Centrifuge Modeling and Large Deformation Analyses of Axially Loaded Helical Piles in Cohesive Soils

  • Author / Creator
    Li, Weidong
  • In the past decades, helical piles have gained increasing popularity in civil engineering practice as
    an option of deep foundations. The design of axial load capacity of these piles relies on an
    appropriate selection of failure mode. Currently, the axial failure modes of multi-helix piles are
    categorized into the individual bearing mode (IBM) and cylindrical shearing mode (CSM). The
    ratio (Sr) of inter-helix spacing to helix diameter is used as the primary indicator of failure mode.
    The industry adopts Sr of 3.0 as the only criteria, but a number of field tests indicate that the soil
    strength and pile embedment depth may also affect the failure mode. However, a comprehensive
    study aimed at all these factors is unavailable yet.
    To improve our understanding of the axial behavior of helical piles in cohesive soils, a
    centrifuge modeling test program for helical piles was conducted. A test frame was developed to
    install and axially load helical piles in flight. The real-time installation torque and axial shaft load
    distributions were measured. One single-helix pile and three double-helix piles with Sr varied from
    1.5 to 3.5 were tested in two types of kaolinite clay with undrained shear strength (su) of
    approximately 50 kPa (denoted as “medium stiff clay” in this thesis) and 120 kPa (“stiff clay”),
    respectively. Each model pile was installed and axially loaded under 20 g centrifugal condition.
    Specifically, the research was aimed at pile installation torque, installation-induced excess pore
    pressure in the soil, and pile failure mechanisms under monotonic axial loads. An analytical
    solution to the installation torque of helical piles in cohesive soils was proposed and verified by
    measured torques. The analysis indicates that the residual su of the soil governs the soil-pile
    interactions during rotation. The pore pressure response to pile installation was monitored near
    two piles at two depths, in the stiff clay. An analytical solution to pile installation-induced spatial
    consolidation was adopted to assess the measured progression of excess pore pressure dissipation.iii
    To observe the failure modes, the model piles were pulled out of the soil immediately after
    loading. Three failure modes were observed, i.e., IBM, CSM and a transitional failure mode (TFM)
    with a cone-shaped inter-helix soil mass. The axial load transfer mechanisms of the tested piles
    were assessed using the axial load distribution measurements. The results show that IBM and CSM
    models may over-predict the axial capacity of a helical pile governed by TFM. In addition, the
    failure modes depend on su and Sr.
    To further explore the axial failure mechanisms of the double-helix piles in a wider range of
    controlling factors, finite element modeling of helical piles in cohesive soils was conducted.
    Because of the large displacement required for pile failure observation, large deformation finite
    element (LDFE) analyses based on the remeshing and interpolation technique with small strain
    were performed. The LDFE model was validated by the centrifuge model test results. The effects
    of Sr, su, and pile embedment depth on the generation of failure mode were assessed. The
    simulation results show that the failure mode changes gradually from CSM to IBM with an
    increasing inter-helix spacing. CSM occurs when Sr is adequately small, and su of the clay is
    sufficiently high. In general, CSM provide greater optimal uplift capacity as Sr increases. However,
    when Sr approaches 2.5, using CSM for axial capacity design may become inaccurate. The helix
    break-out factors of lower helices, which may be affected by the above inter-helix soil collapse
    mechanisms, change with the failure modes. The bearing factors of the lower helices, which may
    not be affected by the inter-helix soil behavior, remain essentially unchanged with the variation of
    failure modes.

  • Subjects / Keywords
  • Graduation date
    Spring 2022
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • 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.