Commonsense on a Curve

Innovation and invention. Could any two words speak more directly of the American way?

The United States is always at or near the technological forefront. The first plane flew here. An American was the first person on the moon. Hard hats, touch screens, parking meters — all American inventions (even if we’re less than thrilled to claim the parking meter). Why then, does the United States trail the rest of the world in microtunneling? Why are developing countries more in-tune with the state-of-the-art in microtunneling than America?

“It’s frustrating,” says Microtunneling Inc. president Timothy Coss of the technological lag in American microtunneling. “There are jobs going on around the world — in India and Malaysia and South America — that are way ahead of what’s going on here.”

Typically, microtunneling in the United States is specified for short, straight runs. “The majority of the market, especially in North America, involves relatively short runs at a reasonable depth,” explains Paul Nicholas, SBU Division general manager at The Robbins Co.

Coss’ record of 1,600 lf in a single drive has stood for 20 years as North America’s longest microtunnel drive. A portion of the Eastside CSO in Portland, Ore., will break that record this year — the drive currently under way has a total length of 3,100 lf. While that’s quite a jump, consider that the worldwide record for microtunneling length is better than 8,000 lf. Drives of 2,000 or 2,500 lf are fairly standard in other countries.

Another specification that is common overseas but has not yet found a foothold in American microtunneling is the curved drive. Contrary to the typical specification here, microtunnel drives can successfully incorporate curved alignments in any direction. “Curved drives are standard practice in the rest of the world,” says Nicholas.

“It’s been done thousands of times in other countries,” agrees Coss. “The state-of-the-art in microtunneling is long drives with curves.”

Bend It Like…

Curved drives offer a great deal of flexibility to a microtunneling project. While flexibility is nice in its own way, it is important to understand that in underground construction, flexibility almost certainly brings cost-savings with it. Employing a curved drive will almost certainly save money.

For example, take a river crossing. If the MTBM needs to be 100 ft below the surface to safely pass beneath a river bottom, the current specification would be to sink a shaft deep enough to allow a horizontal bore path to pass beneath the river. A curved bore path, though, would offer the option of a shallower shaft. Even a shaft 60 or 70 ft deep would be so much more cost-effective than a 100-ft shaft, especially when considering the additional groundwater control expense associated with shafts in the vicinity of a river.

Another example to consider is that of an obstacle in the bore path. Of course the shortest distance between two points is a straight line, but if that line cannot proceed unbroken, other arrangements need to be made. Operating solely with straight line bores, the only way to move around an obstacle is to use additional shafts as pivot points to direct the bore path around the obstacle. So what could have been a simple shaft-to-shaft bore with a slight curve to maneuver around an obstacle becomes more complicated with the additional shaft work.

Shafts add to project costs, obviously, in a significant way. “Owners are forced to spend more to avoid a curved drive,” Coss says. “And a lot of them don’t know any better. They think it’s the only way to get it done.”

But let’s suppose that money is no object. What if adding shafts is not an option? More and more projects are encountering situations in tighter spaces where shafts cannot be sunk at every whim. The Eastside CSO project mentioned earlier is one such project — the primary reason for such a long drive being specified is that there were not any suitable shaft locations available along the bore path.

A project faced with an impassable obstacle needs to be flexible enough to work around that obstacle. Failing that flexibility, the project cannot proceed.

So What’s the Problem?    

“One of the problems is that the engineering community doesn’t know enough about it,” Coss says. “Nobody wants to be first.”

Coss goes on to mention that he has seen engineers back away from curved drive specifications simply because it has not been done in North America. To his credit, Coss has been trumpeting the curved drive cause as part of his microtunneling short course for the past nine years. “I guess I haven’t done a very good job,” he chuckles.

But lack of knowledge does not quite explain curved drives away. Nor does the experience shortage that goes along with a scope of work that has not been performed in America. Indeed, the machinery, supplies and even the crew necessary for a curved drive microtunneling project is a phone call away. Experienced crews can be leased along with the equipment they will need to get the job done.

“Anyone who wants to specify a curved drive should get in touch with me,” Coss offers. “I’ll put you in touch with the right people.”

How Doable Is This?

To be fair, curved drives are more complicated than straight drives. Special guidance systems are required
to properly steer the cutterhead, for example. In a straight drive of ordinary length, a standard laser sight will keep the machine on course. Curved drives typically require guidance systems similar to the type used on longer bores.

If using a laser guidance system, a signal relay might be employed or the laser can be physically repositioned to maintain line of site with the MTBM as it advances. Another option for guidance is a gyro system, which uses instrumentation at the face to report pitch and directional information to the operator.
Several manufacturers offer specialized guidance systems for curved microtunneling, including VMT and Jackcontrol, winner of the innovation award at the aforementioned 2008 Microtunneling Short Course in February.

In 2001, Polish project Zelona Góra was named Hobas Project of the Year at the Hobas Conference in Prague. 

As, the first curved bore microtunneling project in Poland, this project illustrates a successful solution for one of the larger issues with any microtunneling project, let alone a curved bore.

Jacking force is a key consideration when specifying a microtunneling project. If the advance requires more force than the pipe being installed behind the MTBM can handle, the resulting stress will likely compromise the integrity of the pipe. In a curved bore, such stress is a further issue as the force is greater at the bend or intersection of the pipe. The Hobas CC-GRP Jacking Pipe Systems had sufficient ring stiffness to accommodate the necessary jacking force at the curve, where 1-m segments were used as opposed to the 3-m pipes for the straight advances. The total project included 1,149 m of sewer pipe installation, with the curved portion comprising 115.5 m in length.

Another way to address the jacking force issue is to employ intermediate jacking stations, special sections of pipe that fit between pipe installments and allow force to properly transfer along the length of the drive.

Nevertheless, the technology to tackle these concerns is out there, used daily on projects around the world.    


Just as it took 20 years for a drive to break Coss’ 1,600-lf record, it may take time before the right job comes along to force an American team to specify curved bore microtunneling.

Soon enough, as increasingly populated cities need infrastructure support, and shaft access is limited by the crowded surface, such a job will come along. “It’s common sense,” Coss explains. “It’s as simple as digging around or under an obstacle and all that goes along with that vs. simply curving around and continuing on.”

If any industry can appreciate the benefits of not digging, it’s this one. Besides, a curved bore path, ultimately, saves money.

Greg Thompson is assistant editor of Trenchless Technology.
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