Tunnel Design and Construction in a Franciscan Melange
Abstract
This paper discusses the design and ongoing construction of an inclined inlet pipeline, a drop shaft, and a 620 meter outlet tunnel in ground comprised of a variety of rock blocks and crushed matrix of a Franciscan melange. The project, situated in the right abutment of the Lenihan Dam at Lexington Reservoir near Los Gatos, California and within 3 km of the San Andreas Fault, will provide a new water outlet pipeline for the reservoir. The tunnel, designed to provide long-term maintenance access, has a straight-legged horseshoe shape with an excavated width and height of 4.3 m and 3.7 m, respectively, and is being excavated by conventional roadheader and drill-and-blast methods. A detailed description of the geotechnical investigation, interpretation of geologic data, characterization of the Franciscan, and development of a design methodology for the initial tunnel support is given. A significant challenge during the design was to develop an initial ground support system that could be quickly adapted during construction to the chaotic nature of the Franciscan, which can exhibit a full spectrum of rock behavior – from stable to squeezing - within a few excavation rounds. To accomplish this, the design includes a robust but flexible initial support system that has the capability of being modified as excavation proceeds on the basis of geologic observation, pre-excavation probing, and convergence monitoring. The tunnel final lining incorporates elements of the initial support and is designed for long-term squeezing and a groundwater head up to 60 meters.
2008
American Rock Mechanics Association (ARMA) Conference Proceedings
Shawn P. Spreng
Jacobs Associates
Michael T. McRae
Jacobs Associates
Richard M. Nolting
Jacobs Associates
H. Desai
Santa Clara Valley Water District
