Leica Answers Highway Monitoring Call in Minnesota's Twin Cities
Area soils range from peaty with man-made fill to silty and sandy. This variability poses a significant challenge as pipe ramming in these conditions may cause movement at the surface or result in development of a sinkhole. Pavement slumping under these roadways during ramming or the preceding dewatering would be hazardous for commuters.
The project owner, the regional Metropolitan Council, was determined to keep traffic moving while keeping drivers safe. Doing this would require a long-term pavement monitoring solution that provided continuous automated monitoring, as well as real-time alerts that could warn Minnesota Department of Transportation crews to stop traffic, if needed. And, whatever system was used, it would have to work well with non-stop traffic.
Barr Engineering Co., a consulting firm headquartered in Minneapolis, was selected for the project and designed a system incorporating reflectorless monitoring with two Leica Geosystems TM30 total stations.
“Monitoring with a scanner was also considered,” says Joel Swenson, PE, Barr geotechnical and monitoring engineer. “But the continual presence of cars ruled it out. And mounting prisms on the roadway was also thought to pose an unnecessary risk.”
Reflectorless monitoring was the way to go, but it was new to the Met Council. “The council had not previously approved reflectorless monitoring,” Swenson explains. “So we had to monitor accurately, but also be sure that they remained comfortable with this solution.” With the project site minutes from Swenson’s home and on his way to work, the Met Council had confidence that any pavement movement could be readily verified by him during the course of the project.
A Half-Inch Down, a Half-Inch Up
For both project sites, Barr set up two TM30s in secure, weatherproof housings. Leica GeoMoS automatic deformation monitoring software was used to automate the collection of 110 total points for the I-394 crossing, including some low-profile “road prisms” set on shoulders and elsewhere, and 180 points for the Highway 100 crossing, with the majority of these being reflectorless shots on pavement in traffic lanes. Each total station was set to collect 50–90 points every 7-12 minutes. Cycling at this pace was necessary due to the large volume of traffic; in any given cycle, many shots were blocked by cars or trucks, but the rapid cycling and large number of collected points ensured monitoring was always accurate and timely.
To allay concerns about reflectorless shots, Met Council’s surveyor regularly took prism shots on shared shoulder points for comparison and routinely checked on alerts with a prism. He reported no difference in elevation between reflectorless and prism shots, and was also careful to account for potential inaccuracies in horizontal movement due to the lack of a prism. Since elevation was the primary concern, alert thresholds were set to elevation differences, and subsequent checking confirmed that this was an accurate monitoring technique.
Baseline measurement is an important but often overlooked component of monitoring projects. “Really, getting a good baseline for roadway movement should take several months in Minnesota to account for the large seasonal temperature variations we experience,” Swenson explains. “But practically, we often get just a week or two.” For the I-394 project site, monitoring was set up in July 2013, a couple of weeks before dewatering began, and continued until December 2013. Monitoring for the Highway 100 work was set up in just two days in May 2014 following unsuccessful installation of a subterranean monitoring system.
Pavement movement was definitely observed at both pipe jacking sites. During the I-394 project’s lengthy dewatering, roadway settling of about half an inch (and up to one inch in spots) was observed. And during I-394 actual ramming operations, heaves of between a tenth of an inch and half an inch were noted. Movement up to three inches during the Highway 100 monitoring was observed and felt at the ground surface by passing vehicles. All movement over established thresholds triggered email alerts to Swenson, contractors and Met Council observers, and each alert also triggered a visit to the site from Swenson.
“When driving over the I-394 areas of concern, there was never a bump; however, the dips were felt on the Highway 100 project,” he says. “Elevation changes in the roadway were usually small on the I-394 project, and surveying showed the transitions in and out of the affected areas were gradual and didn’t affect drivers. But the dips on the Highway 100 project could be felt in the steering wheel.”
Having the monitoring data available to them, the Met Council and the contractor were both aware of the impact ramming each section of pipe had at the pavement surface and were comfortable moving ahead with the project. Despite many alerts, traffic was never stopped due to roadway slumps or heaves, and the pipe ramming project took place without interrupting commuters.
A Proactive Approach
When long-term monitoring doesn’t detect significant changes, it can seem that project managers have been overly cautious. But monitoring also has a proactive component. At both sites, alerts turned out to be a good source of feedback for the pipe ramming contractors and were sometimes helpful in preventing significant roadway movement because contractors could adjust operations as needed.
“Immediate feedback made the ramming crew more aware and more cautious,” Swenson says. “Real-time information was much better than weekly reports — crews were able to monitor their own work as they were doing it.”
For example, when rammed pipe encountered an unexpected patch of rock at the I-394 site or when the first quarter-inch of movement was measured at Highway 100, immediate alerts were one way contractors knew they had run into something unusual. Had the monitoring system not been in place, noticeable roadway heaving or dipping could have happened abruptly and dangerously, leading to a response that was reactive rather than proactive.
“Based on our client feedback, being able to respond quickly was a great benefit to the monitoring program and helped ease the Met Council’s concerns,” says Swenson.
This project is one of many in recent years that have demonstrated the value of real-time positional feedback on infrastructure projects of all types. As companies like Barr and Leica Geosystems extend the reach and value of real-time monitoring, it is rapidly becoming an indispensable solution for large infrastructure projects.
Angus W. Stocking, LS, is a licensed land surveyor who has been writing about infrastructure since 2002. For more information about Barr Engineering Co., visit www.barr.com. To learn more about monitoring solutions, visit www.leica-geosystems.us/monitoring.