BLASTING AT THE SILVER CLIFF TUNNEL APPROACHES NEAR TWO HARBORS, MN

摘要

The Minnesota Department of Transportation (MNDOT) has an extensive project to improve driving conditions and reduce hazards to motorists on US Highway 61 between Duluth, MN, and Canada along the north shore of Lake Superior. In 1991, the Lafayette Bluff Tunnel was completed just north of Two Harbors, MN, through a joint venture of Johnson Brothers Corporation of Litchfield, MN, and Obayashi Corporation of San Francisco, CA.The latest project, the Silver Cliff Tunnel, locatedapproximately three miles south of the earlier Lafayette Bluff Tunnel, provides for north and south approaches cut into a high diabase ridge, and a 1,340 foot long tunnel with two lanes for vehicular traffic beneath this ridge. The project was let in two contracts. The first was for open cut approaches to the north and south portals. The second for facing up headwalls, tunnel excavation, and installation of utilities prior to paving. Johnson Bros. Corp. was awarded the contract for the excavation and rough grading of the north and south approaches. Total excavation amounted to approximately 425,000 cubic yards. Blasting at the south approach began in November, 1991, and was a through cut into the ridge from existing road level to a height of 150 feet above the tunnel's headwall location. The north portal excavation was a side hill cut with a maximum height of 200 feet, directly above US Highway 61 that remained open to public traffic throughout the entire excavation.