Date of Award

Spring 5-5-2023

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Civil Engineering

Advisor

Aaron Gallant

Second Committee Member

Keith Berube

Third Committee Member

Aaron Bradshaw

Abstract

Helical piles are lightweight deep foundations designed to support both compressive and uplift loads by mobilizing the shear strength of soil adjacent to helical plates that are welded to an extendable shaft. Helical piles are relatively inexpensive, can be installed quickly utilizing non-specialized equipment, and they are suitable for a wide range of soil conditions. However, helical piles are precluded from many applications due to their inability to support substantial lateral and torsional loads. To overcome this limitation, a novel easy-to-install Collar Vane is implemented to augment the lateral and torsional capacity of helical piles.

The Collar Vane consists of four steel fins welded to a hollow steel collar that wraps around the helical pile shaft. The Collar Vane (CV) is structurally coupled to the helical pile via flanges near the helical pile head to transfer lateral and torsional loads through the CV flanges to the soil. Two different Collar Vanes prototypes were manufactured, a `two-piece' Collar Vane (CV2), tested in 2021, and based on the results of this prototype, a `one-piece' Collar Vane (CV1) was manufactured and tested in 2022. Both prototypes were installed and tested in two different well-characterized soil conditions: homogeneous medium-stiff clay soil, and homogeneous dense sand soil.

This thesis presents the field results of an instrumented full-scale helical pile with the Collar Vane subjected to monotonic, lateral and torsional loads; and cyclic, lateral and torsional loads. Displacements and loads were monitored using string potentiometers and load cells, respectively. Strain gauges installed to measure deformations facilitated the computation of the bending moment. The cyclic loading procedure consisted of approximately 1000 cycles at 0.125 Hz to evaluate the effect of cyclic loads on the helical pile which is envisioned to be implemented to support lightweight transportation structures.

The study revealed that the Collar Vane significantly increases the lateral and torsional geotechnical resistance of helical piles. The increase in capacity is associated with the increment in the effective diameter attributed to the mobilization of geotechnical resistance in the upper soil profile. Moreover, strain gauge data suggest that Collar Vane reduces the amount of bending moment on the helical pile shaft by limiting the lateral displacement, reducing the need for a larger diameter pile shaft and hence, making possible a more efficient pile design. In addition, the measured torsional capacity was compared with predicted values using different well-known methodologies. Additionally, the Collar Vane was subjected to different cyclic loading ratios and it was found that the Collar Vane head stiffness degraded as the number of load cycles increased, the accumulated displacement was significant in the first 100 cycles, and a simple approach was proposed to determine the accumulated displacement in function of the number of cycles.

Finally, a numerical analysis is performed using FEM software, to validate field torsional results, and a finite difference program, to compare an standard drilled shaft foundation element to the Collar Vane lateral response. Moreover, an example design of a roadside sign structure is shown to demonstrate the applicability of the Collar Vane.

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