Significant Project Features
- Excavation of impacted material from contained ISSB cells (soil, brick, concrete, former MGP structures)
- Installation of 2,080 LF of insitu soil solidification barrier of varying widths utilizing Portland Type 1 Cement
- Develop a sanitary and storm sewer bypass plan to diver sewer flows around excavation limits; replace sewer lines and manholes upon backfill of work area
- Erection of 96 LF by 115 LF fabric structure with a stone and asphalt pad to house screening and stabilization operations; a carbon vapor extraction air handling system was operated during waste material processing
- Construct, operate and maintain onsite temporary 100-gpm wastewater treatment plant
- Restoration of river bottom and embankments to original grade and river contours
History & Location Details
The Clark Street Former MGP Site is located in Auburn, New York. The site consists of an upland area and a section of the Owasco Outlet adjacent to the NYSEG property, comprising a three-acre triangular-shaped area. The upland area is occupied by a NYSEG electrical substation and natural gas regulator building. MGP operations began in 1901. The site’s infrastructure included a gashouse, oil tank, and a 204,000- and 75,000-CF holder. A 491,000-CF holder was later constructed in the early 1900s. In 1943, gas holders #2 and #3 as well as a tar tank were removed and replaced by a coal storage area and tar shed. Gas operations ceased at the site in 1946. Gas holder #1 was removed in 1958 when the electrical substation was constructed. The McMaster Street MGP was first operated by the Auburn Gas Light Company in 1869 and manufactured coal gas using the coal carbonization process. Sediments and soils in and around the Owasco Outlet were contaminated with MGP-related materials, including coal tar, sheens, and odors from earlier operations. As a result, elevated levels of VOCs and PAHs were detected and documented. After several years of studies, a remedial action plan was developed for the site.
Upon winning this NYSEG contract, Sevenson mobilized and initiated the following remediation operations at this former MGP site:
Site preparation work involved clearing trees and vegetation to facilitate access to the site. Vegetation along the western site area was cleared to allow the installation of the in-situ soil solidification barrier (ISSB). A second clearing operation targeted the far shore of the NYSEG property to facilitate sediment excavation.
A grout-producing mixing batch plant consisting of a Sharpe Lightening Mixer, 30-ton horizontal weight belt cement silo, 200 kW generator, and a 185-cfm air compressor. Sevenson’s temporary on-site waste water treatment plant consisted of two 21,000-gallon frac tanks; two 21,000-gallon Weir Tanks; two feed pumps and sand filters; organoclay filter and granulated activated carbon filters; two backwash pumps; and a transmission pipe. A vapor extraction system was sized at four two-air exchanges per hour and was integrated into the fabric structure to maintain air exchange requirements, as well as collect and treat vapor. The system contained 8,000 lbs. of carbon. The vapor extraction system was designed to treat ambient air within the fabric structure via inline particulate filters, and its vapor carbon filter system removed dust and nitrous oxides prior to release into the atmosphere.
EROSION AND SEDIMENTATION CONTROLS
Erosion and sediment control measures were installed to reduce the potential for offsite material migration due to stormwater runoff in areas subject to erosion. The control measures also effectively limited excess sedimentation associated with on-site drainage pathways.
IN-SITU SOIL SOLIDIFICATION
Sevenson furnished, installed, monitored, and maintained the in-situ soil solidification barrier (ISSB), which was capable of supporting excavation sidewalls, and resisting soil and hydrostatic pressures. The stabilization walls were oriented into five separate geometric polygons, each of which had a different shape and wall segment widths, lengths, and depths. Wall segment widths were determined by the design engineer, ARCADIS, according to the adjacent structures and natural formations of the excavation zone. Wall segment depths were determined by reach bedrock or glacial till.
Pre-trenching prior to ISSB installation was required to locate the groundwater table and any obstructions in the barrier path. Pre-trenching to a depth of three to five feet accomplished water table and obstruction identification. The top two feet of overburden material was removed within the limits of the stabilization barrier to accommodate grout and soil slurry swell during the curing process. A 1.3:1 water-to-Portland Cement ratio was mixed at the batch plant and hydraulically pumped through flexible hosing to the excavation location. Owasco Outlet water and Portland Type 1 Cement were utilized for the grout slurry. The amount of Portland Cement was accurately determined by adding 10% of the soil’s weight to the mix. A bench-scale study indicated the weight of soil per cubic yard.
ISSB sections were installed every other 25 lf to allow proper curing time and prevent excavation collapse. When sections had to be connected, a three-foot overlap was required to prevent cold joints and joint failure. The connection was excavated along the vertical face of the pre-constructed segment to bedrock. Optical monitoring points were also established into the stabilization barrier’s vertical surfaces to monitor horizontal displacement during excavation activities.
Bank soil and sediment excavation reached to the depth of till or bedrock. Impacted sediment and soils were removed from the site for characterization and classification. An estimated 29,000 CY of soil and 9,600 CY of sediment were removed from the banks of the Owasco Outlet. Concrete debris and boulders were decontaminated and crushed for recycling. Excavated soil was stabilized in the excavation zone, characterized, and loaded onto trucks for offsite T+D. Excavated sediment was transported to the material staging area for processing via solidification. Characterization of processed sediments provided final confirmation that treated material met treatment criteria. Material was then loaded onto trucks for offsite T+D.
SOIL EXCAVATION WORK AREA DEWATERING
After the excavation cell opened to the bedrock level, impacted groundwater was extracted and treated. Dewatering was facilitated using a four-inch Godwin Model GHPU10 hydraulic pump.
Imported fill was placed and compacted in one-foot lifts within the excavation cells. Like excavation, backfill activities were progressed such that open excavation size/duration were minimized and clean backfill did not contact the contaminated material. Excavated material along the stabilization wall was backfilled within the same workday. Backfill reached an optimum moisture content of 14% and 95% compaction.
STORM AND SEWER REPLACEMENT
Sevenson conveyed storm and sanitary sewer flows to existing sewer pipes. The original sewer system was removed, relocated, and replaced without discharging flows to local waterways, surcharging upstream tributary sewers, or damaging private property. A by-pass system, consisting of three Godwin Dri-Prime CD300M pumps, was used during the repair and replacement process. The by-pass line was a 24-inch SDR 17 HDPE pipe that navigated the by-pass flow outside of the excavation limits to another sewer manhole.
OWASCO OUTLET BY-PASS
A temporary earthen dam was constructed at the upstream limits of the outlet to effectively isolate the active work area from water flow. The earthen dam was comprised of Type C crushed stone and concrete bin blocks. A hydraulic barrier was installed along the interface of the earthen dam utilizing a layer of 12-mil non-woven geotextile fabric and 40-mil textured HDPE liner. While an existing concrete dam on the outlet’s downstream limit prevented water from back-flowing into the excavation zone, an existing spillway in the concrete dam was regulated using a supersack sand spillway restraint. A by-pass pumping system capable of a maximum flow rate of 100 cfs was installed to re-route the Owasco Outlet channel during sediment removal activities. Six Type 6 (6 – 8-inch) electrical pumps were utilized to efficiently transfer the Owasco Outlet flow around the work area.
WASTEWATER TREATMENT SYSTEM DESIGN
Prior to beginning excavation operations, Sevenson installed a temporary waste water treatment system (WWTS) designed to remove suspended solids, insoluble metals, VOCs, SVOCs, and oil and grease from water generated during construction operations. The WWTS treated all water generated from dewatering and soil/sediment excavation operations.