Sediment Capping

Environmental Dredging

Sevenson uses different capping methods depending on project specifications, project location, water depths, environmental factors, and cap material type.

Sediment Capping by Sevenson
Sediment Capping by Sevenson
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Sediment Capping
Sediment Capping
Sevenson Capping
  • Equipment to place capping materials varies depending on project requirements and site conditions
  • Equipment mobilization costs are minimized by accurately tracking mobilization, demobilization, and expected utilization. This allows for cost-effective mobilization of equipment from the nearest equipment yard or project location

Sevenson typically uses a hydraulic excavator stationed on a modular barge for placing sand and armor stone material. Production rates for mechanically placing armor material vary depending on the material’s availability and physical dimensions.

Sevenson owns and operates custom-built and patented slurry spreader systems for hydraulically placing cap material.

Materials are fed on a dry weight basis to the slurry hopper and mixed with makeup water. A dredge pump is used to convey the slurry. The hopper, screening, and slurry system are typically placed on the shoreline for easy truck pickup and delivery. The slurry system can also be staged on a barge.

A diffuser/spreader barge is attached to the slurry system by a high-density polyethylene (HDPE) pipe. The barge has a discharge apparatus fabricated on the deck, which functions to dissipate the energy in the capping material slurry delivered to the placement barge. Cap material enters the water in a controlled fashion. The barge is equipped with a cable winch system and anchors to facilitate movement.

The barge’s controls are integrated with RTK DGPS and DREDGEPACK® software. Output from the software is used to “drive” the barge along a preprogramed cap lane. The production rate of material placement is based on the pumping distance between the site and the shoreline, as well as the grain size of material.

Sevenson consistently demonstrates the ability to place cap lift thickness ranging from 1 inch to 18 inches per pass with the barge.

Sevenson developed a capping technology using a radial telestacker conveyor system. The system places various sizes of aggregates from fine-grain sand to six-inch diameter cobbles, and covers approximately 15,000 SF per setup. The barge is held in place using spuds and materials are transported to the capping barge via scows. An excavator offloads material from the scows and loads the telestacker feed hopper, which releases a set volume of material onto the radial stacker belt for a consistent feed rate. The telestacker swings on an arc using custom PLC automation integrated with the RTK DGPS system that tracks the conveyor’s boom tip. Feed rates can be adjusted to place designed lift thicknesses ranging from a few inches to several feet.

Among other advantages, the Telebelt system works well when capping subtidal and intertidal areas, as its reach allows work to progress as tides fluctuate. Depending on site conditions, the Telebelt placement method is the ideal alternative to mechanical placement.

We have successfully developed and implemented site specific health and safety plans since 1979. With a full time staff of 30 full-time Health and Safety Officers that include CHST’s and OHST’s, our Site Safety and Health Officers average 14 years of service.

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