Cowlitz Falls North Shore Collector
Owner/Lead Design Firm/Civil Engineer: Tacoma Power
General Contractor: The Natt McDougall Co.
Structural Engineer: CG Engineering Inc.
MEP Engineer: R2 Resource Consultants Inc.
Electrical Engineer: Black & Veatch
Located in the Cascade mountain range, Cowlitz Falls Dam sits in a narrow canyon across the Cowlitz River, where it has served as a power station and fish collection facility for 24 years. The $32.3-million North Shore Collector project upgraded the salmon runs, improving fish passage returns to 95% from 50%.
The project cut two penetrations through the dam: the fish entrance’s 18-ft by 26-ft opening and the 10-ft by 10-ft cut for the reservoir pumpback system. Crews installed about 20 pin piles, or small-diameter pipes, to support the existing fish facility during construction. The new facility also required a 60-ft-tall soldier pile wall with tiebacks and a foundation supported by drilled shafts.
The dam, powerhouse and fish facilities also needed to operate during construction.
The project team designed a 45-ft-deep sheet pile cofferdam to protect the area while crews built the two penetrations. Crews drilled perimeter holes, then cut through with diamond-bladed wire saws before breaking the concrete. This painstaking process helped limit vibrations beneath the functioning power station, says Natt McDougall, president of general contractor The Natt McDougall Co.
“You’ve got a structural facility, and you’re underneath it breaking concrete,” McDougall says. “You’re trying not to cause too much rattling and throw something offline.” The team shoehorned the North Shore Collector in, under and alongside existing facilities without disrupting ongoing operations.