''Horseshoe'' Secant Compression Shoring System For Tunnel Repair-Paper

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A sudden subsidence event during construction of a drainage tunnel beneath I-75 in Detroit damaged the tunnel lining and stranded the tunnel boring machine (TBM) about 79 feet underground. Repair required a roughly 50-foot-wide, 99-foot-deep excavation to remove and replace the TBM’s major components. Because the existing tunnel prevented a conventional compression ring from closing beneath the excavation, the project team developed a specialized support system.<br /><br />The 55-foot-diameter shaft used interlocking secant piles to form a horseshoe-shaped compression structure. The piles extended more than 150 feet deep, including below the excavation invert. Oversized, rock-socketed “king piles” beside the tunnel, together with a heavily reinforced ring beam above it, acted as a frame that transferred loads around the tunnel opening. A reinforced concrete slab provided support at the excavation base.<br /><br />Design combined geotechnical finite-element modeling in PLAXIS with structural analysis in SAP2000, with member design checked using S-Concrete and applicable ACI and ASCE codes. Construction required tight verticality control for deep, fully cased shafts, carefully managed concrete strength and pile sequencing, and detailed quality tracking through a custom GIS system. Monitoring showed maximum lateral shaft movement of less than a quarter inch after excavation, consistent with the structural analysis.<br /><br />Close coordination among the tunnel contractor, designer, and drilling subcontractor enabled the TBM to be recovered and replaced in under six months, providing an exit route for the stranded machine while limiting impacts to nearby infrastructure.

Keywords

Detroit tunnel subsidence

I-75 drainage tunnel

tunnel boring machine

TBM recovery

secant pile shaft

horseshoe compression structure

king piles

finite element modeling

PLAXIS SAP2000

excavation support system