SAN FRANCISCO (AP) Construction crews are preparing to dig a 5-mi.-long (8 km) tunnel beneath San Francisco Bay to bolster an aging water system that serves 2.5 million people.

The new tunnel will replace two old pipes used in the Hetch Hetchy System, which moves water 167 mi. from Yosemite National Park to the Bay Area.

Officials began seeking bids for the $347 million project July 31.

Construction crews will bore a 14-ft. (4.3 m) high corridor through bedrock, sand and clay beneath the bay before running a 9-ft. (2.7 m) high steel water pipe. The project is scheduled to begin next spring and be completed by 2015.

Experts say there is a 63 percent chance of a major earthquake in the Bay Area within 30 years, which led voters to approve the project in 2002.

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