Silver Lake Removal Action and Sediment Cap Installation

Significant Project Features

• Sevenson’s successful pilot demonstration project conducted in 2006 to evaluate the feasibility of installing of a 14-inch-thick sub-aqueous cap in a lake-bottom environment led to the full scale implementation of this remedy
• Sevenson designed and built the cap installation system. The system places multiple thin-layer (one-inch) lifts. Thin layer lift placement minimized sediment disruption
• Geotextile materials were installed to enhance cap integrity and stability were incorporated into
• A turbidity curtain system was installed to minimize TSS and negative impacts on water quality
• Shore line area soils and sediment were excavated and transferred to an onsite storage facility for offsite T+D. Excavation included steel sheet pile used as protective shoring
• Placement of an armor stone layer along the shoreline prevented cap erosion from wave action
• Extensive restoration to disturbed areas. Restoration included a
  walking / bike path

Project Gallery

History & Location Details

Project Description

Sevenson was awarded the contract to implement this remedy based upon a competitive proposal. The scope of work for the project included a series of bank soil excavations at seven residential properties, ten commercial properties, and along the northern and eastern shores of the lake. Near shore sediment removal was performed in specific locations around the lake along with installation of an armor stone erosion protection system and gravel habitat material along the banks. A 14-inch thick subaqueous cap was installed along the entire 26 acre lake bottom. The project incorporated a number of natural resource restoration/enhancement actions, including installation of a walking path with park benches and a series of tree and shrub plantings along the northern and eastern sides of the lake.

Sevenson mobilized personnel and equipment to the site to complete site preparation activities. These activities included the setup of temporary construction trailers, clearing and grubbing activities and installation of soil erosion and sediment controls. Clearing and grubbing was performed in the staging and access/haul road locations, soil and near-shore sediment removal areas, and the recreational areas along the northern and eastern sides of the lake. Sevenson completed the grubbing by utilizing a hydraulic excavator with a rake attachment to remove brush, stumps and other root mass from targeted areas outside of removal areas. Sevenson also constructed material staging areas, a marine/barge access area, and access roads around the site to support material transport and cap placement operations. Stone material from these areas were sampled and analyzed for future re-use.

Sevenson abandoned and demolished utilities around the lake as specified in the contract drawings. For outfalls located below the water line that were found to be inactive, the pipes were considered debris, cut to below grade and plugged with non-shrink grout for abandonment. For above-grade pipes, debris, and remnant bank structures that were determined inactive, were either cut off below grade and plugged or demolished and removed from the bank. 34 outfalls were abandoned during the remediation. Sevenson installed sheet piling at some locations to aid in the dewatering. Dewatering was done utilizing sumps and dri-prime pumps to facilitate proper abandonment techniques in a dry environment.

Sevenson installed turbidity controls in front of the Housatonic River Outfall to prevent migration of fines and cap material outside of the work area. These controls consisted of a system of three individual silt curtains contrasted out of adjustable monofilament curtain. A hydraulic excavator with a Movax attachment was then utilized to drive the steel sheeting necessary to construct a dam capable of controlling the elevation of the water level inside of the lake. Sevenson also installed impermeable silt curtains around bank soil excavations and near shore sediment removal areas with an additional isolation curtain/turbidity barrier to further assist in turbidity control during the sediment cap placement activities.

Bank soil and near shore sediment removal areas with excavation depths greater than 5 ft in depth or 3 ft below the mean water elevation were shored utilizing AZ steel sheet piling. This approach utilized a cantilevered sheet pile system. Sheets were installed with a 100 ton hydraulic crane and a vibratory hammer. Following the completion of the bank soil and near shore sediment work, Sevenson mobilized the necessary marine equipment and initiated removal of in-lake debris. Sevenson deployed a floating barge platform powered by a 75 HP workboat and equipped with a hydraulic excavator with a grapple rake to remove debris objects in 26 target locations. Debris was removed from the lake and placed into custom built scows for transport to shore. Debris was decanted for future load out and offsite disposal.

Immediately after completion of the debris removal operations the sub-aqueous capping operations commenced over the entire bottom of Silver Lake. The slurry barge cap placement system was successfully able to produce, transport, and uniformly install the cap material in one- and two-inch lifts as required by the engineer and client. Sevenson’s integrated cap-placement operation proved capable of installing a sub-aqueous cap of essentially any thickness. 10 one-inch thick lifts and 2 two-inch thick lifts were installed over the 26 acre lake bottom to produce the minimum required 14-inch sub-aqueous cap. The material used for hydraulic capping was generally either a sand/topsoil blend or straight topsoil, with target pre-placement TOC in the 1-2% range. The average blend used across the lake was 31% sand / 69% Topsoil to achieve the target pre-placement TOC range.  The final thickness of the hydraulically-placed cap was a minimum of 14 inches, as indicated by collection of 21 cores in the 26 acre area.  Final thicknesses observed in the 21 cores ranged from 14 inches to 33.5 inches, with an average of 17.5 inches.   All cores met or exceeded target thicknesses and TOC ranges. The success of the cap will be monitored for at least the next 5 years through assessment of thickness, integrity, and chemical composition. The total calculated material placed was approx. 54,000 CY.

To ensure quality control procedures during the cap installation operations, a series of 2 ft x 2 ft sediment collection pans were lowered in front of the barge using a submerged retrieval line. On a daily basis, pans were lifted, measured and visually inspected, to ensure the proper lift thickness has been placed. This testing was done periodically, as needed throughout the capping project to ensure placement methods were working properly and target lift thickness was being achieved. Ultimately sediment cores were taken to verify and record cap thicknesses.

After in-lake capping operations were complete, restoration activities on the northern and eastern banks along and with final restoration of the private parcels were completed. Trees and shrubs were installed in select locations and areas were seeding with conservation and wetland seed mixes as appreciated. 41 woody debris targets were placed and anchored in the near shore areas for fish and wildlife habitats. Nine park benches were also installed along the walking path for pedestrians to view the lake.

Following completion of intrusive work activities, demobilization and decontamination procedures were followed. Equipment, buckets, tacks, transport vehicles, etc. were all cleaned and wash water was collected and transported to the onsite waste water treatment plant. All of the materials used for in water work were removed and decontaminated. Wipe samples were collected for verification. Sevenson was also responsible for all site survey and provision of record drawings. All survey was performed by a licensed land surveyor.