Backhoe / Dipper dredgers are commonly known as Mechanical dredgers. Mechanical dredgers come in a variety of forms, each involving the use of grab or bucket to loosen the in situ material then raise and transport it to the surface. They are stationary dredgers, moved on anchors or spuds. Small Backhoe / Dipper dredgers can be track mounted and work from the banks of rivers and ditches.
Backhoe and Dipper dredgers are a hydraulic type of excavator, consisting of a rectangular pontoon, on which is mounted an excavator unit. The excavator can be either an integral part of the dredger or a proprietary mobile type adapted for marine working.
Material is excavated by the dredger using a bucket of size compatible with the in situ strength of the material being dredged. The excavated material is either loaded into barges or directly placed ashore to the arc of the crane. The older form of this type of dredger - the dipper or face shovel - used a wire-operated integral excavator and was very heavily built to allow for dredging of hard materials, such as rock and masonry.
Wire-operated excavator units have now been largely superceded by hydraulically operated backhoe machines. These operate more efficiently than the face shovel. The size of a backhoe dredger is described by the bucket capacity, which can vary between 0.5 and 13m3. Production is dependent upon bucket size and the hardness of the dredge material.
See below for a video of Backhoe dredger 'New York' with Liebherr P996, working in the port of New York.
Breakout forces in excess of 90t can be exerted by the largest machines and, because of very high horizontal loads developed by the jigging motion, the Backhoe Dipper dredgers work on spuds. These spuds are heavy pike-like structures which can be dropped into the sea bed by the dredger. Two spuds are usually mounted at the digging end of the dredge pontoon, to provide resistance. This type of excavator is very efficient and has good vertical and horizontal control. The units are limited to small operations where water depth is low, governed by the arm of the dipper unit.
See below for a video of a Backhoe dredger with CAT 5230 dredging in Hong Kong.