Los Vaqueros Reservoir

Los Vaqueros Reservoir


Los Vaqueros Reservoir Construction
MSE Precast Retaining Walls
Location: 
Brentwood, CA
Owner: 
Contra Costa Water District
Contractor: 
Barnard Construction Company

 

Los Vaqueros Watershed - Reservoir  Expansion

 

Location: Brentwood, CA

The Contra Costa Water District (CCWD) was formed in 1936 and is responsible for the provision of clean, safe water to approximately 500,000 residents of Contra Costa County in northern California.  The Los Vaqueros Dam was constructed by the Contra Costa Water District in 1998 to create the Los Vaqueros Reservoir.  The Los Vaqueros Watershed is located near Brentwood, California at 37.82025°N -121.7338°E and encompasses nearly 20,000 acres of open space around the Los Vaqueros Reservoir. Not only is the Los Vaqueros Watershed a recreation area for hiking, fishing, and other outdoor activities, it is also a cattle and sheep grazing area, and serves as home to many rare and endangered species such as the golden and bald eagles as well as the San Joaquin Kit Fox.

When salinity levels are low at the Sacramento San Joaquin Delta, water is pumped to the reservoir to be used for drinking water while the delta is too brackish for use.  This occurs periodically when fresh water runoff from the Sierra is reduced, usually in late summer and early fall, and salt water flows into the Delta from the San Francisco Bay.  Fresh water can then be released via pipeline to the Contra Costa Canal for distribution to water treatment plants in the Contra Costa Water District’s region.

In 2011 and 2012, the Los Vaqueros dam height was raised to a height of 226 feet in order to increase the storage capacity of the reservoir from 100,000 to 160,000 acre-ft.  The resultant is an increase in height of 34 feet over the originally constructed dam.  The expansion of the dam was carried out by the Contra Costa Water District to develop water supplies for environmental water management purposes and the improved reliability of the water supply to the San Francisco Bay area.  The cost of expansion of the reservoir will be passed on to the district’s customers.

Barnard Construction Company, Inc. was tasked with performing the work of expanding the reservoir, including increasing the height of the dam, construction of the mechanical and electrical buildings, installation of the necessary instrumentation and controls, and restoration of the roads.  This was accomplished by working double shifts on an accelerated schedule to construct the Mechanically Stabilized Earth (MSE) walls and place 1,000,000 cubic yards of zoned embankment and thousands of cubic yards of roller-compacted concrete and cast-in-place concrete.

Historically, MSE walls have performed very well under submerged conditions and in areas subject to earthquakes such as the Los Vaqueros Reservoir site.  The expansion of the Los Vaqueros dam included the construction of two MSE walls that were used to provide a wider crest of the dam.  The Reinforced Earth Company (RECo) was contracted by Barnard Construction Company, Inc. to design and supply the MSE walls for the project.

The Reinforced Earth Company’s GeoMega™ MSE wall system utilizes precast concrete facing panels with GeoStrap® (high tenacity polymeric strips) as soil reinforcements.  In the past, the GeoMega system has been used on projects in marine environments where potential metal loss conditions make steel soil reinforcements less favorable.  The walls constructed for this project, which reached up to 49’ in height, were designed to be nearly completely submerged during a 100 year probable maximum flood.  Therefore, highly permeable gravel back fill was utilized within the MSE mass to accommodate exposure to fluctuating conditions resulting from changing water levels in the reservoir.  Due to the location of the dam clay core and unknown existing bedrock elevations at the MSE wall locations, the soil reinforcements were designed with a trapezoidal distribution which required field-modification of the soil reinforcements to match the achievable excavation for the construction of the walls.  All of the reasons above led to the selection of the GeoMega system for this project.  Although the achievable limits of excavation were not known when the initial design was performed, close coordination between Barnard and the design staff at RECo allowed real-time analysis of the actual field conditions to enable the contractor to make changes during the construction process while maintaining the project schedule.

RECo was also contracted by Barnard to supply the precast concrete barrier and coping segments that sit atop the MSE walls.  When compared with the traditional construction of barrier and coping atop MSE walls, the use of 10’-0” long precast concrete barrier and coping segments enabled the contractor to save considerable time and resources.  The use of precast traffic barrier sections meant that there was virtually no labor required to construct platforms and fall protection, form the coping and barrier, tie the concrete reinforcement, and pour the concrete in the field.  Instead, precast units were shipped to the job site ready for installation.  These labor and materials savings, combined with the fact that installation of the precast traffic barrier segments is generally not hampered by climatic conditions, made The Reinforced Earth Company’s precast barrier and coping sections a great fit for this project.

Of course numerous challenges such as a fast-paced construction schedule and unknown location of bedrock were encountered during the design and construction phases of this project. The flexibility of the RECo’s GeoMega system, however, allowed the experienced personnel at The Reinforced Earth Company to overcome these challenges to complete another successful MSE wall installation.