''Horseshoe'' Secant Compression Shoring System For Tunnel Repair-PPT Presentation

PPT Presentation

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The “horseshoe” secant compression shoring system was developed to create a 50-foot-diameter, 100-foot-deep access shaft for repairs to a new drainage tunnel north of Detroit, Michigan. A sudden subsidence event in July 2021 sent approximately 1,600 cubic yards of flowing granular soil into the tunnel, damaging the liner and tunnel boring machine. Groundwater drawdown, void grouting, and subsurface investigation helped stabilize the site while the repair plan was developed.<br /><br />The shaft had to provide a 55-foot clear opening, with no feasible internal bracing and a tight schedule. Because the damaged tunnel liner obstructed a complete compression ring, the design used a secant-pile “horseshoe” extending to 150 feet or more. Large king piles at each end transferred loads: a reinforced-concrete ring beam supported the upper ends, while rock embedment supported the lower ends. An invert mat provided stability during tunnel breakout.<br /><br />Design involved staged geotechnical and structural analyses. Construction required exceptionally deep, large-diameter secant piles and king piles, demanding careful equipment selection, verticality control, concrete placement, and cage handling. Sonic caliper and SHAPE checks, along with completed-pile surveys, supported quality assurance. The working platform was designed and monitored for heavy equipment operating over ground affected by subsidence.<br /><br />The system’s deformation and verticality were checked during excavation, and its performance was validated after excavation and backfilling. The coordinated design and construction approach enabled mining to restart six months after the damage occurred.

Keywords

horseshoe secant compression shoring

access shaft

drainage tunnel repair

subsidence event

secant piles

king piles

geotechnical analysis

ring beam

groundwater drawdown

tunnel excavation