grab dredge

A grab dredge is a type of dredger that uses a clamshell bucket to excavate underwater material. Dredges are floating devices used for underwater excavation with several objectives, including maintaining channel depths, creating new land, constructing control works, and recovering valuable subaqueous deposits.12

There are two main classes of dredges: mechanical and hydraulic. Mechanical dredges include:
* Dipper dredge: Similar to a power shovel mounted on a barge.2
* Grab (or clamshell) dredge: Lowers, closes, and raises a single bucket using cables. It can operate at unlimited depths.2
* Ladder dredge: Uses a continuous chain of buckets on a ladder frame.2
* Scraper dredge (or dragline): Uses a scoop suspended from a boom.2

Hydraulic dredges use a centrifugal pump to move a mixture of solids and water through a suction pipe. Grab dredges are particularly useful for working close to existing structures like quay walls with minimal risk of damage.23