Best Civil Works/Infrastructure Project
The $88-million Big Tujunga Dam project was a seismic retrofit to an existing variable radius thin-arch dam.
For approximately 30 years the reservoir's operating level had been restricted by order of the California Dept. of Water Resources' Division of Safety of Dams to an elevation that resulted in reducing its storage capacity by 75%.
In 2008 the Los Angeles County Flood Control District elected to incorporate a complete structural and hydraulic rehabilitation of the dam, which is located at the base of the San Gabriel Mountains. The plan included modifying the spillway, dam crest and auxiliary structures.
One of the project's major components was adding approximately 75,000 cu yd of mass concrete on the downstream face of the existing thin-arch dam, effectively converting it into a thick-arch dam, with a thickness of 20 ft at the crest and 140 ft at the base.
The plan also included adding an overtopping ogee crest spillway with flip bucket and modifying the existing spillway.
Building the project also required 5,880 linear ft of rock anchors, ranging from 45 ft to 70 ft in length; 2,085 linear ft of rock bolts; and 12,615 linear ft of rock dowels, each 15 ft in length.
In addition, the project required installation of new 42-in., 56-in. and 66-in. fixed-cone valves; butterfly valves; a new 24-in. jet flow valve; extensions of the existing penstocks; and controls and equipment with remote data collection and monitoring.
These components of the project were contained in a new state-of-the-art gallery, valve house and control house.
In order to construct the project, the general contractor, Shimmick Construction, had to allow the flood control district to discharge stormwater around the work area without adversely affecting construction.
According to Best Projects judge Mike Gemmill, "This epic project was an engineering and construction feat that resulted in a safer and more reliable dam, while increasing the capacity to serve the growing water needs of Los Angeles."
Owner: Los Angeles County Flood Control District, Alhambra
General Contractor: Shimmick Construction, Irvine
Construction Management: Citadel CPM, Irvine
Engineer: MWH Americas, Walnut Creek