Axial Performance of Continuous-Flight Pile in Frozen Soil
Creators: Gao, Shuai
Publisher: name: University of Alberta Library
Year: 2023

<p>Piles are commonly used in the Canadian Arctic as a foundation solution. Steel pipe shaft pile, which is a conventional pile type in the Arctic, is placed in an oversized pre-drilled hole and then the annulus is backfilled with gravel slurries or grout designed to cure (i.e., a process called freezeback) at cold temperatures. However, the construction is labor-intensive and freezeback is time-consuming. Additionally, conventional steel pipe piles may provide lower adfreeze strength between backfill and frozen soil than other methods. </p>

<p>As an innovative foundation option, continuous-flight piles consist of a steel tube with continuous spiral flights and a tapered lower segment. These piles can be installed by applying a compressive load and torque to the pile head. Owing to the difference in pile construction and soil-pile interface, continuous-flight piles have advantages such as large capacities, rapid installation, lightweight, and reusability in non-frozen soils. Consequently, continuous-flight piles may present a better solution for deep foundations in the Canadian Arctic region. However, currently there is not any design guideline or research on the application of continuous-flight piles in permafrost regions. The short-term strength and long-term creep settlement of the continuous-flight pile in the frozen ground may be different from conventional predrilled steel pipe piles. Understanding the effects of pile shaft features and frozen soil conditions on the axial performance of continuous-flight piles will provide valuable information for the pile design in permafrost.</p>

<p>To address these gaps, a research program was conducted to characterize the installation and determine the axial behaviour of continuous-flight piles in frozen soils. Some specific hypotheses of the research program are: 1) the continuous-flight pile can be screwed into a pilot hole in frozen soil; 2) the water content, salinity, temperature of soils and pile shaft feature will affect the performance of piles; 3) the geotechnical failure may occur locally at the soil beneath the pile threads instead of at the soil-shaft shear surface, based on research of this pile type in non-frozen soils; 4) because of the failure pattern, the short-term capacity of the continuous-flight pile may be greater than conventional piles of similar size, and the long-term creep rates may be less than that of conventional piles.</p>

<p>The research of model pile testing in frozen soils was carried out at the Cold Regions Geotechnical Research Center, University of Alberta. Twelve short-term or constant displacement rate tests, four long-term or constant load tests, and seven installation tests were conducted. Continuous-flight pile segments with a diameter of 89 mm and a height of 300 mm were installed and loaded in frozen soils to investigate the installation torque, short-term axial pile capacities, long-term settlement, and load-transfer mechanism. The effects of porewater salinity (0 and 10 ppt), soil temperature (0 to -5 ℃), soil’s water content (20% to 27%), and pile shaft feature were examined. Strain gauges were installed on the pile surface to measure the internal stress distribution during axial loading and the torque during installation. Pile segments were installed in an undersized pilot hole in warm frozen soils using an electric rotary motor. Results showed that installation was refused at a minimum temperature of -1.88 ºC in 10 ppt soil and -1.0 ºC in non-saline soil due to the limitation of the lab equipment. The test pile carried more load than the smooth pile in the cold frozen soil (-5 ºC). The short-term ultimate pile capacity increased significantly when soil temperature decreased from -1 ºC to -5 ºC. The pile capacity increased with decreasing salinity in cold frozen soils while the effect of salinity on the pile capacity in warm frozen soils (-1 ºC) could be neglected. Cylindrical shearing mode and individual bearing mode were observed for continuous-flight piles in warm and cold frozen soils, respectively. </p>

<p>In conclusion, this research introduces a novel pile type that may be suitable for residential and commercial projects in Canadian Arctic communities. Findings from this research are anticipated to contribute to the safe and economic construction of infrastructure across Northern and Western Canada.</p>