Photo Courtesy of University of Alberta

Roving Pilot Trailing up to 200 meters behind a tunnel-boring machine as it moves through the earth, a surveying robotic total station computes the TBM's geospatial position to within 3-mm accuracy in real time.

After more than six months of full-scale field testing on the construction of a 1,000-meter sewage tunnel, a geomatics-based survey guidance system for tunnel-boring machines shows promise for reducing interruptions in surveying while delivering real-time as-builts of the tunnel's interior.

"We can easily save 10% of total tunnel project costs," says Siri Fernando, engineering manager for the city of Edmonton, Alberta, Canada, which is using the tunnel-survey automation system called Virtual Laser Target Board (VLTB). Fernando is testing it on a $14-million, 8-ft-dia tunneling job. "We've seen good results the whole time," says Fernando, who is wrapping up the tunneling job in the next few weeks. The system's inventor, University of Alberta associate professor Ming Lu, was right in his claims last year of how much work time VLTB saves, Fernando adds.

Traditionally, a specialist survey team is required to do a quality survey of the progress of a tunnel every 200 m or so, says Lu. The process can stop the boring for four to five hours. Further, qualified surveyors are in short supply. VLTB automates a lot of the surveying process and decreases construction downtime.

"I thought of the idea when I was working on a micro-tunneling job," says Lu. With micro tunneling, the small diameter of the tunnel precludes human entrance. Lu says his goal has been to survey a tunnel and guide a TBM without physically going in with a crew.

As the TBM progresses, VLTB uses a total station as a control robot to track and position the TBM automatically and continuously. VLTB charts any line or grade deviation in the tunnel's alignment to within 3 millimeters of accuracy.

"The robot can be fixed on the tunnel wall in any place and doesn't require leveling or surveying calibration," says Lu.

When the robot is set up no more than 200 m behind the TBM, it monitors three tracking points on the back end of the machine. Then, the TBM's three-axis rotation angles can be computed by using what Lu calls point-to-angle algorithms. The algorithms are set to compute this information automatically, and VLTB takes readings continually and in real time. No gyroscopes, inclinometers or any orientation gauges are necessary, says Lu. The robot also monitors its own position changes by checking two fixed reference points inside the tunnel. All the information is computed and turned into a readout for the tunneling operator, who can view on an iPad not only an as-built view of the tunnel but also the machine's accurate position in real time.

The capability for project managers to detect any out-of-tolerance alignment errors in real time has never been available before, Lu states.

The system has caught the attention of TECTERRA, Alberta, a not-for-profit organization whose mission is to help the geomatics industry bring useful new technology to the commercial market.

"During this test, the city of Edmonton is comparing [VLTB] to its traditional survey techniques," says the orgazination's CEO, Mohamed Abousalem. The big difference is the time each method takes.