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Dredging      How did they do that?

At the beginning of the field season during the "mobilization" period, the dredge is launched using a large crane. The dredge was transported to the site on a flatbed truck.

This is a good picture of the dredge's "ladder," which has the "cutter-head" at the end. The ladder allows the dredger to accurately move the cutter-head up and down and left to right so that only targeted (contaminated) sediment is removed.

This picture shows the dredge in action. While there is not much to see above the water surface there are a few items of note here.

1) The dredge now has 2 red masts attached to white antennas. These antennas feed to a GPS system, which allows the dredger to know their exact position to within 1m in the horizontal and 5cm in the vertical. There is also a device on the dredge ladder that measures its angle and position, which then is used to calculate the exact location of the cutter head.
2) There are 3 "spuds" or metal poles that the dredge uses to move short distances. The 2 spuds in front only move up and down, but the spud in back "kicks-out," which allows the dredge to move forward incrementally.

Below is one of two types of cutter heads used. The cutter head disturbs the sediment in the low-pressure zone created by the dredge pump, or vacuum, and all of that material is pumped by pipeline to the dewatering facility.
Dredge Bit

A "booster station" pump is placed on a barge when the dredge is operating at large distances from the dewatering facility. Notice the backhoe on the platform. It is used by the dredging contractor for removal of rocks and debris from the area to be dredged to prevent clogging the cutterhead.
Dredge Booster

Last Revised: Wednesday March 02 2005