Significant Features:
- Stabilization of oily sludge and underlying soil
- 12-foot diameter interlocking soil mix columns
- Dry injection of reagent
- Performed bench-scale treatability study to determine optimum mix design
- Seven acre impoundment, 25 feet deep
Description of Work:
A refinery adjacent to Lake Michigan was required to close a pond (90,000 CY – API sludge; 20,000 CY – soil underlying sludge) which was previously used to contain refinery sludge as well as storm water and other wastes. Contaminants of concern included refinery-related products; petroleum hydrocarbons, and BETX. Oil and grease ranged in concentrations from 8-10% and total petroleum hydrocarbons ranged from 17-21%. A specific breakdown of contaminants obtained from composite samples of sludge includes:
The seven-acre pond was up to 25 feet deep and the sandy soils beneath the pond also required treatment. Off-site disposal was cost-prohibitive, therefore an on-site closure was engineered which included stabilization of the pond and containment using a slurry wall and cap.
Technical requirements for the stabilization specified an unconfined compressive strength of 35 psi in 14 days. Geo-Solutions performed a bench scale study prior to mobilization and a pilot project immediately prior to construction to evaluate the most cost-effective cement addition rate.
The SSM method was chosen as the most cost-effective method for mixing cement with the sludges for stabilization. SSM provided superior mixing quality and the power required to blend the underlying soils and cement with the oily sludge. The SSM rig was specially modified with a dust and vapor hood to minimize emissions. Geo-Solutions utilized a 12-foot diameter SSM mixing rig for the project. Dry cement was conveyed pneumatically to the rig where it was metered into the sludge and blended.
The SSM work was performed around the clock for three months to stabilize about 100,000 CY of sludge and soil. A three-foot wide cement-bentonite slurry wall about 40 feet deep and a jet grout curtain were installed around the pond to isolate the area from the groundwater. The slurry wall was constructed through inactive utility lines and debris. Finally, an impermeable cap of clay and synthetic liners (over 5 acres) was placed over the entire site. Specific constituents of the liner system included:
- 10 feet of random fill above the solidified material
- 2 feet of low permeability clay (1 x 10-7 or less)
- 60 mil HDPE liner – smooth on floor; textured on slopes
- Geo-Composite (net and fabric)
- 2 feet of protective soil
- 6 inches of crushed stone
- 6 gradient control dewatering wells
After final grading, the area supports new oil storage tanks.
SSM is a safe and cost-effective method of stabilization. This project was the first application of SSM using a vapor/dust hood in the United States.
