- Envirocon performed work within a bay of Lake Superior at Duluth, Minnesota, containing coal tar impacted sediments. Envirocon placed a sheetpile containment wall around an 11-acre area which was capped, and subsequently placed a subaqueous sand cap, followed by a sand surcharge cap. The scope of work also included site preparation; spill boom and silt curtain installation; installation of monitoring instruments; performing demonstration lifts; non-woven geotextile installation; subaqueous cap and above water cap and surcharge construction; and demobilization.
- Envirocon developed the project approach with the goals of providing the safest, most cost-effective, efficient, and best quality service possible to meet the stakeholder's objectives. This was accomplished by coordinating our work effectively and safely with the stakeholders, regulatory agencies, and the public to meet the required schedule completion date. The goals were met by providing the following innovative approaches:
- Multi-tasking specific operations to consolidate project schedule;
- Fabricating specialty marine equipment;
- Developing a state-of-the art program using equipment and procedures to control and confirm the rate of placement of the cap and surcharge material;
- Calculating and documenting rate and thickness of subaqueous sand cap and surcharge matterial;
- Performing subaqueous sand cap and surcharge material placement activities using 24-hour operations at 7 days per week;
- Utilizing "as equal" alternate product for root barrier layer material; and
- Protecting air and water quality and minimizing potential impacts to air and water quality.
Envirocon structured the schedule such that a number of work activities were performed simultaneously which ensured the project completion date was met. This reduced overall on-site time required for fieldwork activities, providing savings in management and overhead costs.
Envirocon fabricated specialty marine equipment so the subaqueous sand cap to be spread evenly and uniformly in a manner that could be tracked and confirmed. The introduction of slurry into the water was at reduced velocities, via a state-of-the-art spreader box. Turbulence and sediment disruption was minimized, thereby providing greater safety protection factors for water and air resources as well as the public and surrounding residents.
The marine equipment operated in very shallow water depths. This allowed the subaqueous sand cap materials to be placed according to the specifications, in specified lifts, up to the water surface using the specialty fabricated marine equipment. The control of the location and rate of placement of subaqueous sand materials using this equipment allowed precise monitoring and recording of the depth and location of materials.
Envirocon included a computerized quality assurance (QA) system for monitoring and optimizing the cover placement. The system continuously monitored the cover feed versus the movement of the cover placement equipment. This data stream allowed the QA system to continuously monitor cover thickness as the cover was placed. Based on the QA system feedback, the operator adjusted the feed rate and/or placement equipment movement to produce cover lift thickness as required. A calibrated load cell on the conveyor belt continuously monitored the weight of cover material as it was fed into the cover placement equipment. A GPS unit on the placement equipment provided the equipment's location. A computer was used to gather the input data, calculate the cover thickness, and store the raw data and calculation results in a database.
The geotextiles were assembled in rolls 40- to 75-feet wide and up to 400-foot long. The rolls were installed on a reel fastened to the deck of a barge and unreeled over the advancing side of the barge using an idler roller. The starting end of the geotextile was anchored at the shore. The guide float system was used to keep the geotextile placement in the intended location. The settlement plates were installed immediately behind the non-woven geotextile installation and were installed by clearing the ballast sand prior to installing the settlement plate.
The sand was spread and placed using a special arrangement to ensure that the spreading box traversed the cap area on a linear line. The barge and spreader box were connected by a floating pipeline to the pumping float at the south end of the cap area near the sand stockpiles. The pumping float was a dredge outfitted with a modified hopper pipe intake to accept sand feed from a portable belt conveyor with a variable-rate feed hopper. The belt conveyor had a weight-belt-scale. The weight belt scale was converted using a weight-to-volume relationship for the sand fed into the pump during the day's spreading activity. The feed was at a rate of approximately 200 cubic yards per hour. The sand pump delivered the sand slurry to the spreader barge at approximately 5,000 gallons per minute (3.33 cubic yards of loose sand per minute).
The dredge float delivered a constant flow to the sand box. To initiate a pass across the cap, sand was supplied to the pump at a rate of 3.33 cubic yards per minute. When sand reached the box, the box was moved along the guide system at a rate of approximately 12 feet per minute, spreading approximately 6 inches of sand over the 15-foot width of the box. At the end of the run, sand feed stopped prior to reaching the end (60 to 90 seconds required to empty the pipeline), the two ends of the guide moved 15 feet, and the process repeated.
A Trimble GPS receiver on the sand spreader constantly monitored the location of the spreader box. The Trimble system fed the location signal to a Hypack software system on the spreader barge so that the operator knew his rate of travel along the guide system. The Hypack software also sent the location signal to a separate computer located near the feed conveyor weight belt scale. This quality control computer monitored both the conveyor feed rate and the location signal from Hypack to determine the rate of movement that should have occurred to evenly spread the sand. The QA computer also provided a signal to the spreader barge indicating the required travel rate the spreader operator should have made as he traveled along the guide system and provided notice to the operator whenever the feed to the pumping dredge started or stopped.
Every 24 hours, the database on the QA computer was used along with the positioning control from the Hypack software to produce a map showing the area of the cap layer placed in the last 24 hours and the concurrent statistics showing the rate of placement and rate of travel.
Once subaqueous capping operations were complete, Envirocon began surcharge placement (above the water line) utilizing conventional capping methodologies. Material was loaded from the stockpile location, trucked to the cap area, unloaded, and placed to required lift thickness utilizing a bulldozer. Lift thickness was monitored on a continuous basis.
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