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Dipper Dredges

The dipper dredge is basically a barge-mounted power shovel. It is equipped with a power-driven ladder structure and operated from a barge-type hull. A bucket is firmly attached to the ladder structure and is forcibly thrust into the material to be removed. To increase digging power, the dredge barge is moored on powered spuds that transfer the weight of the forward section of the dredge to the bottom. Dipper dredges normally have a bucket capacity of 8 to 12 cu yd and a working depth of up to 50 ft. There is a great variability in production rates, but 30 to 60 cycles per hour is routinely achieved.

The dipper type of dredge is not self-propelled but can move itself during the dredging process by manipulation of the spuds and the dipper arm. A typical sequence of operation is as follows. The dipper dredge, scow barges, and attendant plant are moved to the work site. The dredge is moved to the point where work is to start; part of the weight is placed on the forward spuds to provide stability. A scow barge is brought alongside and moored into place by winches and cables on the dipper dredge. The dredge begins digging and placing the material into the moored barge. When all the material within reach of the bucket is removed, the dredge is moved forward by lifting the forward spuds and maneuvering with the bucket and stern spud. The loaded barges are towed to the disposal area and emptied by bottom dumping if an open-water disposal area is used, or they are unloaded by mechanical or hydraulic equipment if diked disposal is required. These procedures are repeated until the dredging operation is completed.

The best use of the dipper dredge is for excavating hard, compacted materials, rock, or other solid materials after blasting. Although it can be used to remove most bottom sediments, the violent action of this type of equipment may cause considerable sediment disturbance and resuspension during maintenance digging of fine-grained material. In addition, a significant loss of the fine-grained material will occur from the bucket during the hoisting process. The dipper dredge is most effective around bridges, docks, wharves, pipelines, piers, or breakwater structures because it does not require much area to maneuver; there is little danger of damaging the structures since the dredging process can be controlled accurately. No provision is made for dredged material containment or transport, so the dipper dredge must work alongside the disposal area or be accompanied by disposal barges during the dredging operation.

The dipper dredge is a rugged machine that can remove bottom materials consisting of clay, hardpacked sand, glacial till, stone, or blasted rock material. The power that can be applied directly to the cutting edge of the bucket makes this type of dredge ideal for the removal of hard and compact materials. It can also be used for removing old piers, breakwaters, foundations, pilings, roots, stumps, and other obstructions. The dredge requires less room to maneuver in the work area than most other types of dredges; the excavation is precisely controlled so that there is little danger of removing material from the foundation of docks and piers when dredging is required near these structures. Dipper dredges are frequently used when disposal areas are beyond the pumping distance of pipeline dredges, due to the fact that scow barges can transport material over long distances to the disposal area sites. The dipper type of dredge can be used effectively in refloating a grounded vessel. Because it can operate with little area for maneuvering, it can dig a shoal out from under and around a grounded vessel. The dipper dredge type of operation limits the volume of excess water in the barges as they are loaded. Dipper-dredged material can be placed in the shallow waters of eroding beaches to assist in beach nourishment.

It is difficult to retain soft, semisuspended fine-grained materials in the buckets of dipper dredges. Scow-type barges are required to move the material to a disposal area, and the production is relatively low when compared to the production of cutterhead and dustpan dredges. The dipper dredge is not recommended for use in dredging contaminated sediments.