Significant Project Features:
- In-situ treatment of 5,000 cubic yards of carbon-tetrachloride impacted soils.
- First field implementation of proprietary method of in-situ application of zero valent iron and kaolin clay.
- Large diameter (8-foot diameter) in-situ soil mixing columns to a depth of 35 feet.Solidification of upper 20 feet with incorporation of Portland cement following ZVI and clay treatment.
- Installation of asphalt cap.
Description of Work:
Geo-Solutions was contracted by the DuPont de Nemours and Company to perform In-Situ Treatment and Stabilization of contaminated soils at the DuPont Martinsville Unit I ISM Remediation site. Treatment of the contaminated soils involved implementation of a proprietary method of in-situ application of zero valent iron and Kaolin clay. Zero valent iron was selected to treat the primary contaminant of concern, carbon tetrachloride. Kaolin clay was used as a transport media and to minimize groundwater flow through the treatment zone following the treatment process. Elevated levels of carbon tetrachloride were identified on site in the existing soil and groundwater with shallow soil concentrations as high as 30,000 mg/kg. Other contaminants detected in the soil included chloroform, methylene chloride, dichloroethylene, barium and chromium.
Geo-Solutions’ scope consisted of in-situ treatment of 5,000 cubic yards of soil with zero valent iron and Kaolin clay slurry by the Shallow Soil Mixing technology. Shallow Soil Mixing (SSM) is a soil improvement technique used to treat soils in-situ such that soils can be treated without excavation of removal.
In this application, Geo-Solutions used a Cassagrande Model CM15 crane-mounted drill rig to support the soil mixing operation. A wet or hollow Kelly bar connects the mix auger with the drill turntable and is connected to a hose (via swivel) that carries slurry from the plant to the auger. An eight-foot diameter mixing-auger was used to drill, inject the media at the mixing head and mix to produce a homogeneous soil mix column. Zero valent iron and Kaolin clay were applied at specific ratios in designated areas to a depth of 35 feet. The eight-foot diameter columns were spaced to provide complete coverage of the treatment areas.
Kaolin clay slurry was prepared in Geo-Solutions’ on-site mixing plant to be injected into the soil during the drilling operation. The Kaolin slurry was injected at the mixing head as the mixing auger was advanced downward to the maximum treatment depth of 35 feet to create the appropriate soil mix proportions within the column. SSM columns were laid out to provide coverage of the three specific zero valent iron application rate zones. Once the prescribed amount of Kaolin clay was injected and the column thoroughly mixed, iron was added by driving a steel casing filled with the prescribed amount of iron for the column then pulled out leaving the sacrificial drive point and the iron in place. By proper sizing of the casing for the specific iron application rate, the iron was distributed evenly over the length of the column. The column was then remixed from top to bottom. Cement was incorporated into the upper 20 feet of treated soil to improve the workability of the treated soil so that it could be graded and capped.
A total of 78 columns were successfully installed to treat the designated area. A sampling program was implemented to verify the amount of iron installed. Selected columns were sampled a 10-foot intervals with Geo-Solutions’ SSM sampling tool. Soil mix samples were tested for iron content using a wet wash and iron separation test. Results of sampling indicated the average iron content of each column was greater than required and no individual samples were more than 20 percent less than the required iron concentration.
Control of both carbon tetrachloride emissions and migrant nuisance odors to prevent activation of off-site alarms both real and perceived were a significant concern. Controls implemented included staged mixing and excavation, plastic liners, tarps and application of latex foam. Other work related to the Unit I ISM Remediation included demolition and removal of buried utilities and concrete structures, abandonment of monitoring wells and construction of an asphalt cap.