Upper Gorge Power Plant


Mono County, California

Scope of Work:

  • Geotechnical Engineering
  • Geophysical Evaluation
  • Data Analysis
  • Laboratory Testing
  • Written Report

Ninyo & Moore performed a geotechnical evaluation for the bypass structure to be located at the existing Los Angeles Department of Water and Power (LADWP) Upper Gorge Power Plant (UGPP) located in the Owens River Gorge in southern Mono County, California. The project includes construction of a 4 megawatt (MW) Hydroelectric Bypass Plant at the current UGPP. When the UGPP is offline, the new plant will use water bypassing the UGPP to generate energy that will be delivered to the grid through existing UGPP infrastructure. Currently, the project design includes construction of a new powerhouse to house a Pelton wheel turbine, 4MW generator, switchgear, and control equipment. This equipment will be electrically tied to the existing UGPP step-up transformers.

Project Challenge: A major challenge at this site was that the location proposed for the new structure was located in a 600-foot deep steep canyon bottom inaccessible to vehicles and thus to regular drill rigs. In addition, the site is underlain by a thick rubble fill mostly composed of large (3 to 10 foot diameter) boulders.

Innovative Solution: To access the site, we contracted a portable limited-access, rock-core drill rig and had the rig lifted over the existing structures with a crane. The rig was used to core through the boulder rubble fill at several locations to depths up to approximately 65 feet to observe the condition of the fill and the nature of the underlying quartz monzonite bedrock. Geophysical surveys were also used to evaluate the deep boulder fills.

Services: Ninyo & Moore's services included background review; geologic mapping of exposed bedrock in the vicinity of the proposed construction; performance of a subsurface exploration consisting of the drilling, logging, sampling, and televiewing of rock core borings; performance of a geophysical evaluation consisting of the performance of P-wave refraction profiles and refraction microtremor (ReMi) surveys; and preparation of a report presenting our findings, conclusions, and recommendations regarding the geotechnical design and construction of the project.

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