Could they trust the maps? Were they accurate? Maybe. Complete? Maybe. No one knew for sure — a bad position to be in for the owner of a large wastewater treatment plant preparing for a huge expansion project.
The owner and engineers needed accurate and complete information about underground structures to prepare plans, let bids and manage construction. They needed positions in all three dimensions. Proceeding without the information for all underground features meant risking injuries, line strikes, environmental issues and potentially huge change orders. It just wasn’t acceptable; the risk was too high. But how would they get the data without breaking the bank?
Many technologies for locating underground features have been around a long time. But near the plant, the soils were saturated and full of clay minerals. Ground penetrating radar (GPR) performed poorly in these situations in the past. Electromagnetic (EM) probing was unsuccessful due to the depth of existing infrastructure and surface conditions. The difficulty and cost of hundreds, and eventually thousands of potholes were prohibitive. Time was running out. The hunt for more useful technology was on. The final answer would span some rather old and some very new technologies.
The engineers sought out Cartacoustics LLC, headquartered near Denver. Cartacoustics has patented sophisticated technologies to detect and locate underground features. Its Advanced Sonic Technologies (AST) system uses a technique called echolocation to find and measure depth to underground structures. This approach is like counting the seconds between a lightning flash and the thunderclap to estimate how far away the lightning bolt struck.
AST creates a series of carefully crafted sound pulses so their reflections can be plucked from all the other noise in the ground. The system measures the time from transmission of each sound pulse to the reflections from underground objects to estimate their depth. Since the almost all man-made objects are different from the surrounding soils, they reflect sound very well regardless of their composition. The makeup of the soil does not matter. The moisture in the soil does not matter. Data collected from many points in the area is then aligned onto a single 3D grid and a picture of the underground emerges.
For this wastewater treatment plant project, Cartacoustics collected more than 300 individual soundings. The field work took about a week. Figure 1 shows the site. Each yellow pin is an acoustic probe location or a surface feature used for grid alignment.
To tie the acoustic collections together with the existing BIM model (building information model, a specialized CAD system), Cartacoustics made a high-precision survey of each surface location. Multiple baselines were required because of line-of-sight obscurations. Cartacoustics combined the acoustic findings with the surface location data and created a 3D model showing all the underground features, whether they were known beforehand or not.
The engineers loaded information from the Cartacoustics underground survey into their BIM model, which included data from the existing maps, as-builts, LIDAR surveys. The results are shown in Figure 2.
The acoustic data (red areas) lines up well with the information from the maps, plans and as-builts. This gives the engineers confidence that the acoustic findings are more accurate than the drawings in those places where they do not match. The acoustic information provides SUE Level-B information and can focus attention and resources to specific regions for SUE Level-A investigations, saving time and money, and reducing risk.
Not shown in Figure 2 is a region of acoustic returns in the middle of the open area toward the lower right. The AST acoustic survey showed many returns in this area that did not fit into any cohesive structures. After talking with the plant personnel, they indicated that it may be a debris field from earlier work decades ago. They were impressed that we saw signals of this.
The end result is a map that our grandfathers could not have created and would be proud of. It meets all the current industry needs and goals, created by combining technologies that have been around a long time, a couple of decades and that are brand new.
It is a map that allows the engineers to create the best designs. That supports a smooth and effective construction process. A map for our age that helps protect lives, the environment, infrastructure and deploys scarce public funds to places where they are most needed.