CS35A - Concurrent Session 35A: Underpinning Prairie: Micropiles Support Prairie Du Sac Dam - Part Two

Completed in 1914 and located a little over 1.5-miles due North of Prairie Du Sac, Wisconsin, the Prairie du Sac Hydroelectric Facility comprises Prairie Du Sac Dam (“the dam”) and powerhouse. The dam impounds the Wisconsin River creating the 9,000-acre Lake Wisconsin, and the powerhouse contributes 31 MW of power to Wisconsin Power & Light’s (Owner) customers in Southern and Central Wisconsin. The dam’s spillway, one of the few hollow gravity structures in the United States, is approximately 1,188 lin. ft long with 41 gates, each approximately 40 ft high and 20 ft wide. The dam sits entirely upon a vast network of over 12,000 steam-driven timber piles.

Most of these original timber piles have proven durable over their 105 years of service. However, deterioration of some piles did occur after piles were exposed by historic tailwater recession and associated wetting and drying cycles. The pile deterioration posed the potential for more than 5 inches of settlement (including differential settlement) and concern for spillway cracking. The Owner selected the team of Nicholson Construction Company and GEI Consultants as the Design/Build Contractor to evaluate foundation solutions, conduct design analyses, perform the pre‑production test program, and execute construction. After extensive analyses, the team chose a micropile solution based on a variety of selection criteria. In 2020, Nicholson presented Part One of the project including the micropile analyses and design, the initial testing phase program, and start of construction activities.

This Part Two presentation will outline the project constraints, summarize the installation process, and discuss challenges encountered during the micropile installation process. Although not a stranger to the dam foundation remediation, micropiles are not as commonly used to underpin dams as was done for Prairie du Sac Dam. Micropiles proved to be an effective way to support the existing structure while managing the goals and concerns presented by the Owner.