Millworks HeadStock Helical Piles – The Challenge
A 35–foot tall reproduction of an 18th-century wood coal-mining headstock was going to be erected over a capped mine shaft at the Mid-Lothian Mines Park. The now-defunct coal mine was opened in the 1700s and operated until 1923 when Chesterfield County’s mining industry folded. Records provided by the Department of Mines, Mineral, and Energy (DMME) suggests that the original mine shaft was 750 feet deep and 11 feet by 11 feet in plan.
In the 1980s the mine shaft was permanently abandoned by placing an enlarged concrete cap over the sediment-filled shaft. Approximately 15 feet of earthfill was placed over the cap, and the four corners of the cap were marked on the surface by steel posts. However, the quality and placement of the earthfill layer was not documented, which presented a foundation challenge for the proposed headstock structure.
Millworks HeadStock Helical Piles – The Solution
The project geotechnical engineer, through discussion with Gerald Wilkes of DMME, determined that the shaft was full of sediment when it was capped. It was presumed that the sediment-filled the vertical shaft from top to bottom; therefore, there was no void for the soil surrounding the shaft to collapse into. The project geotechnical engineer and project structural engineer concluded that a deep foundation system and pile caps were required to properly support the headstock structure over the earthfill which covered the concrete cap and surrounding area. Helical piles were determined to be the most appropriate system.
The helical pile system was recommended because of its relative ease of installation and adaptability to the erratic site conditions. The helical pile required to meet structural specifications was designed by the JES engineering teams and consisted of Helical Piles.
This hollow shaft round pile has a 2.875” outside diameter and ASTM A572 Grade 50 x 3/8-inch thick helix blades. The piles were designed to provide 15 kips axial capacity and 10 kips of tension capacity. The torque required to achieve the design capacities was 3,400 ft-lbs. The helical pile components were hot-dipped galvanized in accordance with ASTM A123 [Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products].
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JES installed 24 of the Helical Piles. The lead pile section was 7’ long with 8” and 10” helices. The 8” helical blade was manufactured with a “V-Notch” leading edge to aid in penetrating dense earthfill and isolated cobbles and other buried debris.
To reach acceptable bearing depth 5’ shaft extensions were used. The piles were installed to a typical depth of 23-25 feet, and the required torque value of 3,400 ft-lbs for design capacity (using a safety factor of 2) was achieved.
As directed by the project structural engineer, five piles were installed on a 4:1 (V:H) batter to provide higher lateral resistance. In order to verify individual pile capacity, a “Pro-Dig C440 Torque Transducer” was used to determine that the required torque was being delivered to each pile during installation.
New construction steel brackets (caps) were bolted to the top of the pile shafts and were embedded into the concrete pile caps. To ensure proper bolt-hole alignment, the JES field crew used a “Magna-Drill” attached to an engineered guide system.
JES Project Engineer: David E. Stinnette, P.E.
Owner: TAK Tent, L.P.
Architect: Architects Dayton, Thompson & Associates P.C.
Structural Engineer: Koelzer & Associates
Civil Engineer: E.D. Lewis & Associates, P.C.
Geotechnical Engineer: Atlantic Geotechnical Services, Inc.
General Contractor: Merricks Construction, Inc.