Challenge

March 2012 - A 35-foot tall reproduction of an 18th century wood-frame headstock was planned to be constructed over an abandoned coal mine shaft at the Midlothian Mines Park. The coal mine was opened in the 1700’s and operated until 1923. Records provided by the Department of Mines, Mineral, and Energy, suggested that the original mine shaft was 11 feet square by 750 feet deep. In the 1980’s, the mine shaft was permanently sealed with a concrete cap placed over the sediment-filled shaft. Up to 15 feet of earth fill was reportedly spread across the site and over the concrete cap; however, the fill was not placed in a controlled manner. This uncontrolled fill was determined not to be suitable for the support of the proposed headstock structure. A soil investigation completed at the site included the advancement of five soil borings to a maximum depth of 41.5 feet. The borings generally identified uncontrolled fill with coal and rock fragments to depths up to ten feet. Beneath the fill, inter-layered loose to dense silty sand and stiff to hard sandy fat clay to sandy silt was identified to depths of 20 to 35 feet where the soil transitioned into partially weathered bedrock. Groundwater was observed at depths of six to 20 feet at the time of the exploration.

Solution

Due to the uncontrolled fill and erratic site conditions, helical piles were selected as the deep foundation system to support the headstock structure. The foundation design included eight isolated concrete pile caps, each with three helical piles. The design working loads for the helical piles were 15 kips in compression and 10 kips in tension. The helical pile configuration consisted of the Model 288 (2.875-inch OD by 0.276-inch wall) hollow round shaft with an 8”-10” double-helix lead section. A V-style leading edge on the 8-inch helix plates allowed the piles to penetrate through the fill and into the bearing soils. Five of the helical piles were installed at a 4V:1H batter to resist lateral loads. The piles were advanced to depths of 23 to 25 feet and to an ultimate torque-correlated soil capacity of at least 30 kips (FOS ≥ 2). Installation torque was monitored throughout the installation process using a calibrated torque transducer. The tops of the piles were cut to the required elevation and holes were drilled through the shafts to bolt on new construction brackets.

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Project Summary

Structural Engineer: Koelzer & Associates

Geotechnical Engineer: Atlantic Geotechnical Services, Inc.

General Contractor: Merricks Construction, Inc.

Certified Pile Installer: JES Construction, Inc.

Products Installed: (24) Foundation Supportworks® Model 288 Helical Piles, 8”-10” Lead Sections, Installed to Depths of 23 to 25 feet, Design Working Loads of 15 kips in Compression and 10 kips in Tension