Centrifuge Modeling and Large Deformation Analyses of Axially Loaded Helical Piles in Cohesive Soils
Creators: Li, Weidong
Publisher: name: University of Alberta Library
Year: 2022

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