Micropile Underpinning of the Historic Prairie du Sac Dam Spillway – Support for the Next 100 Years

The Prairie du Sac Hydroelectric Project is an Ambursen type dam on the Wisconsin River northwest of Madison, Wisconsin. Since its original construction in 1914, the tailwater level has dropped between 10 and 12 feet, causing the tops of many timber piles supporting the 1,000-foot-long spillway to be subjected to wetting and drying cycles resulting in deterioration.
Alliant Energy contracted the design-build team of GEI Consultants, Inc., and Nicholson Construction Company to design and construct a foundation system for the spillway that replaces the support provided by the deteriorating timber piles. The remedial system consists of high capacity micropiles that form a stiff truss system and two concrete load transfer beams that can together, support the dam in the event of continued deterioration of the timber piles. This paper discusses the original construction and conditions that led to timber pile deterioration, design and construction challenges, and lessons learned from construction.