Part of the largest Superfund complex in the United States, the Milltown Dam started accumulating mining waste almost immediately following its construction in 1908.
The dam was scheduled to be removed in early 2008 as part of the adjacent Milltown Sediments Superfund Site project, part of the remedy to improve ground water and eliminate a contaminant source associated with the river sediments. Before the dam could be breached to cause river levels to drop, however, the upstream I-90 bridge foundations needed to be stabilized.
Conti Federal worked on a tight schedule and through extreme weather conditions to underpin the I-90 bridge foundations. The team drilled 16 caissons through two existing piers, constructed a cofferdam and poured an enlarged footing for the piers, all on top of a high river and underneath an active highway. Conti Federal used a unique design and implementation of flexi-flow bridges to provide temporary work platforms in the river, allowing for 360-degree access to each pier.
Work included caisson drilling and concrete pouring underwater and took place in cold temperatures within the Blackfoot River, which froze over during the project. The team used ground heaters to melt ice as needed and took necessary safety precautions at all times according to our site-specific Safety and Health Plan.
Conti Federal used a variety of unique equipment and technology due to site conditions. This included LoDril attachments on excavators for caisson drilling, concrete pump trucks and tremie tubes for pouring concrete in caissons underwater and crosshole sonic logging for testing concrete in caissons for anomalies.
Key features of the project included:
“Conti was very proactive in problem-solving and communication. The company always provided cost-effective options and alternatives to any problem.”
– Kurt Anderson, Project Engineer, USACE
