BLASTING A TUNNEL THROUGH FOLSOM DAM

摘要

The Folsom Dam is a 340-foot-high concrete gravity dam with embankment wing dams located on the American River about 20 miles northeast of Sacramento, California. Folsom Dam is operated and maintained by the U. S. Bureau of Reclamation (Reclamation). As part of the Central Valley Project, Folsom Dam is a multipurpose facility providing water supply storage, flood control, power generation, and recreational opportunities.Studies of the dam's design revealed that an existing air intake tunnel was not providing enough air to allow full capacity water releases through the dam's eight internal outlet conduits. Due to El Nino effects, heavy rains were predicted in Northern California for the winter of 1997 and spring of 1998. To prepare the Folsom Dam for the likelihood of record or near record water releases, Reclamation determined that the dam's outlet works required repairs and some improvements. One of these improvements included excavating a small tunnel through the concrete dam in order to increase the air supply to the outlet works conduits. Since the rainy season was due to begin in less than three months, Reclamation concluded that they would need to drill and blast to ensure that the tunnel was completed in time. However, since the tunnel would pass within 40 feet of four of the dam's spillway radial gates and within 32 feet of the trunnion anchors that supported them, there was concern that blast-induced vibration might damage these critical structures.To address these concerns in a fast-track proposal, the on-site contractor retained a controlled blasting specialist to help develop a tunnel excavation and blasting plan. The plan included a test-blasting program whereby strain gauges, accelerometers and velocity transducers, mounted on various critical structures of the dam, measured the effects of small and increasingly larger test blasts. Interesting data was collected regarding the blast-induced motion in various concrete and metal structures of the dam. This paper describes the controlled blasting techniques and monitoring methods used at this project. A partial review of the measured response data and a discussion about how this information was used to prevent damage are also included.