Getting into Deep Water
Like many old parking structures in the United States, the Riverside Drive Parking Deck in Elgin, Ill., is in need of a face-lift. But it’s not just the structure’s mid-1960s façade that needs rejuvenation, it’s also what lies beneath. Nearly all of the 1,000-foot-long by 60-foot-wide structure is situated over the Fox River on hundreds of concrete piles, which are topped by cast-in-place caps and beams. The deck itself is made of precast hollow-core planks arranged in 20-square-foot bays. The planks are covered with 3.5 inches of asphalt.
Structural drawings no longer exist, but what is known is that the precast elements are reinforced with high-tensile steel wire. While this reinforcement technique was common at the time the deck was constructed, the wire’s performance history has been dismal in high-moisture areas. The steel rusts, and since rust occupies a volume two-and-a-half times greater than the parent steel, the resulting expansion causes delamination, spalling and structural failure. Since the 1980s, maintenance on the deck has been ongoing and expensive, and in 2003, a 20-foot by 15-foot section of deck fell into the Fox River.
But before a conceptual design could be developed, the entire structure first would have to be analyzed to see what could be retained and what had to be demolished, and data had to be collected. These requirements meant that new surveys would be needed--including some innovative survey work under the unsteady deck and in the Fox River.
Scanning the River Bottom
The entire project was awarded in 2007 to a team of consultants led by Hitchcock Design Group, Naperville, Ill., with the survey portion of the project contracted to V3 Companies Ltd., a multidisciplined consulting firm based in Woodridge, Ill. “Sure, I was a bit nervous,” admits V3 Survey Technology Manager Grant Van Bortel. “There are a lot of cracks and small chunks of concrete that looked as if they were held up by cobwebs--and there’s the 2003 failed plank already in the river. But our structural engineers gave me confidence that this is not the kind of collapse that happens quickly, but rather slowly and gradually.”
In the summer, the Fox is shallow enough for a person in chest waders to get around. It’s dark and cool under the deck, where piles are arranged in ranks like a lifeless concrete orchard. “There’s a lot of bent rebar and concrete lying on the bottom, most likely from construction and previous maintenance efforts,” says Van Bortel. “It’s all covered by 12 inches of silt, so you can’t see it until you have already stumbled into it.”
Van Bortel and V3 Crew Chief Steve Arnold traveled to the site each morning towing a rowboat, their trucks loaded with a Trimble GX 3D scanner, laptops, heavy-duty marine batteries and all the other equipment needed for a day in such a dismal environment. “We also have a Trimble VX Spatial Station, which is a great instrument,” says Van Bortel, “but the GX was the better choice for the under-deck survey because it gathers so much data so quickly--there was a lot going on under there.”
Scanning was definitely the best approach for the Riverside Drive project; a detailed scan would help engineers determine the exact size and shape of each set of piles and caps and would aid in determining whether they were reusable. Using just a reflectorless total station would have been too slow and would not have collected enough necessary information. Additionally, scanning gathers so much information that revisits are often eliminated--new measurements can be taken as needed from the point cloud. Eliminating revisits is always good, of course, but on this project it was especially desirable.
Scanners aren’t magic, however; and they still need to be used skillfully. Arnold set up on an extra-long-legged tripod mashed into river mud in the deeper water and a miniature tripod closer to the shoreline back wall. “We used resections from the opposite bank to get on our coordinate system,” he says, “and I learned that the GX can’t thread a needle like a total station. It needed a slightly wider window to get a scan of the targets. With a total station, a single straight line measurement is possible. A few times, I got all set up and then couldn’t shoot between the piles to reach our backsight resection points. We had to carefully plan our setups.”
Arnold also dealt with another interesting scanner issue related to the river. At the river’s surface, where the piles meet water, the Trimble GX would record “mirror images” of the piles and caps that were above the water line. These images appeared in the data as if they were under water, and they looked like a footing due to the reflection. “We could actually see the beam bouncing off the water,” he says.
Van Bortel and V3’s project surveyor, Svetlana Koleva, solved the problem with Trimble’s RealWorks Survey office software. “In the [software], we could look at the data and differentiate the objects by their color-coded intensity,” Van Bortel says. “Distinguishing between the actual object and the reflection objects was obvious; I could then visually identify the points under water and segment or delete those points using RealWorks’ topography-based sampling utility.”
By tying the rowboat to beams and piles, Van Bortel and Arnold were able to use the boat as a floating desk. Van Bortel took advantage of this arrangement by bringing a laptop with him out on the water. “We usually use the TSC2 controller with the VX when scanning, and sometimes the GX,” he says, “but for this job we needed to use a laptop.”
The Trimble TSC2 controls the scanner using Trimble’s PocketScape field software, which is adapted to work with the TSC2’s smaller screen. By using a ruggedized field laptop with an outdoor-viewable monitor, Arnold was able to use Trimble’s PointScape software instead, which displays more information and has more tools. This change was another response to setups that could not be repeated, so using PointScape was a way to further verify good results before leaving a point forever. As a bonus, Van Bortel could do office work while wearing a hardhat, safety vest, glasses, life jacket and chest waders--with water up to his waist--as the scanner was collecting data.
“The control setup on top of the deck was standard,” says V3 Project Manager Ed Hedge, PLS, “but extending it down below was a challenge.”
V3 Companies mainly uses Trimble robotic total stations and GNSS receivers. V3 also uses Geodimeter 600 Pro robotic total stations but now operates them with Trimble ACU controllers, which eliminates the need for multiple office software packages and field training. Equipment uniformity allows V3 to take advantage of Trimble’s Connected Site concept, in which all equipment is integrated and designed to work together seamlessly.
In the Riverside Drive Parking Deck project, the firm took the concept even further by using Precision Midwest's Trimble VRS Network, which extends over 1,800 square miles in northern Illinois. V3’s Trimble R8 GNSS receivers are tied into the network and enable highly accurate RTK work without requiring a base station. A local coordinate system using ground distances but connected to state plane coordinates was used on the job so that control could be extended from an adjacent roadway project simultaneously being completed.
“There’s a railroad on the other side of the river,” says Hedge, “so we got permits and flaggers and set multiple points on the river bank, and then we placed targets for resections during the scanning. Being on a common coordinate system for the scanning was a big help in avoiding registration issues.” (Registration is a process where scanning software spatially connects independent scans.)
The survey work on top of the deck was more conventional. A Trimble VX Spatial Station was used to scan features like stairwells and to obtain standard measurements. V3’s Arnold likes to set the Trimble VX up on a tall tripod and level and operate it wirelessly using the Trimble TSC2 controller with the MultiTrack active target and video controller. A tall setup usually means more can be seen from a particular point, which reduces the total number of setups on a project.
V3 used just about everything in the equipment shed on the Riverside Drive Parking Deck project, including auto levels, a Trimble S6 Total Station, and Geodimeter 600s. Trimble software, including Trimble Geomatics Office, was used for downloads and processing.
“Years ago we operated with multiple brands of equipment, which was difficult from an operations standpoint,” says Van Bortel. “We find Trimble to be good equipment with intuitive work flows, while being competitively priced. For us, it made sense to consolidate on one platform.”
In their use of scanning and the data that come from it, V3 Companies and many other firms are a bit ahead of their clients. For instance, on the deck project it would have been nice to develop a complete 3D model of the underside rather than extracting measurements and data as needed. But Van Bortel explains: “Many clients don’t have the software or staff that can utilize that type of product quite yet and are a bit overwhelmed with the amount of information scanning offers. For those that are able to work in 3D, data communication is also a hurdle, as some clients don’t have Internet connections for us to post to an FTP location, much less broadband--you can’t just e-mail over a few million points on a dial-up connection. That’s why we sometimes hand deliver or FedEx a DVD or USB mass storage device.”
But Van Bortel also says this situation is changing: “I’m getting a lot of feedback, and I can see that things will change with time. As civil engineers, architects, landscape architects and structural engineers spend some time with the 3D data, they see the value, and it’s going to become another one of the norms in the surveyor’s tool kit.”
Measurement technology has advanced rapidly in the last decade, and a project like the Riverside Drive Parking Deck shows just how far it’s come. With mostly one-person crews, V3 Companies was able to use multiple scanners, a variety of total stations, GNSS receivers and a real-time network, implementing each tool where it was most efficient and maintaining one consistent data stream in a single coordinate system. All of the work done with the various instruments was assembled into a single model and analyzed with advanced design tools.
But even with exceptionally sophisticated technology, as the Riverside Drive Parking Deck demonstates, the world still needs surveyors who are willing to get a little wet and muddy in order to get the job done.