Gabions
Description
Gabions are rectangular baskets fabricated from a hexagonal mesh of heavily galvanized steel wire. The baskets are filled with rock and stacked atop one another to form a gravity- type wall. Gabions depend mainly on the interlocking of the individual stones and rocks within the wire mesh for internal stability, and their mass or weight to resist hydraulic and earth forces. Gabions are a porous type of structure that can sometimes be vegetated. Gabions are considered to be a "hard" structural solution that has minimal habitat and aesthetic value.
Applicability
Gabions are used to slow the velocity of concentrated runoff or to stabilize slopes with seepage problems and/or non-cohesive soils. Gabions can be used at soil-water interfaces, where the soil conditions, water turbulence, water velocity, and expected vegetative cover are such that the soil may erode under the design flow conditions. Gabions can be used on steeper slopes than riprap and are sometimes the only feasible option for stabilizing an area where there is not enough room to accommodate a "softer", vegetated solution.
Advantages
Some advantages of gabion walls are:
- Ease of handling and transportation
- Speed of construction
- Flexibility (Gabions tolerate movement)
- Permeability to water (Good drainage)
- Gabions offer an easy-to-use method for decreasing water velocity and protecting slopes from erosion.
Disadvantages/Problems
- Gabions are sometimes criticized as being unsightly. They can be made more attractive by use of attractive facing stone toward the front of the wall and by establishing vegetation in the spaces between the rocks.
- Low habitat value.
- Gabions are more expensive than either vegetated slopes or riprap.
- The wire baskets used for gabions may be subject to heavy wear and tear due to wire abrasion by bedload movement in streams with high velocity flow.
- Difficult to install, requiring large equipment.
Planning Considerations
For easy handling and shipping, gabions are supplied folded into a flat position and bundled together. Gabions are readily assembled by unfolding and binding together all vertical edges with lengths of connecting wire stitched around the vertical edges. The empty gabions are placed in position and wired to adjoining gabions. They are then filled with cobblestone-size rock (10-30 cm in diameter) to one-third their depth. Connecting wires, placed in each direction, brace opposing gabion walls together. The wires prevent the gabion baskets from "bulging" as they are filled. This operation is repeated until the gabion is filled. After filling, the top is folded shut and wired to the ends, sides, and diaphragms. During the filling operation live rooting plant species, such as willow, may be placed among the rocks. If this is done, some soil should be placed in the gabions with the branches, and the basal ends of the plants should extend well into the backfill area behind the gabion breast wall.
Several diffierent design configurations are possible with gabions. They may have either a battered (sloping) or a stepped-back front. The choice depends upon application, although the stepped-back type is generally easier to build when the wall is more than 10 feet high. If large rocks are readily accessible, inexpensive, and near the proposed site, then their use in construction of a rock wall may be preferable. On the other hand, if rock must be imported or is only available in small sizes, a gabion wall may be preferable.
Sequence of Construction
- Since gabions are used where erosion potential is high, construction must be sequenced so that they are put in place with the minimum possible delay. Disturbance of areas where gabions are to be placed should be undertaken only when final preparation and placement can follow immediately behind the initial disturbance.
- Where gabions are used for outlet protection, they should be placed before or in conjunction with the construction of the pipe or channel so that they are in place when the pipe or channel begins to operate.
Maintenance
- Gabions should be inspected on a regular basis and after every large storm event.
- All temporary and permanent erosion and sediment control practices shall be maintained and repaired as needed to assure continued performance of their intended function.
- All maintenance and repair shall be conducted in accordance with an approved manual.
References
Connecticut Council on Soil and Water Conservation, Connecticut Guidelines for Soil Erosion and Sediment Control, Hartford, CT, January, 1985.
Gray, Donald H. and Leiser, A. T., Biotechnical Slope Protection and Erosion Control, Leiser Van Reinhold Inc., 1982.
Pennsylvania, Commonwealth of, Bureau of Soil and Water Conservation, Erosion and Sediment Pollution Control Program Manual, Harrisburg, PA, April, 1990.
Massachusetts Department of Environmental Protection, Massachusetts Sediment and Erosion Control Guidelines for Urban and Suburban Areas.1997.
Freeman, G. and Fischenich, C., Gabions for Streambank Erosion Control,
http://el.erdc.usace.army.mil/elpubs/pdf/sr22.pdf