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Delve Underground’s work on hydropower projects extends beyond new tunnels and caverns. Some of the more impactful work we have performed over the years includes safeguarding existing underground hydropower infrastructure.

The resilience and reliability of our energy future depend on the continued performance of this underground hydropower infrastructure. Yet as it ages, unique challenges arise related to inspection, maintenance, and repair—accessibility, outage impacts, changing operational procedures, and difficult working conditions. These challenges and constraints have resulted in deferred maintenance and uncertainty with long-term performance.

Delve Underground has also helped utility clients evaluate underground and geohazard risks to their hydropower assets and prioritize ways to extend the life and optimize the performance of these vital assets. We specialize in comprehensive assessments and innovative restoration/rehabilitation approaches for existing underground infrastructure that ensure safety and longevity. In many cases, our assessments result in the determination that an asset is resilient, giving our clients confidence in continued reliable performance.

Snoqualmie Falls Powerhouse Shaft Infiltration Control
The Snoqualmie Falls Hydroelectric Plant, located in Washington state, consists of two power plants. Plant 1 was completed in 1899 and was constructed 270 feet (82 m) belowground. It is the world’s first completely underground hydro­electric power plant. Puget Sound Energy (PSE) recently completed extensive upgrades to expand the hydroelectric output of this historic facility, including enlargement of the existing Plant 1 intake shaft to make space for a new elevator for improved access to the underground powerhouse. The shaft experienced groundwater infiltration, which needed to be controlled and reduced to allow for safe, independent operation of the new elevator.

Shaft Drains and Shotcrete Mitigation Constructed within an Operating Elevator Shaft

Delve Underground performed a preliminary assessment of the infiltration sources, evaluated alternatives, and presented infiltration control recommendations, which included new drain holes, drain mat covered with shotcrete and plumbed to an existing riser pipe, and acrylic resin grouting of localized inflows. We prepared construction drawings and specifications, assisted PSE in selecting a specialty contractor to perform the work, and provided oversight during construction.

Boundary Powerhouse Transformer Cavern Rehabilitation
The powerhouse, adjacent to the Boundary Hydroelectric Dam in Washington, was finished in 1967. It is completely built underground inside a cliff face that makes up the left abutment of the dam. Seattle City Light (SCL) implemented a major facility rehabilitation and refurbishment program to enhance plant performance, extend operational life, and reduce the risk of major failure. Transmission lines exit from transformer bays in the rock face and extend vertically to the top of the cliff. Groundwater infiltration and seasonal freeze-thaw cycles were causing instability inside the bays. Rehabilitation of the transformer caverns was carefully coordinated with the overall facility improvements.

Unique Facility Arrangement with Transmission Cable Adits Built within a Cliff Face

Delve Underground provided rehabilitation design support to SCL and assisted with specialty contractor procurement. The rehabilitation work included removal of the existing shotcrete lining and installation of new drain holes, drain mat, and fiber reinforced shotcrete inside the caverns. Mitigation included dust control, removal of asbestos-coated shotcrete, protection of other parts of the facility, and protection of the transformers during cavern lining repairs.

Edward Hyatt Power Tunnel Inspection and Unwatering Analysis
This power plant is located beneath the left abutment of Oroville Dam, a large earth embankment dam that impounds the second largest reservoir in California. The Oroville Dam Complex was constructed in the 1960s by the California Department of Water Resources (DWR). The plant infrastructure includes an underground cavern and twin high-pressure, concrete- and steel-lined tunnels that are up to 22 feet (6.7 m) in diameter. Delve Underground planned and performed inspections of the tunnels. Nondestructive testing and ground penetrating radar (GPR) were performed to estimate concrete strength and thickness and to locate potential void spaces outside the tunnels. Delve Underground prepared condition assessments with recommendations for each tunnel.

Oroville Tunnel inspection

Planning included assessing potential structural impacts to the existing penstocks during the unwatering process and then reassessing after the inspections were completed. The additional data helped refine the structural analysis of the existing penstock concrete and steel lining during normal operations and during unwatering. This yielded important recommendations regarding the existing tunnel lining capacity and future unwatering procedures.

Eklutna Access Tunnel Rehabilitation
Chugach Electric Association operates and maintains the 47 MW Eklutna Hydroelectric Project near Palmer, Alaska. The Eklutna Adit Tunnel is a 300-foot-long (91 m), 10.5-foot-diameter (3.2 m) structure that provides maintenance and inspection access to the main 4.5-mile-long (6.2 km) power tunnel that conveys water to the power plant. Portions of the original adit tunnel lining, consisting of steel ribs and timber lagging, had degraded significantly during the tunnel’s 60-year lifespan. Locally, portions of the tunnel had collapsed. The tunnel was inaccessible due to safety concerns.

Lining Deterioration and Ground Instability that Prevented Safe Access to Power Tunnel

Delve Underground designed an adit tunnel rehabilitation to re-establish safe tunnel access. The work included a condition assessment to develop design criteria for a new tunnel lining and the entrance structure at the tunnel portal. The lining alternative selected incorporated the existing steel ribs to minimize workers’ exposure to unstable ground and accelerate the construction schedule. This allowed the project to be constructed within the short, snow-free weather window and minimize impacts to the environment and ongoing operations of the power generation facility.

The finished Eklutna Access Tunnel

Helms Pumped Storage Leakage Assessment and Tunnel Repair Design
Pacific Gas and Electric (PG&E) owns and operates the Helms Pumped Storage Project (PSP) facility in California. It is located about 1,100 feet (335 m) below grade with a series of 27-foot-diameter (8.2 m) concrete-lined tunnels, an inclined pressure shaft, and below-grade steel-lined penstocks. Since the facility’s commissioning in the mid-1980s, leakage has resulted in the rock mass and a major shear zone becoming pressurized, creating hydraulic gradients that drive seepage along preferential flow paths toward the facility’s dry excavations (i.e., access tunnels and caverns). In addition, defects in concrete-lined sections of the power tunnel that can contribute to leakage have developed during the facility’s 40-year operation.

Delve Underground is assisting PG&E with the development of measures to address the leakage issues to increase long-term reliability of the facility and strategically sequence the implementation of measures over a series of future planned outages.

System of Drains and Weirs used to Manage and Monitor Leakage in Helms