Secant Walls are formed by constructing reinforced concrete or steel piles very similar to tangent walls with one main difference; the wall is interlocking, providing a waterproof barrier. Secant walls are used when water retention is required. Using a low vibration system, each pile is drilled several inches into the adjacent pile next to it. This creates the waterproof barrier needed to keep many deep excavations dry during construction.

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Project Details

Davis-Besse Nuclear Power Plant – Borat Water Storage Facility located in Oak Harbor, OH was a project Hardman worked on for Rudolph-Libbe. It consisted of a drilled secant wall for a 25’ excavation with internal bracing and 30’ drilled shaft, designed with loading for a full seismic event. We drilled the secant wall with a Low Mast for part of the project due to overhead power lines.

The City of St. Joseph formed a multi-year strategic plan for updating its drinking water system. One of the first phases of the plan includes creation of a new pump station. The New Pump house building was to be set into a large hill that would need to be supported by ERS for the building to be constructed. A drilled H-pile and wood lagging would be the most economical. The wall would span 180 lineal feet with and have a 24-foot cut. The 24’ cut would not allow for the wall to be a cantilevered ERS system. Tiebacks would be installed to help support the cut. The wall consisted of 12×53 h piles at 8 feet on center with a single row of 100-kip tiebacks. Within the barrier of the lagged wall, a secant wall was constructed to support the pumps that would be 25 feet below the pumphouse building.

Secant walls are a type of earth retention system that creates a water tight barrier. This is accomplished by drilling overlapped concrete piles, with a reinforcing steel beam placed in every other pile. A tremie seal is then poured at the bottom of the wall, ensuring no groundwater can enter the area. This secant wall was 45 feet long x 15 feet wide x 25′ feet deep. The Bottom 5 feet would include a 5′ tremie seal.

Being less than 100 feet from Lake Michigan gave the crew some pretty great views while working, but the proximity to the lake also ended up throwing them some major challenges. First, the excavation depth would be 25 feet total and roughly 15 feet below the lake Michigan water elevation. The secant wall was supported in part by the 14×73 beams at 4.5 feet on centers and a 14X89 waler with 2 cross struts 5 feet below grade.The drilled depth, Soils composition, coupled with the proximity to a large body of water made for constant water issues for the job site. Ground water is the main reason for the ERS selection to be a secant wall was versus other earth retention systems with a conventional dewatering setup. Additionally, the job ran from November-February. The brutal cold and gales off Lake Michigan made for some very harsh working conditions. Even with unfavorable conditions and tight timeline, the crew overcame them and produced high quality products.