Construction of the Dougherty Valley Tunnel, San Ramon, California, USA

摘要

The Dougherty Valley Tunnel and Trunk Sewer project has been constructed by the Central Contra Costa Sanitary District (CCCSD) to provide wastewater services for the new Dougherty Valley residential development, consisting of approximately 10,000 new homes in the southern part of Contra Costa County, California. The tunnel and trunk sewer system conveys wastewater by gravity flow from the new residential development to a pumping station in San Ramon, California. The tunnel was constructed through a northwest-trending ridge between the Dougherty Valley and the San Ramon Valley from a portal site on the west side of Dougherty Road to a shaft located on the east side of Alcosta Boulevard. Cover above the tunnel ranges from about 20 feet at the East Portal and West Shaft to a maximum of about 320 feet near the mid-point of the alignment. The tunnel is 10 feet in diameter and about 4,640 feet long and was constructed through weak sedimentary rock (sandstone, siltstone and claystone). The tunneling was performed with a Technicore Earth Pressure Balance Tunneling Machine (TBM). Steel ribs and timber lagging were installed for initial support. The hole through occurred on April 23, 2002. The tunnel was classified as gassy by Cal OSHA. Three pipes were installed in the tunnel including a 60-inch inside diameter (ID) reinforced concrete sewer pipe (carrier pipe), and two 12-inch ID ductile iron bypass pipes. This paper discusses construction of the Dougherty Valley Tunnel and Trunk Sewer project.