Clean Closure of Five Coal Ash Ponds D.W. Mitchell Generating Station

Significant Project Features

• Closure under USEPA’s CCR Rule included removal of all remaining coal combustible residuals from five (5) ponds
• Demolition of above and below grade piping and concrete foundations
• Dewatering, excavation and transportation of 55,000 tons of coal ash to landfill
• Regrade existing berms within the five ponds to prevent runoff into Lake Michigan

Project Gallery

History & Location Details

Project Description

MOBILIZATION

Sevenson mobilized heavy equipment and installed temporary facilities, which included trailers, a parking area, and a stabilized construction entrance. Following a temporary facility installation, Sevenson proceeds to install a 4,000-LF construction fence along the limits of disturbance, as well as Exclusion Zone signage. The 150-ft.-long by 16-ft.-wide stabilized construction entrance was constructed off of the site’s ingress/egress road.

SITE PREPARATION

Site preparation activities involved constructing an equipment and storage area, and installing a 2,000-LF silt fence along the lakeside of the excavation area, per Sevenson’s Stormwater Pollution Prevention Plan (SWPPP). Silt fencing was inspected weekly, and after significant rain events, for the duration of the project. In addition to the silt fence, Sevenson’s field crews maintained positive drainage at all times toward the excavation and re-grading areas to eliminate the potential for stormwater runoff.

DEMOLITION ACTIVITIES

Demolition of existing structures was required to create safe access points to areas targeted for remediation. Sevenson’s scope of work involved demolishing above-grade and at-grade piping and support structures; drainage units and subterranean piping; and concrete foundations. Pipeline demolition commenced at Ash Pond 1 followed by Ash Pond 2 and Ash Pond 3, respectively. Field crews removed and disposed of approximately 3,350 LF of pipeline from three locations within the generating station. Pipelines were flushed with water to expunge residual ash contaminants and then cut into sections using a Komatsu PC 400. Following removal of overhead pipelines, Sevenson’s crews dismantled steel pipes and used a Komatsu PC 400 to remove over 200 H-beam support columns. Likewise, steel piping and H-beams were subsequently cut and staged for recycling. Next, crews proceeded to demolish three runs of 10- to 12-inch diameter iron and fiberglass piping along the south berm of Ash Pond 3. Additionally, three concrete outlet structures at the ponds were razed and removed for off-site disposal. Removed piping was segregated and loaded for transport and disposal at Summit, Inc. in Gary, Indiana.

EARTHWORK/EXCAVATION OPERATIONS

Prior to commencing excavation activities, Sevenson constructed an on-site discharge infiltration gallery in accordance with an NPDES permit. The infiltration gallery consisted of a polyethylene-covered perimeter berm with a bottom layer of stone, which was constructed by forcing soil up from the interior of the infiltration area. Water was temporarily pumped to Ash Pond 3, the last of the ponds to be excavated if the water in the gallery was not infiltrating adequately. The collected water was occasionally used for dust control purposes.

Sevenson excavated approximately 41,000 CY of ash from the ponds. Based on previous site investigations, Sevenson was aware that the floors of the ash ponds were comprised of a hard, impermeable surface. Sevenson, therefore, removed ash material using mechanical excavation equipment. A Komatsu D61PX dozer pushed excavated ash material into piles for direct loadout and, in some cases, to encourage gravity drainage prior to loadout. A Komatsu PC300 excavator was used to load 55,000 tons of excavated ash into dump trucks for off-site transportation and disposal. Positive drainage was maintained towards the interior of the site during excavation to eliminate the potential for stormwater runoff. Upon completing ash removal operations, Sevenson scheduled “visually clean” inspections for the Engineer and IDEM representative to ensure that site conditions met remedial objectives. Additionally, pre- and post-excavation surveys of the ponds were performed by an Indiana-licensed land surveyor.

BACKFILL AND SITE RESTORATION

Sevenson’s backfilling activities involved re-grading the existing berms around the perimeters of ash ponds into the excavations. The berm material consisted of fly ash, slag, and fragmented kiln bricks. Field crews placed and graded about 58,000 CY of backfill material according to the lines and contours specified in the construction drawings. A Komatsu PC300 (or equivalent) excavator removed the backfill material from the berms and placed it in the pond areas, and a Komatsu D61PX dozer spread the material in 8-inch-thick loose lifts. A smooth drum roller then compacted lifts in single passes. Upon backfilling each pond, a survey was performed to document quantities.

Site restoration activities consisted of removing the stabilized construction entrance and demobilizing all equipment, materials, and personnel.