It is the eighth-busiest U.S. airport in terms of plane traffic, but Minneapolis-St. Paul International Airport also is near the top of the list for ongoing construction--especially tunneling.

MUCKY Equipment, materials and muck disposal formed constant procession. (Photo courtesy of HNTB)

Some $3 billion worth of new terminal, runway, tunnel and station work has been plugging along for seven years, says Gary Warren, director of airside development for the Metropolitan Airports Commission. "We've been spending $1 million a day since 1996," he says.

One notable highlight of all that work lies within level layers of soils filled with boulders, air voids, water and other elements sure to give tunnel contractors sleepless nights. A tunnel boring machine with earth-balanced pressure components has just completed carving twin 7,300-ft-long tunnels for an 11.6-mile Hiawatha light-rail route that will connect the airport to the Mall of America and downtown Minneapolis. Unstable limestone, stubborn glacial till and other obstacles have assured the project's place as one of the region's most difficult earthwork jobs ever.

Planes roared overhead constantly as eight muck cars traveled the 5% grade up out of the bowels of Minnesota earth, carrying some 95 tons of mashed-up soils and mud from the 500-ton, 4,000-hp TBM's last 5-ft push. Some 70 ft below Minneapolis-St. Paul Airport, a pair of construction workers placed and drilled seven puzzle-like precast concrete segments in rapid procession to form a lining ring of the 19-ft-dia light-rail tunnel. They did this almost 3,000 times.

The crews are still working on a major station and cross-passages for a slated completion in early 2005. Working three 8-hour shifts a day, the joint venture of Tokyo-based Obayashi Corp. and Litchfield, Conn.-based Johnson Brothers Corp. tunneled as far as 130 ft a day.

TEAMWORK HNTB’s Rich Johnson, Obayashi/Johnson’s Paul Zick and airport’s Pat Mosites hashed out tunnel issues.(Photo courtesy of HNTB)

Public opinion "forced us to come up with a way to build tunnels without opening up the surface," says Richard M. Johnson, project director for HNTB, Kansas City, designer for the light-rail tunnel and five other tunnels at the airport. HNTB worked with a design schedule of 11 months. "We would've liked two years," Johnson says. Two types of construction methods were offered to give contractors flexibility in bidding–tunnel boring with earth-pressure-balanced methods or a combination of boring and mining. Obayashi and Johnson opted for the TBM/EPB method with cut-and-cover sections in its $110-million contract.

Six teams prequalified for the job, but only two of them submitted bids. MAC disqualified the apparent low bidder, a joint venture of Frontier Construction, Traylor Brothers Inc. and Shea Construction Inc., because it bid as a limited liability corporation, says Johnson. A federal judge supported MAC's decision on appeal, so Obayashi/Johnson won the job.

MAC used a geotechnical baseline for risk analysis of potential claims, with the design report as a reference. The contract documents also called for deductive bid items, escrow bid documents and a dispute review board only for differing site conditions-related claims, says Hugh Caspe, national tunnel director for HNTB. "Using deductive bid items has worked well over the last 10 to 15 years in the tunneling industry," he says. Click here to view graph

Although design-build contracts are used for the rest of the light-rail route, MAC officials decided on design-bid-build project delivery for the tunnel. "Our position was that with the potential risk, including two runways, roads, a flight center, we were not comfortable with design-build and losing the control we felt we needed," says Denny Probst, MAC director of landside development. "We think it was a good decision."

DIGGING IN. The joint venture rented the German 6.5-m-dia Herrenknecht TBM and hit a learning curve. "The machine has been easy to operate," says Paul Zick, project manager for the contractor. But the geology is another matter. A glacial till layer up to 10 ft thick lies on top of weathered limestone about 25 ft thick and 3-ft-thick shales that hold in water. A sandstone layer begins about 60 ft below. In areas where rivers wore away the limestone cap, stability became an issue.

A 500-ft-long buried valley, where sandstones transition to silts, included boulders that got in the way of the TBM. HNTB shifted alignment of the tunnels to avoid boring below an active runway atop the valley. A movable zone of restricted access is applied to minimize air traffic in sensitive areas. The contractor soaked dry sandstone with foam and other chemicals, but material caked inside the TBM cutterhead, says Zick. "We lost some thrust because of the varied soils," he adds. A couple of sinkholes--up to 670 cu yd worth--caused ground loss and voids below the limestone cap.

The TBM's mixed-use face had 42 disks, with cutting foam and a copy cutter extending 2.75 in. radially. Zick says worn-out disks were replaced more than 100 times. Eight ports lubricated the TBM with bentonite and a probe extended up to 150 ft ahead to check for obstacles. The EPB component adjusted rotation and conditioned the muck to ease settlement issues.

Zick estimates that some 2,000 cu yd of steaming, heated muck was excavated daily. Consolidation and compensation grouting solidified the soils or fills subsurface voids "so that the TBM [could] sink its teeth into it," says Johnson. Above ground, computerized gauges monitor soil settlement to aid with the grouting operations. Many of these are perched on a flight center sitting atop the tunneling operations.

A conveyor system traveling 400 ft per minute, 24-ton locomotives and eight muck cars cruised in and out the tunnels, which are strung with utility lines including a 13.8-kv power cord. The precast panels are made of 8,500-psi concrete and were placed by the TBM's articulated ring erector. It took about 30 minutes to erect one ring section. County Concrete Corp., Marathon, Wisc., precast the segments.

When the TBM broke through the end of the southbound tunnel in April, 3.5-ft-dia concrete columns used as temporary supports sat at the end to greet it. The team broke through on the northbound tunnel on Oct. 26.

Despite round-the-clock work in the buried valley section, the team fell about 100 days behind schedule, but has made up all but 26 of that, says Zick.

MAC allowed the joint venture to put the TBM on vehicles and transport it across the airfield to the north portal rather than dismantling it, Warren notes. Although this required midnight use of the airfield a couple of nights, it saved about two weeks of work, he says.

Cut-and-cover box sections connect the bored tunnels at either end. The southern section is 445 ft and connects, in turn, to a 230-ft "boat" section. That is a box culvert with precast concrete slabs that act like a boat sitting in water, under hydrostatic pressure, says Johnson. The 300-ft north cut-and-cover box section connects to a 740-ft boat section that lies about 250 ft from a runway.

The contractor submitted a value-engineered proposal to use shotcrete and rock bolts for cross passages ranging from 18 to 40 ft, connecting the two tunnels. Using this method in lieu of cast-in-place should save about $90,000, says Zick.

The Lindbergh Terminal Station, located just north of the midpoint of the tunnels, is a 30-ft-high, 60-ft-wide, 500-ft-long cavern 66 ft below grade. Crews are dropping excavation from the station down to muck cars in the tunnel below through shafts 4 x 8 ft. Temporary supports of rock bolts, shotcrete and meshing will hold up the station while excavation continues down to the tunnel level, says Dan Webb, a project engineer with HNTB. Rock-bolted precast panels 10 in. thick and about 30 tons each will line the station walls. The completed 30,000-sq-ft station will have a 290-ft center platform and connect to an automated people mover going into the airport. With its 110-ft-high roof, the station will resemble "a mini-Metrodome," says Pat Mosites, MAC tunnel project director.

MAC is almost done with $95 million worth of work in other tunnels--four vehicular tunnels and a pedestrian tunnel, also designed by HNTB. They add up to 1,600 ft and lie under active runways and taxiways, built to withstand plane loads up to 1.3 million lb, says Johnson. The cut-and-cover reinforced concrete tunnels will provide access to an airport area that will be locked in by a new $520-million, 8,000-ft-long north-south runway to be built. The last of the tunnels will open in two years.