Tough Work in a Tight Spot
April 1, 2010
Each day, thousands of commuters navigate the corridor between Minneapolis and Big Lake. Served primarily by Interstate 94 and U.S. Highway 10, the route has become increasingly congested over the past decades.
Several Minnesota governors and other high-level advocates have pushed for a commuter rail system for years. Minnesota Department of Transportation officials also believed that commuter rail was the best way to handle the increasing load. They estimated that building a rail system would be one-third the expense of upgrading existing highways. Still, cost was always an issue for legislators.
A significant cost breakthrough came when it was decided to use existing Burlington Northern Santa Fe Railway (BNSF) lines and rights-of-way rather than installing an entirely new rail corridor. Under the operating agreement, BNSF owns the infrastructure and the Metropolitan Council, the agency providing essential services to the Twin Cities’ metropolitan area, owns the rolling stock. But using existing lines came with its own set of challenges. It meant that railroad sidings would have to be added to allow commuter trains and freight trains to pass each other. Additionally, the commuter system’s stations, platforms, parking lots and pedestrian access tunnels would have to be shoehorned into extremely tight footprints already crowded with existing freight lines.
EVS Inc., of Eden Prairie, Minn., was contracted by Lund Martin Construction to provide construction staking and monitoring services for the new Northstar Commuter Rail station, which would be located in Fridley, Minn. “The site was less than a mile from one of BNSF’s big switchyards,” says Michael Williams, EVS survey office manager. “Five tracks ran through the site, and the freight train schedule didn’t slow down while we were working. There was about one train per hour during the day, sometimes more, coming through at 20 miles per hour. And sometimes trains would park in the middle of the site for several minutes. Since we had to stay in a ‘clear zone’ during those times, we did a lot of waiting.”
The BNSF team was understandably concerned about safety. They required EVS personnel to complete rail safety certification services, and they provided flagmen for all working hours. They were also concerned about possible damage to existing lines. Tunneling, earth moving and trenching were taking place near the rails, and even very small shifts in rail alignment can be disastrous for heavy, fast-moving trains. BNSF officials were interested in any movement over 3/4-inch. “Monitoring was a big deal,” Williams says, “because nobody could get started without it.”
Keeping a Close Eye on the Rails
Pedestrian tunnels had previously been placed beneath lines at Fridley, but elevator shafts and some trenching were still required. In some places, excavation was within 2 feet of active rails, which had to be shored up with a steel sheet and tieback system. EVS was asked to install a 24/7 monitoring system that would automatically alert officials when any rail moved beyond the 3/4-inch threshold. “We’ve done monitoring before, but not so automated as this,” Williams says. “So there was a bit of a learning curve.”
The EVS crew knew it wanted to work with Trimble’s 4D Control software, which takes advantage of the Trimble S8 Total Station’s monitoring capacities and works with Trimble Survey Controller field software to create custom monitoring solutions. In this case, the crew planned to establish a permanent S8 setup onsite and program it to automatically shoot monitored points every hour. Simple enough in concept--but the devil, as they say, is in the details.
For one thing, where exactly--in a crowded, unfenced railroad project--does one place a critical precision instrument so that it can be left overnight and have sight lines to all necessary points? After much consideration, EVS Crew Chief Stan Barthel found a good spot. “There weren’t many options,” he says. “There were deep excavations, as deep as 20 feet. There were right-of-way issues, and the soil was mostly sandy and granular, which meant that vibrations from trains and construction would radiate through the soil to our setup.”
The EVS crew ended up digging a hole 4 feet deep, pouring concrete and setting a 6-by-6-inch wooden post. There was still movement, so four guy wires were added. Then a platform and scaffold were built, with a plywood roof to offer some protection from the elements. “Wind-driven rain was a concern, of course,” Barthel says. “But since we needed about 160 degrees of clear sight for the observations, there wasn’t much we could do about it.” The instrument ended up about 10 feet above the ground, offering some protection against theft and tampering, and the tribrach-set screw was replaced so that an Allen wrench was needed to adjust it.
Even with all that effort, Barthel wasn’t sure the setup would work. “The closest rail, just 35 feet away, had a joint in it, and when trains hit that joint you could feel it a hundred feet away. We did have to re-level a few times. But, overall, things worked out.”
Since there was no permanent power supply for the first month of the project, deep-cycle car batteries were used to power the total station. These were switched out weekly. A Wi-Fi-enabled computer, powered by a generator, was kept in a nearby field office. Wi-Fi was needed so that the 4D Control software could send e-mail alerts as needed. Communication between the Trimble S8 and the onsite computer was by radio.
With the setup established, the EVS crew then set out 72 peanut prisms on L-brackets, which were attached to the tracks with construction adhesive. “After you shoot the targets the first time,” Barthel says, “the S8 remembers and can then shoot them all automatically on a schedule, which in this case was every hour. It took about 20 minutes for a round of shots.” Even with the high setup, prisms were often blocked by trains or equipment. The Trimble S8 and 4D Control handled that by skipping blocked shots when the first attempt failed and returning to missed shots at the end of a cycle. “The S8 was amazingly fast,” Barthel says.
With the help of Frontier Precision Inc., the EVS crew was able to set up the monitoring exactly as required for this project thanks to the software’s capabilities. “All in all,” Williams says, “it’s powerful software that did everything we needed it to do.”
When there were shifts in position of more than 3/4-inch, the software sent e-mail alerts to the crew. There were a lot of these at first until Barthel figured out that normal train activity caused a lot of movement. “When trains move through, the track slides in the direction the train is going,” he explains. “And after the train passes, it settles back into position.” Fortunately, Barthel was able to set different tolerances for different axes of movement. “Since we were more interested in lateral movement and up and down movement,” he says, “we set the tolerances higher for movement in line with the tracks.”
This change cut down on alerts, but one important warning did come through. In the first week of monitoring, ballast shifted near some of the shoring, and tracks sagged unacceptably. The EVS team alerted BNSF personnel immediately. Machines were brought in to raise the track, and stabilizing piles and ballast were reset. No trains passed over the shifting track until it was repaired, and the shoring held for the rest of the project.
EVS ran the monitoring full time for three and a half months with no significant outage of service. “This type of monitoring is more common in mines, and this may be the first time it’s been used by a railroad,” Williams says, “but now that BNSF has seen it in action, they’ve said that they may start requiring it more often.”
Staking in Crowded Conditions
By coincidence, the EVS crew had done the design survey work for this site in 2005. They found four control points still in place from that work, and they quickly got on basis. After locating the tunnels and laying out elevator-shaft entrances, they adjusted the plan grid for the station platforms to match the as-built tunnel location. This required shifting station plans several tenths of a foot north while maintaining platform alignment parallel to existing tracks.
A Trimble R8 GNSS receiver and base station were used to lay out storm sewers, catch basins, rough grading and light poles. A Trimble 5603 Total Station was used for all other staking chores. Stakeout points were taken from CAD files, processed with Trimble Geomatics Office software and loaded into the total station’s faceplate.
The BNSF-provided flagmen were in contact with the dispatch office at all times, and incoming trains communicated directly with them, as well. When trains came through, the EVS crew had to move to clear zones while they were onsite. “We tried staying far enough away that we didn’t have to take down the instrument when trains came through,” Barthel says. “But we definitely had to check our setups after each one. Sometimes we had to reset, but not too often.”
“Nobody could really tell us how to do this,” Williams says. “Setting the S8 up on a tower, leaving it on 24 hours a day, leaving it unsecured overnight--it was all new to us, and we felt like we had to learn it as we went along.” One big issue at the beginning of the project was liability for a damaged or stolen instrument. Ultimately, Anoka County officials agreed to take responsibility, but no surveyor ever wants to lose an instrument. Fortunately, no losses occurred during the project.
The Northstar Commuter Rail Line began service on Nov. 16, 2009, and is expected to serve 5,600 commuters each day in 2010. By taking a chance, learning to use new tools and software, and coming up with innovative ideas such as perching a survey instrument on a 10-foot pole for three and a half months, EVS was able to build a key component of an important transportation system--without disrupting existing rail service.