THE WORLD'S LARGEST-DIAMETER SOIL TUNNEL

When completed, the Mt Baker Ridge Tunnel now under construction in Seattle, will form part of Interstate Highway 90 and will be the world's largest diameter soil tunnel with an inside diameter of 63 ft. Over its 1332 ft length it will accommodate five traffic lanes comprising three westbound lanes, two bus/carpool lanes and an overhead pedestrian/bicycle path. All five lanes were placed in the single bore for environmental and minimum right-of-way acquisition requirements. The bored tunnel will be excavated through a 200 ft high ridge of glacially overconsolidated clay, silt and sand, by a "stacked-drift" method in which an articulated or flexible tunnel lining, forming a compression ring is constructed, followed by removal of the interior soil core. The method allows the use of relatively inexpensive machines for excavation and also minimises the size of excavation open at any one time, so reducing disturbance and settlement problems. (TRRL)