Helical Piles for Support of Station Platforms in a Cohesive Soil Deposit

Amtrak is implementing Americans with Disabilities Act (ADA) improvements to existing stations across the country to provide access to transit for persons with disabilities. In most cases, existing substandard platforms are being replaced with new low-level platforms and wheelchair lifts for ease of boarding. Amtrak has successfully supported new platforms on drilled helical piles at several stations. Advantages of helical piles are that they can be installed with relatively small equipment with low overhead clearance, and they do not require support of excavation adjacent to existing tracks as would be required for spread footing excavation. This paper presents the details of the design and construction of foundations supporting upgrades to Amtrak's Fargo, North Dakota, station. Test borings drilled at the site encountered surficial existing fill overlying medium to stiff lean clay and fat clay. The clays are glaciolacustrine in nature deposited by the nearby Red River and historic glacial lake Agassiz. Based on the proposed platform loading, helical piles with four helices would provide an allowable compressive capacity of 15 kips (67 kN) in the glaciolacustrine clays. As part of the specifications, the contractor installed three sacrificial helical piles close to test boring locations, and one location was selected for static load testing. After the initial load test failed, one possibility was to switch to a much more costly deep foundation system. After some discussions with Amtrak, and the contractor, a second load test was suggested and successfully performed to confirm design capacity. The paper will denote how installation methods and soil setup contributed to the capacity of the helical pile.