Keeping a Puzzle Together
In some ways, excavating rock is the reverse of assembling a jigsaw puzzle. Rock, after all, is not a big monolith. Instead, it’s full of breaks and cracks, discontinuities that can cause problems during tunneling or other excavation. The trick is to keep the assembled pieces from falling apart, a challenge given the three-dimensional nature of the puzzle.
Using a laser scanning system, engineers are mapping the rock surface during excavation for a tunnel bypassing the landslide-prone Devil’s Slide along California’s Highway 1. From visualizations (left and center of the excavation surface and right of the tunnel face) derived from the point cloud produced by the laser scanner, they can determine the discontinuities present (arrow on left) and how best to keep the rock in place. Courtesy of Kiewit Corp.
Engineers, unfortunately, see only the 2-D representation of that puzzle. If two seemingly unrelated but visible cracks connect at some hidden point within the rock, the whole thing may be unstable and need support.
Traditionally, engineers have predicted the unseen part of discontinuities by manually mapping and measuring what can be seen. That presents problems, said Joseph E. Dove, a civil and environmental engineering research professor at Virginia Polytechnic Institute & State University in Blacksburg. “You have the problem of hazards. You may not be able climb up to a point where you really need to get a measurement, or you may not be able to get up to the ceiling of a tunnel.
This dangerous and sometimes tedious job may be getting easier, thanks to laser-based scanning systems and visualization software. Together, they produce a map of the rock face, providing engineers with a means to safely measure and record discontinuities.
The technology is being used to complement traditional methods in the building of a kilometer-long tunnel along California’s Pacific Coast-hugging Highway 1. Kiewit Pacific Co. of Vancouver, Wash., is handling the construction. The tunnel will bypass Devil’s Slide, one of the most landslide-prone stretches of highway.
Creation of the point-cloud map begins with a series of laser-ranging measurements. Dove has used terrestrial lidars from Optech Inc. of Vaughan, Ontario, Canada, for this. In the case of the work being done at Devil’s Slide, engineers are using a tunnel-scanning system from Dibit Messtechnik GmbH of Mils, Austria, to generate the data.
Both systems fire laser pulses and collect the return to build up a picture of the rock face. Both can work from known safe locations and can map potentially unsafe areas, reducing risk to the operator. The scan takes a few minutes, with additional time required for subsequent data processing.
If the measurements are to be absolute, the position and orientation of the scanner must be known. That can be a problem when operating underground. The Dibit system solves this through the use of theodolites – prisms that are placed a distance away from the scanner and whose location is known.
The laser-produced map is accurate to a distance of ±5 mm. From this data, software developed by Dove and Dr. Jeramy Decker, who is now at Kiewit, creates a point cloud that allows visualization of the tunnel. Dubbed gVT, (geotechnical visualization tool), the software offers some advantages. “I can get more discontinuity-orientation data from looking at the 3-D visualization in the same amount of time than I can from looking at the actual excavation,” Decker said.
He added, however, that the visualization complements manual mapping but does not replace it. And because some data, such as rock hardness, is difficult to obtain by 3-D visualization alone, it may be necessary to obtain that information manually.
Dove said that he has used spacing between collected points of a few millimeters but that, most of the time, such fine resolution is not necessary. An advantage of tighter spacing is greater resolution; the downside is a larger data file, which makes manipulation slower.
As for the scanning technology, he noted that it works well and is ideal except when distances of more than a few hundred meters are involved. Although most tunnel work is done in proximity, Dove does see cases where distance limitations could be a problem.
“If you’re in a long tunnel, and you’re shooting down the axis, that could be an issue,” he said.
Contact: Joseph E. Dove, Virginia Polytechnic Institute & State University; e-mail: firstname.lastname@example.org; Jeramy Decker, Kiewit Pacific Co.; e-mail: email@example.com.
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