University Link project will add more than three miles of subway light rail between downtown Seattle and the University of Washington.
In late 2008, Sound Transit began its University Link (U-Link) project, a future 3.15-mile (5.07km) subway light rail extension in Seattle, which will connect downtown Seattle with the University of Washington via Capitol Hill. The extension will expand upon the existing light rail system, which features two lines that connect other parts of the city. To place concrete for the extension in the narrow underground tunnels, the team at Stacy and Witbeck, Inc. are using Putzmeister’s Mixkret 4 and a Thom-Katt® TK 50 Trailer-Mounted Concrete/Shotcrete Pump. In addition to the two bored tunnels, the project also includes two cut-and-cover stations: one at Capitol Hill and one at the University of Washington campus.
Prior to concrete placement, the two tunnels were bored using tunnel boring machines. An earth retention system consisting of drilled soldier piles and lagging with whaler supports were used to span each tunnel shaft. The tunnels were then lined with pre-cast concrete segments, and jet grouted.
One of the most notable challenges for the U-Link project is the limited access points in which materials, equipment and personnel can be staged and utilized throughout the construction process.
“Unlike a typical construction project, this one does not have a tunnel portal through which you can easily reach the work zone with conventional truck delivery systems,” explained Josh Pategas, construction manager for Stacy and Witbeck.
Instead, there are three openings through which the crew can lift equipment into and out of the excavation, which range from 60-100 feet (18-30m); two of the openings have active underground station structures being built. The remote access means the type of machinery used on the project must be equipped with picking eyes, which, according to Pategas, is one of the reasons the team at Stacy and Witbeck chose the Mixkret 4.
Once the equipment is lowered into the tunnel, it is then transported to the active work zone, which can be up to two miles away. This creates a unique logistical challenge.
“A conventional job site allows for materials and equipment to be delivered precisely where they will be used,” said Pategas. “In a tunnel, there is very limited square footage for materials to be stored and accessed during the construction process, which makes planning and timing much more critical.”
Not only does working in a tunnel create a challenge in terms of getting equipment down into the excavation, but it also poses a challenge when the equipment finally gets there. In this case, the crew had to maneuver in a tunnel diameter of approximately 19 feet (6m).
“Another reason the Mixkret 4 was selected for the U-Link project is due to the size constraints found in the tunnel,” said Pategas. “Every inch counts when you are working in a tunnel—and the Mixkret 4 allows for greater mobility.”
The low-profile Mixkret 4 concrete mixer is equipped with a 6-cylinder, 174 hp engine with ACERT technology and 5.2 cubic yard mixer drum designed specifically to mix, transport and discharge concrete. It can also be equipped with an exhaust scrubber to minimize fumes, which is a big advantage when working in tunnels.
“The machine is compact and versatile,” added Pategas. “It allows for great traction and driving capacity and has helped to increase our payload while still ensuring stability during transport.”
The crew is currently performing concrete pours every other day, which allows enough time to build out the track and form for the next pour without having to work a second shift. To arrive at the pour location, the concrete is trammed from one of the tunnel access points to the Thom-Katt TK 50 Trailer-Mounted Concrete/Shotcrete Pump, and then pumped through steel/rubber slickline up to distances of 700 feet (213m).
“We are only pumping concrete in the horizontal plane, so we needed to make sure we had a machine that had enough power to pump long distances,” explained Pategas. “With its high horsepower, the TK 50 has performed really nicely on this particular job considering the unique challenges we’ve faced in the tunnel.”
Also contributing to it’s ease of use, the TK 50 features an angled hopper and can pump a variety of materials at up to 54 yd³/hr and handle harsh mixes.
Because the concrete sometimes needs to be transported up to two miles, a special mix was also required. The mix design approved for this project used concrete admixtures to extend the life of the material and allow the machine ample time to tram to the pour location. Additionally, a smaller nominal size aggregate of 3/8 in. (10mm) and a higher plasticity range helped make the concrete mix more conducive to pumping long distances.
Upon completion in 2016, U-Link will serve the three largest urban centers in the state of Washington–downtown Seattle, Capitol Hill, and the University District. The new line will add 71,000 riders to the system by 2030, bringing the system wide total to 114,000.
Putzmeister America, Inc. manufactures a complete line of truck-mounted concrete boom pumps, seperate placing booms, truck-mounted telescopic belt conveyors. ready mix trucks and trailer-mounted concrete pumps as well as mortar, grout, shotcrete, plaster and fireproofing pumps and mixers, industrial pumps, tunneling machinary and pipeline systems. Some of the industry's best known brands, such as Tellebelt, Thom-katt, and Tommy Gun are part of the Putzmeister America family. The company's workforce is dedicated to the hands-on customer support and advancing the industry in design and technical innovation.
Owner/Developer: Sound Transit – Seattle, WA
General Contractor: Stacy and Witbeck, Inc. – Alemeda, CA
Equipment Owner: Stacy and Witbeck, Inc. – Alemeda, CA
Ready Mix Concrete Provider: Concrete Company – Seattle, WA
Equipment: One Putzmeister Mixkret 4, and one Thom-Katt® TK 50 Trailer-Mounted Concrete/Shotcrete Pump.