Long-duration energy storage is creating new construction opportunities around specialized civil and underground infrastructure, with Granite taking an early site-development role on a major California project. Granite has been awarded an approximately $31 million early works grading package for Hydrostor's Willow Rock Energy Storage Center in Kern County.
The planned Willow Rock facility is a 500 MW advanced compressed-air energy storage project located north of Rosamond in Kern County. The California Energy Commission approved the project in December 2025, and the development is currently in the pre-construction phase.
Granite's package covers early infrastructure across the approximately 89-acre site. The work includes preparing a nine-acre pad for underground construction, improving offsite roads, building stormwater basins and establishing laydown areas for construction materials.
A major component of the subsequent underground work will involve excavating a cavern with approximately 1.3 million cubic yards of material at a depth of about 2,000 feet. The site preparation will support the subsurface contractor as well as contractors working on the above-ground energy storage facility.
Granite began its early works grading activities in July 2026, with completion planned for March 2027.
Willow Rock is designed to store electricity using advanced compressed-air energy storage technology. The planned system is rated at 500 MW and is designed to provide up to eight hours of electricity, equivalent to 4,000 MWh of storage capacity.
The project is expected to become Hydrostor's first utility-scale U.S. facility. The developer has also secured electricity offtake agreements as construction activity advances, including a 60 MW agreement announced in August 2026.
The project is designed around underground compressed-air storage and above-ground turbines. Its development is part of California's broader effort to add long-duration storage capacity to support grid reliability.
Granite's early works package positions the contractor at the beginning of a complex energy infrastructure development involving surface grading, transportation improvements and underground excavation.
For construction owners and civil contractors, the project illustrates the range of site and infrastructure work required before specialized energy-storage systems can be installed. The California Energy Commission lists Willow Rock as a 500 MW project in pre-construction, providing a regulatory milestone for the development as site work proceeds.
Source: Granite Construction.
