The existing reinforced concrete slab and buttress type dam, built between 1926 and 1928, is 265m long at the crest, and has a structural height of 42.4m. The dam regulates flows along the lower reaches of Stony Creek and stores about 61.7Mm3 of water for irrigation purposes.
Stony Gorge dam was built on steeply tilted sedimentary rocks consisting of thick, massive sandstone with interbedded strata of conglomerate and shale. It was found later that an inactive fault in the streambed crosses the dam axis. The dam was built prior to current standards for earthquake resistance coming into force. A study conducted by the USBR in 2002 indicated that the dam's structure could fail in a lateral, or sideways-moving earthquake of magnitude-6.0 or greater. Analysis showed a need to provide additional lateral support at the top of the buttresses as well as at the base of the dam during a seismic event.
After more studies, analyses and environmental studies, USBR awarded a US$23,956,706 contract last year to Shimmick Construction Co. of California. The ongoing contract is to provide increased lateral support by the addition of diaphragm walls, additional struts, and capital struts. The diaphragm wall, placed in each bay is a 1.8m thick wall with a maximum height of about 27m. The additional strut is a 0.6 x 0.6m strut located at the midpoint of the buttresses. At the top of the buttress, 2.4 x 2.1 x 0.5m capitals with a 0.6 x 0.6m strut will span across each bay. There are a total of 45 bays with approximately 5352m3 of concrete needed to place all of the diaphragm walls, additional struts, and capital struts.
Completion date for the seismic upgrade contract is September 2009.
