Significant Project Features
- Remediation of upland soils and sediments contaminated with MGP residuals (TPH) from the Illinois River and adjacent property
- Excavation, T&D of 51,900 T of contaminated soils from depths ranging 2-ft. to 25-ft. BGS
- Mechanical dredging of 11,300 T of contaminated sediments from the Illinois River from depths of 2-ft. to 15-ft. BGS
- Installed 2,500 LF of temporary sheet piling during excavation activities at depths up to 25-ft BGS. Installed 450 LF of temporary sheet piling during sediment dredging operations at depths up to 17-ft. BGS
- Designed, constructed, operated and maintained a dewatering system (pumps and wells) and 500 GPM temporary WWTP on-site; treated >20M gallons of wastewater to date
- Utilized 286 T of Portland Cement to dewater and stabilize 7,154 T of soils and sediment
- Completed site rehabilitation activities and placed backfill material in all remediated areas
- Sevenson completed over 42,000 safe workhours without a lost time incident
History & Location Details
Manufactured gas operations at the MGP Site occurred from 1872 and ended in 1931. In the mid-1950s, Central Elementary School was constructed on the Site. Central Elementary School flooded in September 2008, resulting in permanent closure of the school. The City of Ottawa purchased the property and demolished the buildings and structures in 2013. The MGP site has undergone remedial activities prior to redevelopment including the removal of MGP residuals from former tar tanks, tar well, and gas holder areas; and construction of an engineered cap over the majority of the MGP Site.
Site Preparation and Mobilization
Prior to site excavation activities, Sevenson obtained the approval of applicable pre-construction submittals, designs and permits. Crews mobilized 6 excavators, including 2-long front excavators, PC450, PC400 and CAT 345 excavators; 2 loaders; 2 haul trucks; a bulldozer; a compacter; and a custom-fabricated, 100-CY scow. Crews constructed a debris pad, a WWTP pad, a dredged material management pad, and an unloading platform along the bank of the Illinois River. There were 8 Sevenson personnel on-site including a PM, General Superintendent, Site Superintendent, Project Engineer, SSHO and 3 foremen. Sevenson managed 4 subcontractors for surveying, geotechnical compaction testing and dewatering activities.
Sevenson excavated impacted soils from two locations: the northern upland soil removal area and the southern upland soil removal area. Crews began excavations in the northern upland area and removed impacted soils with Total Petroleum Hydrocarbon (TPH) concentrations greater than the Site-specific remediation objective of 5,700 mg/kg at depths of 2-ft. to 25-ft. BGS. Due to deep excavations, water table elevation, and the presence of bedrock, Sevenson installed 2,500 LF of temporary steel sheet pile shoring around upland soil excavation areas at depths up to 25-ft. BGS. Crews pre-trenched the alignment of the sheet pile at a depth of 5-ft. BGS to remove any debris and obstructions and utilized a hydraulic excavator mounted pile driver to install the ESZ20-700 perimeter sheeting with internal bracing that consisted of steel walers and hydraulic struts. The top of the sheet pile was installed to grade and was placed above the 100-year flood limit elevation. Crews installed sheeting first in the northern upland area. Upon completion of excavation and backfill, sheeting was removed and installed around the southern upland area. Crews utilized a CAT 345 excavator and a PC400 long-front excavator to concurrently remove and load out materials for off-site T&D. Following excavation and backfill of the northern upland area, crews excavated the southern upland area at depths ranging from 2-ft. to 22-ft. BGS. Prior to load out, soils were stabilized with Portland Cement. Sevenson managed off-site T&D and shipped all materials under a non-hazardous waste manifest to the approved disposal facility. Crews excavated, transported, and disposed of 51,900 tons of contaminated soils.
To expedite the timeline, Sevenson performed sediment removal operations concurrently with the excavation of the northern upland area. Prior to dredging, Sevenson performed preconstruction bathymetric surveys of the proposed limits of disturbance. Sevenson installed temporary shoring along the shoreline to complete dredging and backfill operations within the dredging prism. Crews utilized a crawler crane and vibratory hammer to install 450 LF of temporary steel sheet pile along the shoreline at depths up to 17-ft. Sheets were driven to bedrock and supported using tie-rods and toe pins. Tie rods were installed on the landside of the sheets and penetrated the top portion of the sheet pile. Toe pins were then installed. Crews utilized marine based equipment including a PC450 long-front excavator equipped with a 1.5-CY bucket, and a PC450 long-front excavator with a 2-CY closed environmental conventional bucket, secured to a 40-ft. x 60-ft. sectional modular spud barge to remove sediments. Crews utilized custom-fabricated 100-CY scows to convey dredge and capping materials. Both excavators were equipped with Trimble Marine’s Teledyne PDS Dredge software which was utilized to streamline production and monitor the real time position of the bucket to prevent over/under dredging. This was critical due to the stringent horizontal and vertical tolerances of this project. Excavators mechanically dredged sediments and placed the material in the 100-CY scows which transported the material downstream to the unloading platform where crews decanted water and utilized a land-based excavator to unload the sediments onto the material management pad for dewatering/stabilization. All decant water and water from dewatering was filtered and discharged through the WWTP. Sevenson mechanically dredged 11,300 tons of impacted sediments with concentrations >5,700 mg/kg TPH at depths ranging from 2-ft. to 15-ft. BGS.
Sevenson utilized turbidity control measures and BMPs to minimize resuspension and migration of sediment. Crews installed semipermeable Siltdam turbidity curtains around the work area and upriver to reduce the migration of sediments and allow more water to pass through the curtain to reduce wear and tear on the curtain. Crews monitored the curtain and turbidity daily to ensure there were no gaps, tears or breaches in the curtain.
Sevenson initially designed, constructed, operated and maintained a 100-GPM temporary on-site WWTP to treat wastewater generated during excavation and sediment removal operations. The WWTP was constructed on a lined pad for containment and bounded by a berm to contain any potential spills or rainwater. During excavations, wastewater was transferred from the excavation dewatering system, consisting of well points, around the exterior of the sheeting and from sumps within active excavations to two 21,000-gal. frac tanks and two weir tanks. Decant water from sediment removal was collected at two locations: the 100-CY scow and the dredged material handling pad. A pump was positioned at the unloading platform and pumped decant water from the scow to the WWTP. This reduced the saturation of the sediment prior to placement on the dredge material pad. Drains were installed on the dredge material pad and transferred additional water from the sediment to the WWTP. Water was subsequently treated through the WWTP and pumped into an effluent tank prior to discharge into the Illinois River. Wastewater was treated and discharged in accordance with the IEPA NPDES discharge permit. Monthly compliance and weekly process sampling were performed prior to discharge to ensure all treated water met permit criteria. Sevenson had one exceedance for benzene and subsequently changed out the filtration media to meet discharge criteria. Crews determined a higher volume of water needed to be treated than originally anticipated and subsequently constructed an additional 100-GPM WWTP. The two 100-GPM WWTPs were ultimately replaced by a 500-GPM WWTP. To date, over 2M gallons were treated and discharged through the on-site WWTP.
Combined Sewer Relocation
During excavation and backfill of the southern upland area, Sevenson installed and operated a temporary bypass pumping system for the combined sewer system that existed on the southern portion of the Site. Upon completion of excavation and backfill, Sevenson installed a new combination sewer line and manholes, including 24-in. PVC piping and precast concrete manholes. Crews utilized a PC300 excavator to excavate a trench for the installation. Trench boxes were used, as necessary, to provide safe shoring. Sevenson installed 680 LF of new piping, 2 manholes, and 2 dog houses. Upon completion, Sevenson performed the necessary leak tests and removed the temporary bypass system to allow the new combination sewer to function.
Crews restored the excavation areas to the previous grades utilizing 47,040 tons of imported fill including half granular and half common fill, topsoil, and seeding. Asphalt and concrete structures were restored. Sevenson performed capping operations following dredging operations and placed 7,560 tons of course aggregate and 588 tons of rip rap.