McMahon & Mann Consulting Engineering and Geology, P.C.

Clifton Court Forebay Dam

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Clifton Court Forebay Dam
Project Details
Location: Byron, California California Aqueduct
Client: HDR Engineering, Inc.
Client Contact: Mr. Sam Planck, P.E. HDR Engineering, Inc. 916-817-4859

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Clifton Court Forebay is a shallow 28,653-acre-foot reservoir at the head of the California Aqueduct, in the southeast corner of Contra Costa County about 10 miles northwest of the City of Tracy, CA. Earth Reinforcement Testing, Inc. served as a subconsultant to HDR, Inc., GEI, and the California Department of Water Resources as part of the inspection team and forensic investigation following failure of one of the Tainter gates along the concrete intake structure. The Tainter gates rotate with respect to a pair of hinges and are supported within the gate bays by hoist cables, and trunnion beams. The trunnion beams each support a pair of hinges, and are fastened to the gate structure walls by a set of four prestressed steel tendon rods anchored into the concrete walls. Tasks included nondestructive testing and evaluation of tendons and anchor assemblies for existing conditions, and preparation of a report describing the existing condition of the trunnion beam anchor assemblies, estimating remaining service life and recommendations for future operation and maintenance of the facility.

NDT and condition assessment were in general accordance with the protocol and test methods described in the National Cooperative Highway Research Program (NCHRP) Report 477, “Recommended Practice for Evaluation of Metal Tensioned Systems in Geotechnical Engineering.” Based on the results of NDT performed by ERTesting at this site, the remaining in service tendons do not appear to be fractured, or subject to loss of cross section greater than 25 percent of the initial cross-sectional area. Other information obtained from the results of NDT indicate loss of performance and potential deterioration of the original tendons installed during construction of the forebay. This includes relatively low remaining prestress in two of the tendons supporting Gate 3, and incomplete grout coverage surrounding all these tendons near the anchor head. Results from electrochemical testing indicate electrical connectivity between some tendons, as well as potential differences with nearby tendons that may have facilitated corrosion of tendons prior to the failure of Gate 2

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