These days modular steel wire-mesh (bottom right) has replaced wicker, and stones are used instead of soil, but the underlying strength of gabions - and their advantages over conventional rigid structures in engineering works - are unchanged. Subjected to alternating tension and compression, the intrinsic flexibility of a gabion structure enables it to bend rather than break, thereby preventing loss of structural efficiency. Being a deformable structure, any change in shape due to subsidence of foundations or internal stress is a functional feature, and not a fault. It adapts therefore to small earth movements and, in the process of deforming, will remains structurally sound, without fractures.
Since gabions are bound together, the wire mesh is extremely strong under tension, where concrete is not. A gabion structure will withstand a degree of tension that would severely test a dry stone construction and be downright dangerous with plain concrete and masonry. The wire mesh shell is not simply a container for the stone filling, but a reinforcement of the entire structure. And a well-made gabion can withstand years of punishment: one used in Italy for protection of river banks, more than 100 years ago, was recently examined and found to be still in perfect condition.
Vertical diaphragms. The gabion shape has evolved and is reinforced along the edges of the wire mesh with larger diameter wires. This helps support the sides of the boxes during construction, makes tying operations easier and generally strengthens the gabion structure. Vertical diaphragms fixed to the base of the gabions are designed to restrict internal movement of the stone filling and provide further reinforcement. The hexagonal "wire mesh" itself is double twisted and galvanized to resist stress and corrosion hazards.
Stones within the gabion shell provide a great degree of permeability throughout the structure, eliminating the need for a drainage system. In river works, pressure and counterpressure on the banks due to variations in water depth between flood and low water are therefore also eliminated.
AGLW says a gabion structure costs much less than a conventional structure. Low grade or even waste stones, usually found close to the work site, can be used to fill the wire cages, with no specialized materials or labour - such as formworkers, masons or ironworkers - being required. Labour costs are minimal as unskilled labour, under the supervision of a few skilled workers, can be trained quickly to erect the gabion baskets, fill them and tie them together with galvanized iron wire. All of which makes this simple technique easy to introduce to rural people, who can also be actively involved in both the construction and future maintenance of hydraulic structures.
Gabion baskets can also be manufactured - literally, by hand - at village-level. This is a double advantage: it lowers the purchase cost of the baskets and creates a small rural industry, using unskilled local labour. This is in line with trends towards increasing use of labour intensive techniques in modern development projects.
Embankments and spillways.
AGLW has long experience with gabion construction in various countries of the developing world, including Botswana, Ethiopia, Niger, Nigeria, China, Viet Nam and Haiti, where water development and irrigation projects have all made use of either imported or locally-made gabion baskets. The structures most frequently consist of an earthfill embankments and gabion-based anti-erosion protection and a surface spillway made from gabions.
The spillway is an essential component of the structure, since it is the part in direct contact with water flow. A properly designed spillway should be able to control the discharge of excess water in a reservoir and protect the earth embankment against scouring and erosion. However, AGLW cautions, while building hydraulic gabion structures is fairly simple, basic engineering rules must always be respected in order to ensure the stability, and thus the sustainability, of the structure. In particular, gabions are often associated with earth cuts or fills - therefore, the static stability and the intrinsic resistance of the whole structure and each of its components, individually, must be ensured.
The design and construction of gabion structures have not always been up to standard and, as a consequence, have caused partial or total failure of the works due to the excessive subsidence of foundations and, most often, the progressive infiltration and leakage of water along the interface between the gabions and adjacent material earth and foundations. This has sometimes led to progressive breakdown of the whole structure.
AGLW is now preparing a set of practical guidelines and norms for field engineers for the design and building of structures using earth and gabions. Particular emphasis will be placed on the interface between the two construction materials, precisely where the greatest risk of failure has been found. The publication will be useful in designing gabion spillways for small hillside dams, intake weirs for gravity irrigation schemes, groynes, river bed training works and for protection against hydraulic erosion. A set of programs for computing construction stability will be provided, together with users' manuals.
Published December 1998