ductile behavior is evident when soil temperature is above -0.2˚C, as the peak load is reached at larger displacements of about 1D.
Dielectric sensor measurements show that unfrozen water content decreases with temperature, with the most significant decrease occurring between 0 and -2˚C for the compacted silt. The axial capacity of helical piles increases linearly with decreasing temperatures below 0˚C, while it increases exponentially with decreasing degrees of saturation. For the experiments conducted at the same temperature but different strain rates, the slower pull-out rate results in higher axial capacity. Although the differences in pile capacity are very slight, this behavior is unexpected because viscous effects from the unfrozen water should increase the capacity at higher strain rates. Slight variations in temperature and unfrozen water content can explain this, indicating that those parameters may have a more significant effect on axial capacity. For the experiments conducted at the same strain rate but different thaw rates, the test with a faster thaw rate reaches a higher axial capacity. This may be due to more prevalent water migration in the test since slow thawing may lead to higher water contents at the center of the tank where the pile is located.
Axial capacities from experiments are compared with predictions from theoretical helical pile capacity equations. Estimates of undrained shear strength from existing temperature correlations are employed in the theoretical calculations. It was concluded that predicted axial capacities show a good match with experimental results at temperatures below 0°C, but not for the fully thawed condition at room temperature. It is possible that for unfrozen soil, the undrained shear strength could not be predicted using the existing linear correlation since it is governed by other parameters, i.e., particle shape, pore size distribution, and water content, when ice is not present.
The analyses and conclusions drawn from this study using the data sets from empirical observations, theoretical models, and numerical simulations will significantly contribute to design guidelines of helical piles installed in frozen soils and will be instrumental in adaptation strategies for sustainable development in cold regions.Submission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2023-05-01The student, Sofia Fernandez Santoyo, accepted the attached license on 2021-04-28 at 09:08.The student, Sofia Fernandez Santoyo, submitted this Thesis for approval on 2021-04-28 at 09:27.This Thesis was approved for publication on 2021-04-28 at 13:44.DSpace SAF Submission Ingestion Package generated from Vireo submission #16597 on 2021-09-16 at 20:14:41Made available in DSpace on 2021-09-17T04:06:56Z (GMT). No. of bitstreams: 2
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Helical pile behavior in a silty soil at varying thermal and hydraulic conditions
Get PDFSubmission published under a 24 month embargo labeled 'Closed Access', the embargo will last until 2024-12-01The student, Kamran Nawaz, accepted the attached license on 2022-12-08 at 13:29.The student, Kamran Nawaz, submitted this Thesis for approval on 2022-12-08 at 22:05.This Thesis was approved for publication on 2022-12-09 at 13:01.DSpace SAF Submission Ingestion Package generated from Vireo submission #18779 on 2023-04-12 at 11:37:34This thesis focuses on the axial behavior of single- and double-helix helical piles installed in ice-poor unsaturated Bonny Silt layers. A physical modeling approach was conducted to develop an experimental setup for estimating the uplift capacity of helical piles in permafrost in laboratory conditions. A series of tensile load tests were performed to predict the uplift capacity of piles. Physically meaningful relationships were established between the results from pile load tests and their corresponding ambient environmental conditions. The effect of degree of saturation, embedment depth, and temperature variation on the pull-out capacity and adfreeze bond were investigated. Laboratory experiments were used to determine what mechanism causes the variability of helical pile capacity in Bonny Silt. Physical scale models of single and double helix helical piles were embedded in a silty layer inside a test tank. Pull-out tests were conducted for the frozen and unfrozen conditions at different temperatures and volumetric water contents. The water contents were measured using a capacitance sensor embedded in the silt layer. This experimental campaign consists of 30 tests at varying soil temperatures, four degrees of saturation (Sr), and embedment depth ratio. The displacement rate (0.5 mm/min) was applied and kept constant throughout the testing campaign to sustain a uniform loading condition. The results from the pull-out test revealed that strain softening was the main stress-strain behavior observed for the pile in frozen conditions, while strain hardening was prevalent for the piles tested at room temperature. The initial degree of saturation significantly affected the pull-out capacity of the piles. The pile capacity in frozen conditions was higher for higher Sr and decreased with increasing Sr for the unfrozen conditions. The adfreeze strength was predicted from the load-displacement curves for the first time in this study. As expected, the adfreeze strength was observed to be higher for piles with higher Sr. When the embedment/depth ratio decreased, the piles exhibited lower strength for both single and double helix helical piles. The results from the experimental campaign and main conclusions drawn from this thesis directly contribute to the helical pile design in a changing subsurface in cold regions and will be instrumental in developing sustainable built environments in which the impact of climate change is considered at-large scales
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Coupled Heat Transfer and Water Flow in Soil-Borehole Thermal Energy Storage Systems in Unsaturated Soil Profiles
Get PDFThis research focuses on understanding the key variables affecting the performance of soil-borehole thermal energy storage (SBTES) systems installed in the vadose zone. In particular, this study seeks to understand how the coupled thermal and hydraulic properties of unsaturated geomaterials and coupled heat transfer and water flow processes may be exploited to enhance heat injection and heat retention in an array of geothermal borehole heat exchangers. Specifically, heat injection into unsaturated geomaterials may be enhanced through convection of liquid water and water vapor as well as through latent heat energy transfer associated with water phase change. Further, heat retention in unsaturated geomaterials may be enhanced by the reduction in thermal conductivity associated with the thermally-induced drying expected during heat injection. Multi-fidelity numerical models are employed in this study to consider the roles of different mechanisms of heat transfer during a cycle of heat injection and ambient cooling. A conduction-only model was found to be useful for evaluating scalability issues and for the geometric design of a full-scale SBTES system installed in Golden, Colorado, but required a low soil thermal conductivity to match collected field data. A numerical model the incorporates convection in the liquid and vapor phases as well as phase change was validated using the results from a full-scale SBTES system installed in an unsaturated rock layer in San Diego, California. The numerical model was calibrated using coupled thermo-hydraulic soil properties from element-scale experiments and parameters governing water vapor diffusion and phase change rates from tank-scale heat injection experiments. A comparison of the simulation results from this models with another model that does not incorporate vapor flow confirmed the significant effect of water vapor convection on the permanent drying of the subsurface during heat injection. The results from the validated numerical simulations and field experiments confirm the potential benefits of siting SBTES systems in the vadose zone. The validated numerical model considering convection in the vapor phase can be used in future studies to evaluate the evolution in the energy extraction efficiency over multiple heat injection and extraction cycles for SBTES systems in the vadose zone
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Prediction of creep behaviour of geogrid reinforced fill structure using numerical analysis : a finite element method
No full textVarious types of geosynthetics are utilized in conjunction with concrete panels, gabions encased in steel wire mesh, or as a wrap-up solution to stabilize steep slopes. Geogrids have become a prevalent choice for stabilization and reinforcement in the environmental and civil sectors. However, due to their characteristics, they are susceptible to experiencing increased displacements in the long term as a result of creep deformations due to ageing under sustained loads. The degradation in the stiffness of reinforced fill structures is in fact primarily attributed to the impact of creep behaviour. Visco-elastic geogrid model and hardening soil model can be combined and applied in the prediction of time dependent deformations of the geogrid-soil composite. In this study, extensive numerical analysis has been conducted to examine the deformation behaviour of geogrids over longer periods. With the information of technical details regarding geogrids from the manufacturer, we can have input parameters used by Plaxis2D, and then with the help of the Visco-elastic (time-dependent) model, we can predict the long-term behaviour of the geogrids. The Pull-out tests supported the interface values between the geogrid and the soil. The scope of this paper has been limited to showing preliminary results of a first stage of a Ph.D. study by predicting the creep strain of geogrid in long-term. The results of this study will aid in predicting the future creep behaviour of reinforced fill structures through the utilization of the Finite Element software, PLAXIS 2D
Dynamic mechanical behavior of frozen Ottawa sand subjected to high strain rate loading
No full textSubmission published under a 24 month embargo labeled 'U of I Access', the embargo will last until 2027-08-01The student, Cocou Davis Ruben Aza-Gnandji, accepted the attached license on 2025-07-14 at 16:04.The student, Cocou Davis Ruben Aza-Gnandji, submitted this Dissertation for approval on 2025-07-14 at 16:14.This Dissertation was approved for publication on 2025-07-17 at 06:39.DSpace SAF Submission Ingestion Package generated from Vireo submission #22553 on 2025-10-21 at 10:06:05Increasing temperatures in the Arctic region raise environmental concerns, but also bring new opportunities for civil engineers including infrastructure development, such as roads, buildings, and pipelines, etc. Extensive research focusing on the behavior of frozen soils under quasi-static loading resulting in small strain rates exist; however, limited studies have explored their behavior under high strain rate loading, which is relevant in construction and resource extraction, etc. This research aims to investigate the dynamic mechanical behavior of frozen sands under varying thermal conditions and to characterize the impact of high strain rates on the overall stress-strain response. Frozen Ottawa sand samples having different dry densities and initial degrees of saturation were tested at temperatures of -15, -10, and -5°C to characterize the mechanical behavior. These samples were subjected to strain rates ranging from 400 to 1500/s using both traditional and modified Split Hopkinson Pressure Bar (SHPB). A temperature-controlled chamber was designed and attached to the SHPB setup to maintain constant temperatures during the experiments. A high-speed infrared camera was integrated to monitor temperature variations during the impact tests for estimating the thermal energy during the tests. The stress-strain curves of frozen Ottawa sand at different temperatures were obtained, and the results indicated that the stress-strain behavior was significantly influenced by the strain rate, temperature, and initial degree of saturation of the frozen sands. Specifically, the stress-strain curves exhibited peak stresses followed by pronounced strain softening when the strain rate was less than 900/s. However, at strain rates above 900/s, relatively more brittle response was observed. The results also revealed that the strength of the frozen Ottawa sand increased as the temperature decreased, due to the enhanced bonding between the ice and soil particles. To further evaluate the behavior of frozen sands under various strain rates, numerical simulations using LS-DYNA were performed. Two numerical methods available in LS-DYNA, namely, the Finite Element Methods and the Smoothed Particle Hydrodynamics, were employed to perform the numerical simulations of the SHPB tests. Holmquist-Johnson-Cook material model was employed in the simulations. Both numerical schemes produced results that were in good agreement with the experimental results. They also revealed the need for the development of advanced material models for the simulations of the dynamic behavior of frozen soils under extreme loading conditions. Key experimental results of this study can contribute to the design of infrastructure and protective structures that may be subjected to high-strain-rate deformations, impact loadings, or explosions. Future research will build on this study to develop a material model with temperature-dependent parameters for simulating the thermo-mechanical behavior of frozen sands under different thermal and extreme loading conditions
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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