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The City of Milwaukee, Wisconsin, Manages a Remedial Excavation with GIS
Maintaining Accurate Data During Brownfield Site Redevelopment Excavation
The City of Milwaukee, Wisconsin, planned to redevelop a Brownfield site (i.e., land known to contain pollutants or contaminants) just outside the central business district as 53 single-family housing units. The property historically consisted of approximately 80 parcels with various land uses, including residential, commercial, and industrial, dating to the late 1800s. In the 1960s, the site was cleared and filled for a proposed freeway that was never built. Most recently, the site was used as a neighborhood park.
With the various historical site transformations, the soil on the site could be classified into three different categories:
The varying levels of contamination and destinations of soil disposal presented the following problems: tracking where the soil had been, determining where it was going, and calculating how much was to be moved. In addition, the data had to be collected rapidly to not interfere with the excavation, as well as have results to report the next business day. Finally, with a site that has multiple historic uses with varying levels and sources of contamination, 89 cells had to be created to be able to quantify excavation depths at specific locations.
The City of Milwaukee had available Milwaukee County Automated Mapping Land Information System data of the entire site, but it needed someone to plan and manage the remedial excavation activities. Following extensive research, the city retained Symbiont (formerly Triad Engineering), a full-service engineering and consulting firm headquartered in West Allis, Wisconsin.
Symbiont used ArcView along with the ArcGIS 3D Analyst extension to calculate and manage data pertaining to the site. "Without using GIS, we could not have sustained such a rapid excavation pace while maintaining accurate data," says Michael Kumbera, former GIS specialist with Symbiont.
Utilizing this data, a Leica subcentimeter GPS was used to map soil-boring locations on the site. After the soil boring data was analyzed, geologists derived areas of contamination and formulated cells based on depth of levels of contamination. These cells were digitized in ArcView and x,y coordinates were calculated at the cell corners. These coordinates were then exported to the GPS unit, and the cells were physically staked out on-site.
With the site staked out, the excavation process was ready to begin. During the process, as many as two backhoes and 10 dump trucks were operating simultaneously to transport soil. Depending on the category of soil being excavated, soil was transported to either the on-site stockpile area, an industrial fill site, or the landfill location. During the excavation, the GPS unit was used to verify the elevation of the excavated soil. Therefore, site engineers could accurately determine how deep to excavate with subcentimeter accuracy. When the excavation for the cell was completed, data points were collected at several locations to verify the elevation of the entire cell for each specific soil type. This process continued rapidly with backhoes operating on different sections of the site.
After the excavation activities concluded each day, the GPS data collected was imported into a geodatabase using ArcView. Since the excavation site was local, the data could be manipulated in an office using a desktop computer. Using the ArcGIS 3D Analyst extension, the coordinates collected were used to construct triangulated irregular networks (TINs).
"The TINs were very representative of the site," says Kumbera. "We were able to clearly decipher historic foundations, manholes, and variations between cells." With the TINs created, cut/fill analyses were performed and the volume of soil excavated was calculated by cell and type. This enabled the excavators to know the daily volume totals of soil type as a measure of progress and as quality control.
Telling the story of a site excavation in a meaningful manner can prove to be difficult. Verbally stating how deep soil was excavated at a location fails to provide a comprehensive picture of what really happened at the site. Using GIS to generate before and after topographic maps summed up the result with only two images. Displaying the TINs in one-foot intervals created a colorful yet meaningful project summary. Furthermore, being able to overlay planimetric data of the existing ground surface and the proposed site plan gave the ability to determine impacted cells relative to existing and proposed features.
The results of using GIS to collect and manage excavation data proved to be very successful. The final volume of soil relocated was within one percent of initial preexcavation engineering estimates. Utilizing a subcentimeter GPS was the quickest and most accurate way of collecting data. It gave the excavators the opportunity to work at an optimal level, as well as provided a detailed log about what was being excavated and when. The presentation-quality graphics, provided by ArcView and the ArcGIS 3D Analyst extentsion, allowed those who could not physically be at the site to experience what had occurred and visually see the physical changes. The use of ArcView on this project saved time and money while producing highly accurate results. Project engineer Chris Gdak states, "Looking back at the project, I don't think there is any other way we could have done it."
For more information, contact Dave Misky, environmental scientist, City of Milwaukee (e-mail: email@example.com, tel.: 414-286-8682); David Holmes, P.G., senior hydrogeologist, Symbiont (e-mail: firstname.lastname@example.org, tel.: 414-291-8840); or Michael Kumbera (e-mail: email@example.com).