CHANGING BUNYARD FOR THE BETTER

This article describes the design and construction of the Bunyard Tunnel in Arkansas, USA. It will carry part of the relocated limited access Highway 71 through the Ozark mountains. It was decided to tunnel, as the alternative was a long, very deep and wide cutting which would have destroyed the local scenery. There will be two parallel reinforced concrete lined, mined rock tunnels. Each will be about 1375ft long, and run nearly 200ft below Bunyard Ridge; it will accommodate two 12ft traffic lanes and two shoulders within a 38ft wide roadway, and require an excavation approach 45ft wide and 29ft high. The preliminary design was conducted in 1992, and used subsurface excavation borings to 300ft deep. The rocks include limestone, sandstone and shale, and there is a thin surface layer of clayey silt with sandstone cobbles. As originally presented, the construction project was to use the New Austrian Tunnelling Method (NATM) in four classes of ground. The contract was awarded to J.F. Shea and excavation at the south portal began in April 1996. The excavation procedure was changed from NATM's multiple heading drift and bench system to a full steel support system with a full top heading. Bench excavation began in both tunnels in March 1997. The project is due to be completed on time, on budget, and with an excellent safety record.