TWIN PASSAGES BURROW UNDER THE LAND OF LAKES AND AIRPLANES : MINNEAPOLIS-ST. PAUL LIGHT-RAIL LINE REQUIRED TOUGH DIGS

Renovation and construction at the Minneapolis-St. Paul International Airport includes extensive tunneling work. One job that recently broke through was the digging of twin, 7,300-ft. long tunnels for a light rail line connection the airport to Minneapolis' downtown and the Mall of America. Still to be completed is a major station and cross- passages. Work is being done in three 8-hour shifts. Officials used a design-bid-build project delivery because the airport didn't want to cede control because it involves two runways, roads and a flight center. The first tunnel boring machine was stalled by the challenging geology at the site. Glacial till lies on top of limestone and thick shales that hold in water, with sandstone below that. Stability and moisture were issues. Grouting was used to stabilize the excavation face and computerized gauges aboveground monitored soil settlement. Pre-case concrete panels were installed as liner. Mucking out was messy and wet. Fitting out the tunnels with rail and electronic components and construction of the at-grade portions of the route are proceeding next. Five other tunnels, four for vehicles and one for pedestrians, are almost complete; they are made with cut and cover reinforced concrete and capable of withstanding plane loads of up to 1.3 million lbs.