Uplift capacity of helical piles in thawing permafrost

This 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.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste