When the 12.5-mile stretch of Interstate 70 passing through Glenwood Canyon in Colorado was finished in 1992, the highway was hailed as the jewel of the interstate system. More than 30 engineering and design awards, including the prized Outstanding Civil Engineering Achievement Award from the American Society of Civil Engineers in 1993, honored this environmentally sensitive achievement. Forty viaducts and bridges and the 4,000-foot-long Hanging Lake Tunnels carried the interstate through the air, offering spectacular views and preserving the canyon below.
However, rockfalls challenged this engineering marvel, causing a 70-foot-long crack two-thirds of the length of the ceiling slab over the eastbound lanes of the Hanging Lake Tunnel. The Federal Highway Administration discovered the crack during a routine inspection in July 2006 and kept an eye on it while the Colorado Department of Transportation banked funds for an anticipated repair. When the crack lengthened to 80 feet, widened to 1-1/2 inches, grew 4-1/2 feet deep and leaked, CDOT closed the tunnel on March 30, 2007, and negotiated an emergency contract for repairs with Lafayette, Colo.-based Concrete Works of Colorado Inc.
Because Concrete Works was already scheduled for full-depth concrete pavement reconstruction, replacing aging asphalt sections and bridge deck rehabilitation work in Glenwood Canyon, the firm was able to ramp up immediately and meet with CDOT on the crack repair solution. The highway rehabilitation project between the No Name overpass and Grizzly Creek rest area began May 7 and finished 22 days early in late June, earning a $132,000 early completion incentive payment. Both this project and the crack repair shared the same traffic control detour, shifting the estimated 20,000 daily vehicles and extending the detour to direct both lanes of traffic through the westbound tunnel bore.
"We responded on the fly, and have been working on the repair puzzle piece by piece," says Dick Brasher, Concrete Works vice president. "This project's out-of-the-box thinking and solutions have required a collaborative approach between the CDOT and Concrete Works team."
Marc Lenart, owner of Concrete Works, has demonstrated his company's commitment to this important project by taking charge of the construction since day one and being instrumental in developing solutions to the complicated excavation effort and other project details.
The repair sequence involved installing three strain gauges to monitor any changes in the crack, installing structural steel shoring, excavating the rock and dirt, grouting the crack, stabilizing the slab with a top and bottom slab, modifying drainage, and then returning the site to its original condition with backfill up top and restored tunnel surfaces below.
Expected and Unexpected Challenges
One might question, "Why is it taking so long?" Gary Coffey, Concrete Works site superintendent, hosted RMC on a tour to witness first hand the enormous undertaking and better understand the logistics.
The Hanging Lake Tunnel cuts through two rock ridges. In between the ridges sits the Cinnamon Creek Complex, consisting of a five-story buried traffic control center and transition tunnels. The crack is in the ceiling of a transition tunnel over the eastbound lanes. Probable cause of the crack is 500 cubic yards of rocks, some as big as a van, that fell on top of the concrete slab ceiling of the transition tunnel in 2002, according to Joe Elsen, P.E., program engineer with CDOT Region 3. Elsen has a long history with the tunnel and the Glenwood Canyon highway from working 11 years on Interstate 70 construction through this sensitive environment.
Removing 25 million pounds of rock to reach the slab in this limited access area and repairing the damage would challenge both the engineers and the contractor. "In this limited space, everything from the equipment to the repair processes has to go single file," Coffey remarks.
A number of temporary structures would need to be built for the repairs to go forward. The first temporary structure was shoring. Concrete Works immediately called on Denver structural engineering firm SGD to design a shoring system to stabilize the slab during excavation. Massive 24-inch steel beams were pressed up against the bottom of the slab. Four vertical 12-inch and 14-inch steel columns extending down to the roadway surface were welded to each transverse beam. Concrete Works crews installed 210,000 pounds of structural steel in these 12 shoring bents every 8 feet on center.
"The only access to the slab above is by railroad cars and a steep billy goat trail," Brasher explains.
This necessitated more temporary construction — a road down to the railroad tracks. Concrete Works had to size the excavating equipment that included a Volvo 30-ton and a 40-ton truck, Caterpillar 345 and 365 excavators, and Cat 953 crawler loader to fit through the railroad tunnels. The Cat 365 was too big, so they removed the bucket and the stick and moved the tracks in on the sides. Indeed, shipping the rock out by railroad car was simpler than bringing in the equipment.
To protect the workers from rockfall from the sheer cliffs towering over the excavation area, Concrete Works stationed a spotter in the doghouse, an outhouse-like structure overlooking the excavation. Fortunately, the bullhorn never sounded to warn of falling rocks, according to Coffey. The excavators dug as deep as 82 feet from the ground surface to the top of the damaged slab. Average depth of this rock and soil cover totaling 16,000 cubic yards was 40 feet to 46 feet. Because this rock and soil would be needed for backfill once the repair was completed, it was stockpiled up to 40 feet high in the limited working space on site.
"When we finished the excavation, the slab actually lifted up from the shoring, as the tremendous weight was deflecting it downward!" Brasher exclaims.
"The repair Staff Bridge designed is actually a concrete sandwich," observes Pete Mertes, P.E., CDOT resident engineer.
Above the 4.5-foot slab is a 12-inch-thick poured reinforced concrete slab and below a 6-inch-thick reinforced shotcrete slab. From July through August more than 1,000 2-inch-diameter holes were cored through the 4.5-foot slab to accommodate the No. 7 reinforcing steel in that holds the "sandwich" together.
A complication encountered in setting the rebar is that the ceiling slab actually varies in thickness from the originally planned 4-1/2-foot thickness. This meant every piece of rebar had to be custom cut so it's all at the same elevation above the slab. Then an elbow joint was added and the horizontal rebar trimmed to fit, according to Coffey. In all, 813 holes were 4.5 feet deep, 140 holes 6 feet deep and 140 holes 2 feet deep.
Concrete Works built another temporary structure, a working deck, to grout the rebar and build the bottom shotcrete slab. To prevent air bubbles, the rebar was grouted from the bottom up. A rope was threaded through an adjacent hole so a worker up top could signal the worker below when to stop pumping grout. After the last of the holes were grouted, communication from the top side was by walkie talkie.
Once the rebar was grouted and horizontal rebar mats tied in, the top 12-inch-thick slab was poured and a 6-inch slab shotcreted below. The concrete pumper sat in the tunnel below and concrete was pumped up through a 10-inch-diameter hole to the top.
Subcontractors working with Concrete Works of Colorado include CPSS (Concrete Pavement Saw and Seal), coring holes, and WLH (Warren Harrrison Construction Co.), grouting crack, core holes and the shotcrete effort. "We feel we have pulled together the most qualified team possible to accomplish this unique repair and couldn't be happier with quality of the work being done and the great teamwork being exhibited," Brasher observes.
The new concrete slab is waterproofed internally. The CDOT Class D concrete for the top slab contains an admixture called Xypex®. This additive should provide nearly 100-percent waterproofing, according to Brasher. Shotcrete containing Xypex was also applied to the sides of the existing slab to seal it from water infiltration.
At the end of the project, Concrete Works brought in a Powerscreen by railroad to produce 2-inch-minus aggregate for the first 3 feet of backfill. On top of this crews placed a 10-foot cushion of high density Styrofoam, then the remainder of backfill including layers of reinforcing fabric.
To lessen the occurrences of future rockfalls, modifications to the Cinnamon Creek drainageway, located directly above the tunnel and the repair area, were made in conjunction with the repair effort. Workers installed 450 linear feet of 48-inch reinforced concrete pipe along with new manholes and water control diversion gates. The concrete for the drainage work came in 1-yard buckets by helicopter, and the pipe came in by rail.
The entire work effort has required close teamwork by all involved, including CDOT project staff: Dana Christensen, P.E., and Mike McMullen, P.E., of Staff Bridge; Roland Wagner, P.E., project engineer; Adam Conely, assistant project engineer; Ty Ortiz, of the CDOT Rockfall team; Will Mischlich, project inspector; Fred Kirschbaum, materials technician; and the CDOT Hanging Lake Tunnel maintenance staff. Joe Elsen adds the names of two instrumental Parsons Transportation Group (PTG) engineers, Ralph Trapani, P.E., and Brad Doyle, P.E. (On the 1980s–90s construction of I-70 through Glenwood Canyon, Trapani worked for CDOT as program engineer.)
Concrete Works completed the repairs on schedule and reopened the tunnel to traffic on Oct. 3 while work continued up on top to correct drainage issues and backfill and seed the site. CDOT was then estimating the total cost to be $6 million.