Excavating and building twin tunnels to extend the Sound Transit in Seattle, Washington, is an extensive process requiring durable, versatile equipment that can work productively on the challenging job. Each of the twin tunnels created for the University Link Extension are 3.15 miles long and 26 feet in diameter, ultimately joining downtown Seattle with the University of Washington to its north. Contractor for the project was Traylor Frontier Kemper JV,which specializes in deep shaft and tunnel projects for highways, railroads, subways and regional transit systems. On the Seattle project, the company employed a sequential excavation method, using two pulverized face tunnel boring machines to dig the tunnels, while a Gradall XL 4300 III excavator played a key role in the process that created 16 cross-passages between the tunnels.
Once it was determined where 16 cross-passages between the tunnels would be created, the ceiling and walls of the main tunnel segments had to be reinforced and supported with propping rings on each side of the planned passage openings. Made of steel reinforced concrete, the 32 propping rings secured the finished tunnel segments while the cross-passage openings were created. After the passages were completed, the propping rings had to be carefully removed without damaging the finished stone interior of the tunnels.
"The Gradall rotating boom could precisely position a hydraulic hammer on the entire propping ring - on the ceiling, walls and floor - which was very beneficial for this part of the process," said Chris Chartrey, the Traylor Frontier Kemper equipment superintendent. An XL 4300 III Gradall excavator with 220 degrees of full boom tilt was able to efficiently demolish the 32 rings over the course of several months. A small excavator moved the debris to a box that was hauled away.
"We could have used the Gradall for the entire process, including both the demolition and the debris removal, but we would have had to stop and change from the hammer to a bucket and back," said Chartrey. "It was more important to keep the Gradall doing the demolition work. It's really the only excavator of its size with a low working profile and boom movements to fit into the tunnel and demolish the entire ring."
The low profile of the XL 4300 III also contributes to its exceptional stability, allowing it to work in any direction without the need for outriggers. "We like the mobility of the Gradall, too," said Chartrey, noting that it can move into position easily with its rubber tire undercarriage.
Chartrey had kind words for John Monaco, the Gradall regional sales manager, who worked with Pape Machinery, the Gradall distributor located in Kent, Washington. "John got us an extra-heavy-duty hydraulics hardware package for the Gradall, especially designed to handle vibrations in very tough environments like metal mills," he said. "This is a well constructed, heavy-duty machine and it performed very well."
"This machine is a great fit for the work being performed," added operator Jack Locke. "There is excellent mobility and stability on the tires, and it controls very well."
For more information about Gradall excavators call (800) 445-4752.