Control of 24-h blasting noise during construction of light rail tunnels in Portland, OR

In 1993, the Tri-County Metropolitan Transportation District of Oregon (Tri-Met) began construction of the Westside Light Rail project in Portland, OR. The project included the construction of a 3-mile-long twin tunnel through the hillside west of downtown. The original plan called for the tunnels to be excavated from the east using a tunnel boring machine that was supposed to be quick and quiet. However, soon after excavation began, the composition of rock along the tunnel path began to cause problems for the boring machine. Excavation fell behind schedule. Tri-Mets contractor decided that a second excavation team needed to move from the west end of the tunnels toward the boring machine to ensure the project would stay on schedule. Tri-Met requested permission to allow the contractor to conduct 24-h blasting in the vicinity of residences to help catch up in the excavation phase. Daly-Standlee & Associates, Inc. (DSA) assisted in defining noise criteria that could be used to ensure blasting noise would be acceptable at residences and, once blasting began, DSA assisted in developing mitigation measures that could ensure the criteria were met. This paper presents the experience gained by the firm during the tunnel boring project.