CS36A - Concurrent Session 36A: From Severe Deterioration to a “Butte”: Rehabilitating an 1890s Dam with Post-Tensioned Anchor Bars and a Concrete Overlay

Abstract Description: Basin Creek Dam No. 1 was originally constructed in 1897 as an 88-foot-high curved masonry dam. The dam has undergone several modifications over its lifetime. In 1913, mass concrete was added to the downstream face and crest, and in the 1930s, an earthfill embankment was placed on the downstream slope to enhance stability. In 2006, additional work was completed on a new concrete spillway with an operable crest gate and rehabilitating the outlet works.

During the 2019 5-year inspection of dam, significant freeze-thaw deterioration of the mass concrete was identified. This subsequently resulted in a 5-year undertaking to rehabilitate the dam to address stability concerns and prevent further freeze-thaw damage to ensure the safety of Butte, Montana’s primary water source.

Owned and operated by the City and County of Butte-Silver Bow (BSB), Basin Creek Dam provides 60% of Butte’s drinking water. To safeguard this vital resource, BSB partnered with Great West Engineering and Gannett Fleming to design and implement an effective, cost-efficient design for the rehabilitation.

The design team evaluated both post-tensioned anchor strands and bars. Due to the shallow depth of the critical failure plane and the small anchor design loads, bar anchors were determined to be the most cost-effective option. Given the challenging site access, the team opted to perform sampling and testing during construction by coring pilot holes during PTA installation. The assumed conditions of the concrete, masonry, and bedrock were confirmed prior to installing the PTAs. The concrete overlay was installed to protect and prevent additional freeze-thaw deterioration of the mass concrete portion of the dam while also protecting the heads of the PTAs.

To facilitate the work, Basin Creek Reservoir was lowered, and barges were used for demolition and construction access. A helicopter was utilized to place the anchor bars, significantly reducing instillation time. Construction was successfully completed in one construction season, avoiding a winter shutdown, and minimizing disruptions to BSB’s water supply.

This presentation will discuss the challenges of rehabilitating a historic 1890s dam, including difficult construction access and unique design elements, while trying to minimize the impacts to the water supply. Lessons learned from this project provide valuable insights for the rehabilitation of aging infrastructure critical to community resources.