Enough research uncovers record breaking performances in any number of events. For example, on July 19, 2007, Hou Jin of Changchun City, Jilin Province, China, set a record for the longest motorcycle ride through a tunnel of fire (178 ft, 10.4 in.). Unfortunately, a great number of engineering and construction feats go untracked and unheralded. Currently under way in Portland, Ore., a microtunneling project for the City’s Bureau of Environmental Services is looking to find a place in history as the longest drive in North America.
The Project
While far from the rainiest city in the United States, Portland receives more than its fair share of rain. Rain falls roughly 100 days each year in the city, which causes wastewater and stormwater volumes to exceed the capacity of the sewer system. When sewer capacity is exceeded, the overflows run into the Willamette River, a tributary of the Columbia River. The East Side Combined Sewer Outflow program is part of a massive program that seeks to eliminate combined sewer overflows into the river.
The West Side CSO portion of the project completed tunneling in 2005 and went online in 2006. When the East Side CSO is complete in 2011, the reduction in combined sewer overflows into the Willamette will be reduced by 94 percent.
The $381 million East Side contract to Kiewit-Bilfinger Berger JV (KBB) includes six miles of 22-ft lined diameter tunnel running between 85 and 165 feet below the surface. Also included are seven shafts, 6,000 lf of microtunnel and 3,000 lf of open-cut pipeline construction.
The microtunneled portion of the project is divided into eight drives of varying lengths. To date, three drives have completed and the fourth drive is under way. At 3,050 lf, this fourth drive is the longest, both of the project and in North American history. TBM spoke with representatives from the design team, the contractor and the MTBM manufacturer to gain insight on this record-setting MTBM drive.
The Drive
Herrenknecht is no stranger to record-setting microtunnel drives. According to Julian O’Connell, microtunneling sales and technical consultant for Herrenknecht, the longest microtunnel drive worldwide was roughly 2,600 m (1,400 lf), performed with a Herrenknecht machine. The drive was in Germany, which is important to note because microtunneling is defined differently in Europe than it is in North America.
Microtunneling in North America describes the process by which a pipeline is installed, while in Europe, the term refers to pipelines of a given size. In simple terms, European microtunnels are smaller tunnels (typical point of delineation is 36- or 48-in. diameter. North American microtunneling describes a process by which the boring machine is controlled. Regardless of diameter, if the machine is controlled remotely and steerable at the face, it falls into the North American definition of microtunneling.
North America has not seen many, if any, microtunnel drives longer than 2,000 lf. A typical North American microtunnel drive numbers in the hundreds, rather than thousands of feet. This shorter typical drive length is due to any number of causes, but understanding the specific reasons for the specified length of Drive 4 on the Portland East Side CSO is key to appreciating the record drive.
According to KBB operations manager Scott Wimmer and KBB microtunneling lead superintendent Matt Roberts, the soil facing the machine on Drive 4 is predominantly sand/silt/alluvium and fills. Drives 1 through 3 included encounters with Troutdale Formation, which consists of gravel and cobbles with intermittent fine materials.
“We didn’t have many surprises on the first drives,” says Wimmer. “We encountered some wood that the MTBM handled well.”
“We have also used a centrifuge to assist in soils separation when tunneling through fine-grained soils, such as clay and silts,” adds Roberts.
The location of the drive alignment is the primary reason for such a long span between shafts. “Surface access is very restrictive,” says KBB project director Bill Mariucci. “Including the drive running parallel to railroad tracks and a train yard.”
The alignment also runs near enough the river to cause groundwater concerns. “The shoreline has seen extensive filling and industrial development over the past 100 years,” says John Charles Horne, engineering manager in the Geotechnical and Tunneling Technical Resource Center for Parsons Brinckerhoff. “We were concerned about encountering poor soils and construction debris in the upper soil horizon with open trenching.
“Having seen success on the microtunneled portions of the West Side CSO,” Horne continues. “We were confident specifying microtunneling for this portion of the East Side CSO.”
The Machine
“We believe Herrenknecht was chosen for this project,” says O’Connell, “for its reputation in general, but also for the success we had on the West Side project with a similar machine.”
The MTBM is a Herrenknecht AVND2000 slurry machine with a cutting diameter of 104.5 in. Key features include variable flushing modes to suit different ground conditions, a highly effective cone crusher and access to the cutting wheel for tool replacement. Additional consideration and adjustments were made in light of the length of the drive.
“We fitted an electrical trailing sledge with a transformer in the tunnel to accommodate the length,” O’Connell explains. “Also we installed a Herrenknecht gyro and water level system for guidance.” O’Connell goes on to explain that the drive length is out of range of the more-typical laser guidance system.
Another feature that suits this particular machine to a longer drive length is that the face can be reached through an access door. Cutter wheel changes can be through that access in a pressurized air chamber. “Of course, if cutters become worn and no longer mine properly, drive length is compromised,” O’Connell says.
The Process
“Another major contributor to the success of this drive will be the use of several intermediate jacking stations,” notes Wimmer. An intermediate jacking station is a fabricated steel cylinder fitted with hydraulic jacks incorporated between two pipe segments in a pipeline to distribute the jacking load over the pipe on long drives.
KBB has also deployed a Herrenknecht Automatic Bentonite system to ensure good lubrication to the exterior annulus of the pipe.
“The key to a drive of this length is keeping the friction low and maintaining low jacking forces,” O’Connell explains. “I was on site [on March 10] and witnessed a jacking load of 420 tons. The tunnel was approximately 1,400 lf (430 m) along.”
The pipe being installed behind the MTBM is 84-in. ID class 4 concrete jacking pipe. By incorporating the intermediate jacking stations and the bentonite system, the MTBM will be able to complete its drive into the record books.
Microtunnel drives are not designed to break records. The drive, machine and pipe specifications are dictated by ground conditions and availability of access points among other factors. That said, as longer and longer drives find success in North America, drives will continue to lengthen when necessary. Owners, contractors and designers are finding that the right machine with the best team can achieve drive lengths that would have previously been thought impossible.
Greg Thompson is assistant editor of Tunnel Business Magazine.
