Field Testing of Helical Piles and SPT-based Method for Estimating Axial Capacities in Sand

• Author / Creator

  Zhang, Yiwen

• Helical piles comprise of at least one helical-shaped bearing plates appended to the square or round shafts. In the past decades, helical piles have gained significant popularity across Canada in various foundation applications such as power transmission towers, bridges, solar farm, and residential and commercial buildings. They provide a few advantages including quick installation, cost effectiveness, low level of noise, minimal vibration and reusability. Estimating the axial capacity is crucial to the helical piling industry. Because the Standard Penetration Test (SPT) is a popular and economical method in subsurface exploration, SPT-based direct method is one of the direct methods commonly used to predict the capacity of piles. The pile shaft friction and end bearing resistance are empirically correlated with the results of SPT for conventional driven piles or cast-in-place piles. Inspired by the methods of conventional piles, the industry of helical piles also needs a direct method that can estimate the pile capacities with the results of SPT to effectively design the piles.
  In this study, twelve full-scale load tests of helical piles, including six axial compressive tests and six tensile tests were carried out in a sandy site. Before conducting the load tests, two SPT tests were carried out in test site to investigate soil stratigraphy. The samples were obtained to proceed laboratory testing. Chin’s hyperbolic method was applied to the interpretation of the load vs. displacement curves for the piles that exceeded the loading frame capacity before reaching the axial displacement of 10% of the helix diameter D according to the industry convention. Chin’s method also used to clarify and correct some abnormal observations in regard to curve segments. Results from the present study and the literature were compiled to construct a helical pile load-test database. The database includes the test results of 47 single-helix piles with various dimensions in sandy soils. From this database, a direct method based on SPT blow count N for estimating the axial capacity of single-helix piles is proposed. The proposed SPT- based method established simple correlations between the average N over the pile shaft length (Nbar) and the unit shaft resistance qs, and between the average N around pile base (Nb) and the unit end bearing resistance qb. The shaft resistance is taken into account, and this modification is critical for a long pile whose shaft carries a considerable amount of load. The proposed SPT-based method was verified again by back-analyzed results.

• Subjects / Keywords

  • Helical piles
  • sand
  • ultimate axial capacity
  • Standard penetration test
  • direct method

• Graduation date

  Fall 2021

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-1hvb-w957

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.