The conventional clamshell bucket-type dredge is widely used in harbour maintenance operations and can be acquired for $250,000 to $500,000. The typical configuration is a production crane mounted on a barge which is positioned using anchors or spuds. The clamshell or grab ranges in size from 1m3 to 200 m3 with 4m3-10m3 being the most common. At depths down to 50m production rates of 5km3/day to 10km3/day are common. This is entirely dependent on the skill of the operator and the size of the grab. The high production rate can be deceptive due to possible over excavation. Spillage can be high due to less than 100% coverage and losses from the grab itself[i]. Again, all of the material is excavated to the surface for separation and the environmental impact will be high with regard to material displacement and surface plume. Bucket recovery induces a sporadically loading the surface separation plant causing operational problems and oversizing to meet peak demand.
[i] Vlasblom W.J. (2005, May). Grab or Clamshell Dredger, WB3408B Lecture Notes. Retrieved from http://www.dredging.org/content.asp?page=105