TUNNELING IN VARIABLE ROCK CONDITIONS: FLEXIBILITY IS KEY

摘要

This paper describes the subsurface explorations and design process for a water pipeline tunnel in highly variable weathered granite. Definitive contract language and contractor flexibility produced success in spite of highly variable ground conditions. The Lake Pleasant Pipeline project included 1,700 linear feet (518 m) of tunnel below a City of Phoenix mountain park in the northwestern part of the Phoenix metropolitan area. The 79-in. (2 m) internal diameter steel water supply pipeline required an approximately 10 ft by 10 ft (3 m by 3 m) tunnel envelope beneath the park. Subsurface investigations revealed a highly variable weathering profile in the granitic bedrock at the tunnel horizon. This necessitated a rigorous analysis of the project subsurface data in order to quantify the variably weathered bedrock into definitive "baseline statements" in the project Geotechnical Baseline Report (GBR) (Lyman Henn, 2005). The project tunnel horizon was broken into reaches, based on the severity of rock weathering, and the degree of weathering and associated tunnel length for each type of weathering was provided to bidders in the contract documents. Associated groundwater inflow rates were also provided.The successful bidder used a highly flexible approach to tunneling: roadheader excavation from the west shaft through highly weathered granite, and drill-blast-muck excavation from the east portal through variably weathered granite. Light steel ribs were used in conjunction with shotcrete to provide temporary ground support in the highly weathered granite on the western portion of the tunnel. Light steel ribs, wood blocking, and some wood lagging were used for most of the eastern portion of the project. The contractor switched to rock bolts and mine straps for some of the eastern portion of the tunnel drive. Using these two excavation and support methods resulted in a highly flexible construction approach, highly efficient excavation in the highly weathered granite and needed flexibility in excavating and supporting the variably weathered materials.