Building a Secant Pile Ring Wall for the I-75 Access Shaft
Just north of Detroit, Michigan is a stretch of freeway approximately eighteen miles long where the planned reconstruction of I-75 Modernization Project lies. Recent years have seen flooding become a major issue, and the Modernization Project will help prevent flooding by replacing an outdated drainage system.
The I-75 reconstruction plans include the construction of a drainage/storage tunnel. Approximately 22,370 feet long, the proposed tunnel lies below the ground one hundred feet along the northbound I-75 service drive. This required the use of a Tunnel Boring Machine (TBM) known as Eliza. Eliza came to a complete halt in Madison Heights during its voyage. After considering several options, the engineering consultants decided a vertical rescue shaft would be necessary for safe access to remove Eliza and repair the damaged tunnel liner. Eliza would be encased in a vertical shaft that would support excavations up to 100 feet in depth.
Having built the rescue shaft for Bertha in Seattle and our experienced with difficult drilling conditions, Malcolm was contracted to construct a large diameter secant wall ring comprised of fifty-six secant piles approximately 150 feet deep.
The secant wall ring was constructed using 46 each, 5-foot diameter drilled shafts down to the tunnel crown. In front of, and to the sides of Eliza (TBM), the secant shafts were extended to just above the bedrock, approximately 150-feet deep; creating a horseshoe of support extending 50-feet below the planned excavation invert. Two 6.67-feet diameter “King Pile” shafts were drilled immediately to either side of the tunnel, overlapping into each end of the secant “horseshoe”. The king pile shafts, together with a heavily reinforced ring beam above the crown elevation, functioned as a door frame to transfer hoop loads from the incomplete secant arc into reaction supports above and below the tunnel. These oversize king pile shafts were socketed into rock over 70-feet below the tunnel invert.
To maintain secant shaft overlap to depths exceeding 150-feet, tight installation tolerances were required. The innovative access shaft design relied on maintaining drilled secant shaft verticality tolerances of less than 0.5% at depths exceeding 150-feet. Secant shafts must be drilled plum from the ground surface and cannot be steered down-hole during the advance. Despite this method having been widely used for excavations reaching 60 to 80-feet in depth, I-75’s secant piling method is rarely used for elements with the depth required. To ensure the strict vertical tolerances, we employed temporary steel casings, a robust guide wall and surveyed the piles using the SoniCaliper instrument.
This unprecedented challenge could only have been effectively accomplished with the involvement of the entire team throughout the planning, design, and implementation processes. From the moment Eliza was stranded to the day the final element in the shaft system was installed, it took less than six months for it to have an escape route.