The backhoe dredger is the vessel of choice for projects where real muscle is required. This type of dredger uses a backhoe and a hydraulic crane with an excavation bucket to mechanically dig up the soil. The backhoe dredger is a stationary dredger that can handle a range of different soils and materials, including rocks, boulders and underwater dams.
The backhoe is also suitable for dredging compact clay, sand and materials that contain pebbles or fragmented rock. Therefore, it can be mobilised for all kinds of dredging projects. Since backhoe dredgers can dredge extremely accurately, they are often used for projects where dredging is needed close to solid structures, such as in ports and channels.
Main features of a backhoe dredger:
- Stationary – backhoe dredgers can be self-propelled, but they do not sail while dredging
- Accurate dredging with good control on vertical and horizontal position
- Can easily handle clay, sand, pebbles, rocks, boulders and a mixture of soil types
- Dredged material is transported by barge
- No need for anchors, so interference with shipping traffic is limited.
How does a backhoe dredger work?
Backhoe dredgers operate much like a backhoe on land. They dig up the soil by lowering the bucket into the sea or riverbed, then raising it again while making a backward scooping motion. Once the bucket is above water level, the backhoe turns towards the transport barge in which the bucket is emptied. The effective dredging area in a certain position depends on the length of the crane, the swing angle and the dredging depth.
During excavation, large digging forces come into play. The center of gravity shifts forward when the bucket enters the soil. That’s why backhoe dredgers are anchored and kept in position using a spud system. The spud system of backhoe dredgers is different to that of cutter suction dredgers and consists of three spud poles. Two are situated at the front of the pontoon, near the crane. The third is located at the back of the dredger and is fitted with a spud carriage. The spud poles keep the dredger in place and they are also used to move the dredger backwards after a full cut has been made.
The backwards movement is achieved by first raising the rear spud and bringing it into position using the spud carriage. Once the rear spud has been lowered into the seabed again, the front spuds are raised and the dredger moves backwards by riding in the spud carriage of the rear spud. Then the front spuds are firmly planted into the seabed. The dredger can partially lift itself out of the water on the spuds to anchor them more firmly. As soon as the dredger is repositioned, the dredging can continue.
Spuds: three spuds keep the backhoe dredger in place while dredging. The spud system is also used to reposition the dredger after a full cut has been made.
Turntable: the backhoe is placed on a turntable, so that it can swing around to dredge a certain area and to empty the bucket in the transport barge that is moored alongside the vessel.
Bucket: the bucket acts as a scoop to collect the soil. It is controlled by hydraulic cylinders in the crane.
Hopper barge: a backhoe dredger has no means to transport the dredged soil. The material is transported to the disposal area by barge.
Initial investment costs of a backhoe dredger
Backhoe dredgers are generally custom-built. They can measure between 25 and 70 metres in length and, based on the required installed power, dredging depth, bucket size and accommodation needs, their price can therefore vary widely.