Prairie Du Sac Dam
A unique solution deployed at a uniquely-constructed dam
Prairie Du Sac Dam sits on a vast network of more than 12,000 steam-driven timber piles. These piles have proven to be durable for more than 100 years, but historic tailwater recession left them exposed to open air, which caused significant deterioration. A detailed look into the severity of the situation made it clear that remediation was needed to fully restore the historic dam’s integrity.
For more than a century, the Prairie Du Sac Dam has been a reliable source of energy for the people of Southern and Central Wisconsin.
The Prairie Du Sac Dam has been harnessing the waters of the Wisconsin River’s energy for more than 100 years. Located approximately 23 miles northwest of Madison, the Prairie Du Sac Hydroelectric Facility includes the dam, which impounds the Wisconsin River to create Lake Wisconsin, and the powerhouse, which contributes 31-MW of power to Wisconsin Power & Light’s (Alliant Energy) customer base of 470,000 people.
The dam’s spillway, one of the few hollow gravity structures in the country, is approximately 1,188 LF in length and contains 41 gates that are approximately 40 feet in height and 20 feet in width.
The entire dam rests upon a vast network of over 12,000 steam-driven timber piles, which were extremely durable for an astounding 105 years of service. But, historic tailwater recession along with subsequent wetting and drying of the piles led to deterioration as they were exposed to open air.
The Owner initiated a program to investigate the severity of the deterioration and a plan to remediate.
A thorough vetting process showed that micropiles were the best means to support the century-old dam.
The Nicholson/GEI Consultants Team was chosen as the Design/Build Contractor to perform the pre-production test program and production installation.
The dam had been under observation for tailwater regression almost immediately following its construction in 1914. Half of the regression, a full five feet, happened between 1915 - 1925. Another five feet of regression occurred between 1925 and present day.
The current efforts to remediate the dam were not the first, with a lower-downstream apron installed in 1935 to reduce scour and maintain tailwater levels necessary to submerge the pile caps and prevent deterioration. Fifty years later, in 1987, a grouting program was initiated to prevent deterioration, but investigations showed that voids remained beneath the dam’s apron.
While the full extent of the damage was unknown, the potential for settlement and, ultimately, dam failure, posed a major threat.
Many different techniques for remediation were assessed, but micropiles were chosen for several factors, including the precision with which they can be installed, high compression and load capabilities, ability to carry loads horizontally and vertically, fewer number of elements required due to higher capacity of the elements and many others. Ultimately, micropiles would create the least amount of disturbance, thus maintaining the Owners “do not harm” goal for the dam.
The scope of work included the installation of micropiles through the dam and the construction of a reinforced concrete transfer beam as a means of permanently supporting the structure.
Testing for this scope of work included two compression and eight tension load tests during the verification phase to verify assumptions and capabilities. Load tests were performed to failure or to limits of the stressing frame and the Type A micropiles’ tension capabilities were tested to 133% of design capabilities.
A total of 984 micropiles (7.625-inch diameter) would be installed in a variety of orientations and loading combinations. Micropiles would be installed through each of the four gates and installed through the three main areas (the operator deck, the spillway toe and the spillway apron. Additionally, a concrete transfer beam would provide a positive connection to the existing dam piers and buttresses and would be installed from within the dam gallery.
While the main scope of work for the project was micropile installation, the need for 248 spin-lock anchors to be installed through the piers and buttresses became apparent during an inspection. The anchors installed through piers would tie the piers together across construction joints present due to the sequence of the original construction. The anchors installed through the buttresses would pin the buttresses to the footing of the dam. Ultimately, the anchor design would “sew” the existing structure together so that if settlement did occur, it would occur evenly as one unit.
The verification and design phase of the Design/Build contracting method allowed the team to verify assumptions prior to production and to provide an efficient solution.
Though micropiles are not commonly used for dam remediation, they were the right choice for the Prairie Du Sac project.
The installation of micropiles for the remediation of Prairie Du Sac Dam was somewhat of a unique choice, but ultimately, micropiles proved to be the right choice for supporting the structure and maintaining the goals of the Owner.
Nicholson has a long history of dam remediation across the country. It’s great to be able to look into our past experience and to tap some of our in-house experts to come up with solutions that meet the unique challenges that our clients face.
DAN THOME, P.E.
Senior Vice President, Midwestern Region