Significant Project Features
- Mechanical excavation of approximately 17,000 tons of MPG-impacted soil from two separate areas of the Former Bay City Manufactured Gas Plant
- Monitoring of natural methane gas levels and work-related odors during remediation operations to ensure that the project was executed with zero exceedances
- Sevenson constructed, operated, and maintained a temporary 100-GPM Waste Water Treatment Plant (WWTP), which treated and discharged over 4.6M gallons of impacted water
- Sevenson completed over 8,000 safe workhours without a lost-time incident
History & Location Details
Located in Bay City, Michigan, the Consumers Energy Company’s Former Bay City Manufactured Gas Plant was used for converting coal and oil to gas for heating and lighting. As a result of industrial activities, MGP-related contaminants impacted the area’s soil. Consumers Energy purchased the plant in 1922 and officially terminated plant operations in 1942.
As the prime contractor, Sevenson worked in conjunction with Consumer’s Energy Company and Soil & Materials Engineering, Inc., to execute environmental remediation activities at two contaminated sections of the property, Area A and Area B.
SCOPE OF WORK
Before commencing excavation operations, Sevenson’s project management procured the required permits, including the MDEQ/USACE Joint Permit, the Waste Water Discharge Permit, and the Access Agreement with Bay Riverfront, LLC. Sevenson also obtained the Encroachment Permit from the City of Bay City, the hydrant permit, and the Soil Erosion and Sedimentation Control Permit.
After obtaining all required permits to perform work, Sevenson mobilized the following equipment to the site: a conventional long-front excavator; a front-end loader; pumps; bulldozer; a vibratory hammer; and components, materials, and tools to construct an on-site temporary WWTP.
Major site preparation operations included installing the required sediment and erosion controls, which consisted of installing a 200-LF silt fence, grate bags in catch basins, and wattles/silt sock. Additionally, Sevenson constructed, operated, and maintained an on-site temporary 100-GPM WWTP to treat and discharge all impacted site water. The WWTP’s system components included two 21,000-gallon equalization tanks, two granular activated carbon vessels, and two bag filters. Over the course of the project, Sevenson WWTP treated over 4.6M gallons of contaminated water.
SHEET PILE COFFERDAM
Sevenson installed a temporary sheetpile cofferdam to ensure MGP-impacted soil was removed in a safe manner. Sevenson removed topsoil and overburden, and then mobilized a crawler crane and vibratory hammer to install steel sheets to a depth of 60 ft. BGS. To facilitate shoring operations, field personnel constructed and a work platform and access to facilitate the movement of work crews and heavy equipment. Field crews excavated soil to a depth of 18 ft. BGS to install two rows of walers and spreaders.
WELLPOINT DEWATERING SYSTEM
Sevenson engaged and managed a specialty subcontractor to perform the installation of a wellpoint dewatering system. The system served to maintain a groundwater level of approximately 22 ft. BGS. PVC wellpoints were placed strategically around the perimeter of the excavation area to facilitate the installation of the wellpoint system. This system allowed Sevenson to perform soil excavation in relatively dry conditions without being impacted by water infiltration. The water removed from the wellpoint system was pumped and conveyed to the on-site WWTP before being discharged into the City of Bay City’s sanitary sewer.
After installing the wellpoints and sheet pile shoring, Sevenson proceeded with full-scale soil excavation of Area A and Area B. Primary excavation of Area A encompassed a 190-ft. x 40-ft. section of Bay Riverfront Development property. The second excavation addressed the 190-ft. x 30-ft. section of Area B, located adjacent to Area A. Sevenson mobilized a long-front excavator that removed contaminated soil from depths ranging from 10 ft. to 22 ft. Excavation activities proceeded from the west to east end of the cell. Excavated soil was then transported by a subcontracted trucking outfit to an approved offsite landfill.
Particularly challenging during excavation was removing contaminated soil around active utility lines. To address this issue, Sevenson utilized a subcontractor to install six inclinometers around the high-voltage electric lines located between Areas A and B. The inclinometers were monitored for movement during excavation to protect existing utilities and to ensure safe installation of temporary shoring. Field crews also performed hand digging and hydro excavation around the high-pressure gas main to facilitate safe soil removal around live utilities.
Another challenge involved mitigating MGP-related odors and monitoring natural methane gas emissions. To abate odors, field crews applied RusFoam® on excavated soil. Sevenson crews implemented personal and area monitors to control methane gas emanation and alert field personnel to potential exceedance hazards. These monitors were set to activate an alarm if methane gas exceeded 10% of the lower explosive limit. Observing these precautions, and addressing preventive measures in daily safety tailgate meetings, Sevenson completed this project with no methane gas exceedances or alarm events during operations.
BACKFILL AND SITE RESTORATION
After Sevenson’s project engineer obtained confirmatory soil samples and verified that cleanup criteria had been achieved, field crews commenced backfill operations. Following soil excavation, Sevenson installed an impermeable HDPE liner from the top of the excavation to the bottom on the northern wall in Area A. The rolls of HDPE were suspended beneath the bottom waler and extended along the entire 190 LF of the north wall. The bottom of the HDPE liner was extended to the bottom and anchored to prevent any movement during installation of backfill materials. During this process, field personnel removed the walers and struts before backfill was placed.
Upon completing backfill operations, Sevenson removed the access road and work platform; removed overburden; and regraded topsoil to restore previous elevations and contours. To prevent erosion and promote germination, Sevenson placed an erosion control blanked and applied hydroseed to the restored areas.