A 144-Inch-Diameter Composite Steel-and-Concrete Tunnel Liner Designed for External Heads Up to 1200 Feet

The 144-inch-diameter composite-steel-and-concrete tunnel liner for the Arrowhead tunnels below the San Bernardino mountain range in Southern California (the cover over the crown of the tunnel is as much as 1590 feet), 9 miles long, was designed for external heads up to 1200 feet (520 psi) and maximum internal pressure of 80 psi. The steel cylinders resist the internal pressure and a portion of the external pressure while the concrete cores resist the major portion of the external pressure. The special liner design required high concrete compressive strength with high modulus of elasticity and low creep properties to ensure that there would be no overstress in the steel cylinder for the long term conditions. The composite-wall tunnel liner also required concrete core reinforcement designs to withstand all pipe handling and installation forces. Structural analyses of the pipe, with a few preliminary reinforcement schemes including embedded studs, were conducted assuming a line-bearing support with 50% impact. The analysis included calculations for maximum pipe deflections and maximum stresses due to moment and thrust at the pipe invert and springline. Shear and radial tension was also checked. Two test pipe sections were manufactured (based on the final proposed reinforcement scheme) and were subjected to load-deflection tests to verify the pipe design. In this paper, the overall design concept, the summary of the structural analyses results, creep and modulus qualification tests, pipe-deflection test results, and the steel cylinder fabrication testing requirement will be presented. The advantages of this unique concept tunnel-liner design for high external heads will be discussed.