MANY BORES FORM A SINGLE TUNNEL: RECORD-DIAMETER SOFT-GROUND JOB HAS MULTIPLE PERIMETER DRIFTS.

Workers are mining out the world's largest diameter soft-ground tunnel using a novel technique and bringing the job in early at less than half the cost estimated by the Washington State Department of Transportation. The 63-ft-dia bore through Mt. Baker Ridge in Seattle provides for widening Interstate 90 through a congested area. Despite early problems, the contractor expects to complete the 1,332-ft tunnel early in February, six months ahead of schedule. Passing 110 ft under the crest of the ridge with only 10 ft of cover at the ends, the tunnel runs mostly through highly consolidated silty clay varying from laminated to massive stiff material. Designers opted for multiple drifts around the perimeter, lined and filled with concrete to form a compression ring. Since drifts are 9 1/2 ft wide, the outside diameter of the ring is 82 ft. Drifts have 5-ft contact surfaces between them and are not connected by reinforcing steel, so the ring is somewhat flexible.