- SPECIAL REPORTS
- THE MAGAZINE
Bombs, GPS and an unusual experience for one surveyor.On April 28, 1973, I was taking my land surveyors exam at California State University when a large blast rocked the Sacramento campus. I continued the test, but was interrupted by additional blasts throughout the day—sonic booms, I thought. Later, I heard that 21 freight cars carrying 7,056 Mark 81 250-pound bombs for Vietnam had exploded in the Roseville railyard, just 15 miles east of Sacramento. The explosions lasted 12 hours and completely obliterated the surrounding area of Antelope, but no one was killed or seriously injured. It was later found that a “hot box”—an overheated wheel bearing—had set one of the boxcars on fire on the 6,000-foot descent from the Sierra Nevada. The story made front page news for days as officials sought to quickly repair the damage, re-lay the tracks and ensure the area was clear of all unexploded munitions. Little did I know that 25 years later I would survey that very area using technology unheard of back then: the Global Positioning System (GPS). And little did I know what history I would then find.
The largest trainyard west of the Mississippi, the J.R. Davis Yard in Roseville, Calif., is a major Union Pacific (UP) switching center built in 1907. Ninety years later the 850-acre yard was ready for an update. UP’s 1997 $130 million project included modernizing the yard, making it more automated and increasing its capacity by replacing 86 miles of track with 136 miles of new track.
UP awarded contracts for the yard rehabilitation project to Jim Dobbas Inc., Roseville/Newcastle, Calif., as general contractor for demolition, track removal, surface prep and track laying; and Gabe Mendez Inc., Newcastle, Calif., for underground work and sitework. Ultimately, Mendez moved more than a million cubic yards of earth and installed nearly 70 miles of utility lines. UP surveyors set the control points and laid out the track; Mendez awarded a six-figure contract to my company, California Construction Control (CCC) of Diamond Springs, Calif. for all construction staking.
The Roseville yard was approximately six miles long and nearly one mile wide in some places; its sheer size posed a challenge for conventional survey methods. The job’s scope required different phases to be done simultaneously, while the trainyard remained active. Sight lines to backsights and points to be staked and stability of control stations were seriously impacted, at least potentially, by slow-moving or stationary trains and mounds of earth. And the wide range of Sacramento-area atmospheric conditions, 30 degrees F and fog in winter, 100 degrees F and higher in summer, was expected to seriously affect the progress and efficiency of the work.
Before starting the job, CCC owned only conventional total stations. But conventional staking over those distances would have required me to hire more people or devote all my resources to the job, very possibly losing other clients. As a business owner, I am constantly seeking ways to increase efficiency and productivity, and had been tracking GPS survey technology for years. However, GPS survey equipment required carrying a backpack and I didn’t like the fact you need to be tethered to the backpack with cables. But when Trimble (Sunnyvale, Calif.) came out with survey-grade GPS equipment that integrated everything into the range pole, eliminating the backpack, I was interested. In December 1997, CCC purchased Trimble’s GPS Total Station 4800, an all-on-the-pole, real-time kinematic (RTK) survey system. After on-site training of less than eight hours of hands-on use, we were ready to go.
We quickly found that the large scale and wide-open areas of the Roseville job made it a natural for GPS. Throughout the 18-month job, we were able to complete a week’s survey requirements in two to three days with RTK GPS, with ample time to fulfill other clients’ obligations. We more than doubled our productivity; in fact, the 4800 literally paid for itself on that one job.
On the Job with GPSFor each job phase, UP would post the required grade control and alignment points on its bulletin board. We would download and process the data, load it in Trimble’s TSC1 data collector and stake out the points as needed. The half-dozen control points UP had set throughout the jobsite were the basis to calibrate the GPS.
An immediate advantage to using GPS became apparent when we didn’t need to set any control points in pairs close together, because GPS, unlike conventional total stations, doesn’t require a backsight. With GPS, if visibility was obstructed for any reason, such as dozers, scrapers and trains on the site, it wasn’t a problem; intermediate control points weren’t necessary. The GPS base station remained at the same location throughout the project.
Additionally, sight limitations inherent with traditional surveying due to atmospheric conditions were not an issue with GPS. During a heat wave or in fog, optical instruments would be limited to two or three hundred feet or less in order to obtain the vertical accuracy needed. The satellites are overhead at all times and the GPS base station centrally located, so line-of-sight and visibility is not an issue. Wherever we were on the site, whether close in or three miles away, it didn’t matter; we still could set the points needed for the task at hand.
Increased production was another advantage of RTK GPS. The sheer volume of stakes we were able to set was amazing. With RTK GPS, we found that whether we were staking for grading or setting stakes for utility lines, the tasks get done efficiently and accurately.
Speed was one factor; RTK also allowed the party chief to be right where the stake was set, cutting miscommunication or misdirection to zero. Because the chief was at the rover, he could direct what to write on the stake, which way to face it, and whether it should be set at all, in view of imminent destruction by construction activities.
Before RTK, GPS provided raw data that needed post-processing, and I found it wasn’t useful for jobs that required immediate results, such as construction work. RTK gives you the solution in the field, which means GPS can now be used on all kinds of survey jobs.
In fact, RTK GPS opened up other avenues of work I didn’t anticipate when I purchased the system. Today, much of CCC’s work is in wetlands mitigation and fiberoptics route survey. These both became practical with RTK because RTK is faster and better for wide areas. With the miles involved in those kinds of jobs, standard optical total stations are no longer economically practical to use.
Increasing Efficiency On-the-FlyWith six miles to cover on the UP project, we often needed to drive from one part of the site to another. But with RTK’s on-the-fly initialization, we lost no time. The project proceeded largely from west to east. At times, we needed to set rough grade in one area, ditch grading in another, storm drain or electric in yet another. If we put the GPS system in the truck—or lost satellite lock for another reason—by the time we got out of the truck and walked to the general area to stake, the GPS system had automatically locked onto the satellites and initialized or completed the re-determination of the real-time corrections necessary for RTK positioning.
In contrast, with a conventional total station you have to stop and start over, hoping nothing blocked the backsight or foresight. That is the real convenience of the 4800: with everything on the rover pole, you can set up your base station and move all over within 10 kilometers.
As a backup or on jobs using both conventional and GPS, the data collector provides a worry-free data management between both types of instruments. With the data collector and a PC card, a surveyor can carry data for several jobs, manipulate more data while in the field and easily download it to a PC.
I found using GPS gives me greater confidence and control on the entire job. Today we use conventional equipment on projects only when GPS can’t be used due to obstructions to the sky or when accuracy requirements are greater than a couple of centimeters, such as laying out a building.
Even with these enhancements, survey training is still required. In fact, because the actual ease of use of GPS is great, the need to employ good surveying practice is as important, or more so, than ever, to counter the “magic black box” mentality.
Bombs AwayWhen we had completed about five percent of the work, Mendez’ grading crews uncovered a cigar-shaped hunk of steel about four feet long. At first they thought it was part of a refrigerator, but experts quickly identified it as a 250-pound bomb, one of the Mark 81 bombs from the 1973 cargo.
Officials called in U.S. Navy Moffet Field UXO (Unexploded Ordnance) personnel, who moved the bomb to an isolated part of the yard, buried and detonated it after 400 nearby residents voluntarily evacuated.
Because of the large amount of metal debris in the trainyard, an electromagnetic survey proved unsuccessful. A week later, grading continued; within 24 hours, grading crews uncovered eight 250-pound buried bombs as well as chunks and fragments of bombs. UXO personnel again supervised the detonation; this time, several residents refused to leave their homes.
UP stopped work in a 150' x 1,000' zone and issued an RFP to recruit contractors to help clear the area of any remaining bombs. USA Environmental (USA, formerly CMS Environmental, Tampa, Fla.), which specializes in UXO removal, was selected for the job.
Underground SearchUsing a Geonics EM61 (Geonics, Mississauga, Ontario, Canada), a time-domain electronic magnetic sensor, USA began work. The EM61 measures changes in the electro magnetic field in the ground caused by buried metallic debris; each type of buried object produces distinctive responses that a trained operator can identify.
A typical EM61 survey is performed by pulling the instrument along predetermined, equally spaced survey lines and uses an axle-mounted odometer to determine the spacing of each data sample along the survey line. However, this method proved impractical to produce sufficiently accurate positions due to the rough terrain and mud from the wet winter. USA’s vice president/owner John Adams sought other navigation tools and chose a Trimble GPS mapping system for on-site navigation. Using a combination of the mapping system rovers and a Trimble GPS reference station, the spatial component of the survey was accurately recorded for each of the geophysical survey data samples. “GPS was the most cost-effective option given the open space and clear view of the sky,” he said.
The EM61 is a digital geophysical survey instrument that records and displays digital time-domain electro-magnetic (TDEM) survey data using an integrated data logger. “For this survey the EM61 collected geophysical survey data at a rate of eight readings per second and GPS updated and recorded the positional data every second,” Adams said.
The data files from the EM61 and the GPS were then merged into a single data file and the positional data for the intervening EM61 data samples was interpolated between the GPS readings. Combining the data and interpolation of the spatial coordinates was easily accomplished and proved to be very accurate.
USA then sent the data to Sanford Cohen and Associates, Boston, Mass., who created a computer model from the data, performed enhanced computer-based analysis of the data and produced target lists. The target lists included the spatial location of each target and the estimated depth and mass of the individual targets.
“To successfully model the individual targets, you have to have precise spatial references for each geophysical survey reading,” Adams said. "The points are meaningless by themselves unless you can accurately position each of the readings from the geophysical survey instrument.” Overall, USA targeted 1,131 areas that had the electromagnetic signature of a 250-lb bomb.
Following completion of the data analysis, USA used the same GPS mapping and reference station systems to precisely locate the targets and dig up the objects. Within several weeks crews found two more 250-lb bombs and seven additional fragments with high explosive (HE) residue, which they shipped to Sierra Army Depot in Susanville, Calif., for disposal. UXO removal then stopped due to extremely wet weather.
In mid-March 1998, grading work started again. Within a month, the crews found a small fragment with HE on it and another intact 250-lb bomb. Grading again stopped.
For the next six weeks, USA provided additional UXO operations under U.S. Army Corps of Engineer (COE) and Sacramento County oversight. By early June, UXO operations had uncovered four more intact 250-lb bombs; all were stored in magazines and shipped off-site to Laidlaw’s Louisiana facility for disposal.
“We investigated more boundary areas around the original target site and found items that were deeper this time,” Adams said. “We were very satisfied with the results overall.”
In all, recovered material included 16 unexploded MK 81 bombs; seven partial fragments containing explosive residue; 8,625 pounds (4.31 tons) of bomb fragments; and 131,560 pounds (65.78 tons) of ferrous material. Experts found the bombs were not fused (armed), making them less likely to accidentally detonate, but obviously not items that you would like to have in a railyard.
USA and COE completed their quality assurance operations and released the site to UP. Due to all the digging, grading crews had to bring in backfill to return the overexcavated site to railroad subgrade elevation, the survey points reset and the grading completed. This time, we noticed lots of bomb fragments; it was like finding rocks randomly scattered on a sandy beach. Chunks and fragments were all across the yard; they could’ve been rocks but you knew what they were.
In many ways, my involvement with this job was ironic. In 1973 when the bombs blew up I was taking my land surveyor’s exam, preparing for a career as a professional surveyor. At that time, HP-35 calculators were the “latest and greatest,” distance meters came in suitcases, total stations were in the realm of speculation, and GPS for surveying was unheard of. Now 25 years later, I’d surveyed the very area where the bombs exploded and used equipment few would have dreamed about. To me, that’s pretty cosmic.