June 23, 2017
The San Vicente Dam Raise Project, located approximately 25 miles outside San Diego, was an emergency water storage project designed to increase the capacity of the existing reservoir by 152,000 acre-feet. The existing 220-foot concrete dam was raised 117 feet to 337 feet using the roller-compacted concrete technique (RCC). During the final stages of completion and certification of the dam raise, the California Department of Water Resources, Division of Safety of Dams, required coring and testing of the RCC to verify that design standards were met.
San Diego County Water Authority, Crux Subsurface
Crux provided geotechnical drilling and sampling services prior to the dam raise, and was selected to return to participate in the post-construction coring program. The scope of work included 6-inch diameter wireline coring, core extraction, development of a customized core transport system, and design and installation of specialty platform setups.
Coring began with an initial test program to determine the appropriate drill bit and rotation speed for subsequent coring. Due to the variable access conditions at the dam, multiple handling procedures and equipment types were employed to complete the coring program. Crux completed three borings with depths up to 167 feet on the left side of the spillway. The left abutment required helicopter supported drilling equipment, and helicopter portable mini cranes to assist in core handling and drill movement from one site to the next. Core samples were transported in 10-foot sections (300 lbs.) across the crest of the dam by a specialty pulley system designed and implemented by Crux.
Three additional borings were completed, two vertical and one angled, from the downstream face of the dam. Crux employed a customized drill platform designed and installed by Crux using crane support. The remaining 10 borings on the right abutment allowed for conventional access equipment. In total Crux completed 16 borings, ranging between 56 and 167 feet, for a total of 1,586 feet of drilling.
Extracting unfractured 10-foot core sections was essential to the RCC testing program. Crux had prior experience drilling 6-inch diameter cores with a triple tube coring system. However, it was determined during the test program that the industry standard core barrel system had larger tolerances that adversely affected the RCC core, causing mechanical fractures from the drilling action. Crux worked directly with the tooling manufacturer to develop custom sized inner tubes with tighter tolerances. These tubes successfully held the RCC sample stable during the drilling process.
The project was heavily monitored by San Diego County Water Authority and the Division of Safety of Dams, and required high attention to detail to ensure a safe and successful outcome. As an increased safety measure, Crux designed and installed a CALOSHA handrail system across the 280-foot spillway to facilitate crew access from the right abutment to the left. The owner chose to purchase this railing system for continued use on the dam after this phase was complete.