Located in Sterling Heights, Michigan, the sewer tunnel was 11 feet in diameter and approximately 50 feet below grade along a stretch of 15-Mile Road.
The original sewer was constructed with unreinforced concrete, up to 18 inches in thickness. As part of the re-lining project, it was decided to locate construction activities in a utility corridor to minimize disruption to traffic. The utility corridor contained multiple overhead high-voltage power lines and was located off to the side of 15-Mile Road. Ground conditions dictated the use of a cutoff wall; however, the overhead power lines restricted the equipment which could be used. The resulting design consisted of an access shaft with two different wall types.
The upper 42 feet was composed of ring beams and wood lagging, while the lower section included reinforced concrete secant piles. Conceptually, a large rotary drill rig could fit within the upper shaft in order to construct the lower circular secant pile shaft.
Schnabel Geostructural Design & Construction was hired by Oscar Renda to install the 34.5 foot inside diameter secant pile shaft. This work consisted of 60 overlapping secant piles ranging from eight feet to 51 feet in depth. Shallow drill depths of 8 to fourteen feet followed the top radius of the tunnel. Once clear of the tunnel to the sides, 38 feet deep primary shafts and 51 feet deep secondary shafts were drilled below subgrade to ensure basal stability of the excavation. Reinforcing steel consisting of W21x182 wide flange sections were placed in secondary shafts.
ASSEMBLY & JOB PREP
Months of planning from an equipment and safety standpoint went into this project. The unique construction technique of low-overhead secant piles had many obstacles to plan around. First was lowering a 200,000 pound drill rig in sections into the upper shaft. Second was assembling the rig with a 70 foot tall mast inside of a 45 foot wide shaft. Third was constructing the secant piles with limited access for people, support equipment and material installation.