Case study | Geotechnical Engineering
Ameren MGP Site Centralia, IL
Completed April 2012
Remediation of impacted soils was required for the Ameren Manufactured Gas Plant Site in Centralia, Illinois. The remediation site was approximately 5.2 acres. Tar-impacted soils along with a decommissioned gas holder and other buried structures were removed, resulting in a maximum excavation depth of about 35 feet. It was desired to make near vertical, unbraced cuts along the perimeter of the excavation in order to remove the environmentally impacted soils.
A total of five phases were utilized during the remediation of the site. Each phase had an overall duration of 10 to 12 weeks. The deepest portions of the excavation were only open for a maximum of about three weeks prior to initiating the backfill process. The excavation was backfilled with lean clay. An excavation plan, designed by a Professional Engineer licensed in the State of Illinois, is required for personnel entry into excavations deeper than 20 feet. KEG provided excavation plans for the various phases of the project.
A tent was used to enclose the area of the excavation to limit the amount of odorous fumes emitted during removal of the contaminated soils. The tent was about 130 by 220 feet, supported by metal posts on 16-foot centers along the perimeter. Each post had a vertical design load of about 3 kips, transferred to the ground by a metal plate, approximately 16 by 24 inches. Air handlers used for vapor mitigation are enclosed in trailers, approximately 8 feet wide by 40 feet long, located around the outside perimeter of the tent. The trailers typically weigh about 40 kips each.
The Phase III through V excavations were complicated by the presence of an existing box culvert. The culvert is a cast-in-place rectangular box that runs along the east side of the remediation site, turns at a 45-degree angle towards the southwest for a distance of approximately 60 feet, and then runs along the south side of the remediation site for the remainder of its length. The inside opening is 14 feet wide and typically about 6 feet high for most of its length.
For the Phase III and IV excavations, the impacted soils were removed from beneath the culvert by making slot excavations. In several cases of the Phase III Excavation, the second round of slotted excavations was required to remove deeper impacted soils from beneath the culvert. The slot method worked well for the Phase III and IV remediation and was also used for the approximately 80 lineal feet of the culvert impacted with the Phase V work.
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