Bridge River Generating Station No. 1 (BR1) is located on the shore of Seton Lake in Seton Portage, BC. The generation station was constructed in 1948, where the powerhouse is fed by four penstocks. The water is supplied from Carpenter Lake created by Terzaghi Dam, from the tunnels bored through Mission Mountain. As the penstocks exit the mountain, they are constructed within trenches that have filled with soil and rock debris over time. To maintain the penstocks, the debris requires removal and stabilizing the trench slopes. Ecora (Engineer) collaborated with Norpac (Main Contractor), BC Hydro (Client) and TDC (Tsal’alh First Nation Development Corporation to deliver a successful project which commenced in 2022 and concluded in 2025.

The terrain has an elevation difference of 300 m over a horizontal distance of 400 m. The steepness of the site required innovative engineering solutions. As an example, a temporary flume system was constructed to transport approximately 5,000 m3 of accumulated debris down the hillside through gravity. Other practical and economical solutions were employed to address further challenges due to the steepness of terrain and the remote nature of the site.
After the debris removal, mitigation measures were required to combat erosion within the trenchers. Initially, ground anchors and shotcrete were designed to be the only methods for stabilization. As the project progressed varying ground conditions were encountered. Due to the nature of a Design-Build project and challenging terrain, Ecora and Norpac collaborated to create innovative designs for the ground stabilization.

The following treatment methods were designed and constructed:

• Stabilization of rock slopes using fibre reinforced shotcrete and ground anchors.
  
• Stabilization of soil slopes through stable excavations by Spider Excavator.
  
• Stabilization of steeper soil slopes using soil nails and wire mesh reinforced shotcrete.
  
• Shotcrete debris deflection walls.
  
• Rock bolting.

The primary objective was to stabilize steep trench slopes and remove accumulated debris around the penstocks so that the penstock maintenance could be undertaken. The challenges posed by the terrain demanded innovative engineering solutions.

The stabilization the trench slopes required long-term structural integrity and a design life of at least 50 years. Initially, the plan involved using fully grouted ground anchors and fibre reinforced shotcrete for stabilization. However, as the project progressed, unforeseen ground conditions required the design team of Ecora, Norpac and BC Hydro to collaborate closely, employing a range of adaptive solutions. These included stabilizing rock slopes with fibre reinforced shotcrete and ground anchors, and employing a Spider Excavator for stable soil excavations. Additional measures addressed steeper soil slopes, with soil nails and wire mesh reinforced shotcrete applied for stabilization, along with shotcrete deflection walls to divert debris. The deflection walls were constructed by drilling micropiles and shooting shotcrete on temporary wood formwork reinforced by threadbar and reinforcing bars. Finally, rock bolting was introduced to secure rocky sections, ensuring comprehensive slope stabilization. The project’s Design-Build approach enabled flexibility in adapting to varying ground conditions, making this innovative engineering solution efficient and cost-effective.

The debris removal and trench stabilization were completed successfully to ensure penstock maintenance started in 2024, which included concrete footing and steel repair.

The following quantities were used:

• Ground anchors (EA): 2,745
  
• Fibre reinforced shotcrete (m3): 1,260.1
  
• Soil nails (EA): 688
  
• Wire mesh reinforced shotcrete (m3): 232.0
  
• Rock bolts (EA): 22
  
• Shotcrete debris deflection wall anchors (EA): 269
  
• Shotcrete debris deflection wall shotcrete (m3): 94.7
  
• Weep holes (EA)