Environmental Engineering

Installation of a wood chip blanket on an attenuation pond embankment in Gauteng

Designing and building bio-engineered structures

Gabion baskets and gabion mattresses are among the oldest and most proven environmentally engineered structures. At times, however, the strategic use of plants and geotextiles can work equally well on their own, or in combination, says Louis Cheyne, managing director, Gabion Baskets. By Alastair Currie

While serving effectively as mass gravity retaining wall structures, gabion baskets and gabion mattresses are also ideal for slope and riverbank stabilisation. Their main purpose here is erosion control – an especially important intervention given increasing stormwater runoff within urban centres.

Previous land use, as well as deforestation, fires, droughts and the drying out of wetlands are also all major causes of erosion in both urban and rural areas. This is compounded by the invasion of alien species, like black wattle, which displace indigenous plants that were naturally suited to anchoring soils via their water-wise roots.

“Over the years, there’s been increasing appreciation for the role that plants play as nature’s bio-engineers. While we’ve traditionally focused on supplying solutions for ‘hard’ engineering designs, we are seeing a growing interest in the ‘soft’ side of environmental engineering. Here, the use of plants can complement, enhance or even replace conventional techniques. Either way, today’s emphasis is on green designs that blend in and coexist with their surroundings,” says Cheyne.

Permeability

The one major advantage of gabion baskets is that they are formed using double-twisted galvanised wire mesh filled with rocks that are permeable to a greater, or lesser, degree, according to the design. This means they are also perfect for establishing plants, as well as forming a habitat for fauna like insects and lizards. The soil contained in the gabions for this purpose is held in place by geotextiles to prevent fines loss. The use of tickey creepers is also ideal for growing up and over gabion structures.

“Essentially, there are endless variations of soft and/or hard interventions,” says Cheyne, adding that environmental engineering specialist Gabion Baskets provides installation training for industry. This includes SMME and community-based projects.

This environmentally engineered solution combines a range of interventions that include gabion weirs, and a geocell-lined channel planted with vegetation. Soil blankets on the channel banks have been seeded with plants for further erosion protection (Credit: HansKingSRS)

River Willow anchor system: a hybrid approach

A classic example of a system that combines hard and soft approaches is the employment of shrubs to either green or help anchor retaining systems. Globally, the use

Young tree cuttings planted to grow through and above the gabion retaining wall

of young tree cuttings, like river willow, is especially popular.

“Each plant has its own characteristic tensile root strength, so that’s a factor to consider. Choosing the right tree cutting is equally important. Willows, for example, are flexible, woody plants and their roots help increase bank stablilisation. In contrast, stiffer plants should be avoided, as these might act as levers as they grow and work against the gabion retaining structure,” says Cheyne.

In terms of installation, these willow cuttings – typically measuring up to 2.5 cm in diameter and preferably less than a year old – can be inserted in-between the 1 m x 1 m gabion baskets that form a conventional retaining wall and then planted into the rear soil embankment. As the trees mature, their roots firmly gain hold and increase the shear strength. In the meantime, the front face will become progressively greener as the shrubbery grows through and around the gabions. The top of the gabion wall can also be covered with a geotextile and topsoil, and then planted to complete the overall effect.

A soft approach

The other option is to build a similar retaining structure where gabions are not used. Instead, stepped-back, high-strength geogrid material, covered with compacted soil, forms the horizontal layer works to create the retaining structure. Again, live cuttings are positioned at intervals to grow back and through the external face.

In some cases, it is also possible to design an erosion control slope with loose rocks resting on a soil blanket. Sausage gabions can also be employed, which are secured with live tree stakes to hold everything together.

Where the intention is to form gradually sloping terraces, another option is the GabBloc system. These are mini gabions used to retain embankments that can be interspersed with flower beds. “We’ve also supplied gabions with a pocket inside. Once these pockets are planted, the external wall is attractively vegetated,” Cheyne explains.

Erosion control blankets

Within the soft engineering space, there are a host of other interventions, such as the deployment of erosion control soil blankets where the main aim is to achieve soil protection. An example would be the covering of the slopes bordering a river or attenuation pond.

Their use is also employed when greening gabion mattress slopes or the top of a retaining wall. Overlying the gabion structure, the blanket is covered with topsoil and planted. The blanket’s main purpose is to keep the soil moist and cool during the initial plant growing phase. A further advantage is that these blankets degrade into the soil over time, enriching the nitrogen content.

Architectural gabions

There are times, however, where gabions are used purely for aesthetics. A good example is the architectural market, where Gabion Baskets is experiencing strong demand for

A slope stabilised with a gabion mattress. Note the return of the mattress into the top of the bank, which is essential to ensure it is firmly anchored and doesn’t slide down. A planted topsoil layer promotes the growth of vegetation A few weeks later, the vegetation is now well established

its welded-steel square mesh range. Unlike woven mesh, which is designed to exhibit some degree of flex, welded mesh products are intentionally rigid. Typical applications include internal and external building cladding, fencing, and property boundary walls.

Welded mesh can also be attractively greened. Examples include plants designed to cascade downwards from the top of these structures.

Strategic partnerships

“The science of environmental engineering is definitely experiencing a resurgence, and the impact of extreme weather due to climate change is one major factor,” says Cheyne. “Another is a desire to reverse the harsher elements of our concrete landscape by going green.

“Combined with training, our strategy is to work closely with our network of contractors and consulting engineers to provide them with fit-for-purpose solutions,” he continues. A recent example is a new strategic alliance with Western Cape-based consulting firm HansKingSRS, a leading expert in the field. In addition to river stabilisation, HansKingSRS provides hydraulic and hydrological consulting services, and specialises in the planning of soil conservation structures.

“A crucial component of environmental engineering is understanding hydrology and water velocity. That’s the starting point for deciding on the optimum bio-engineered design,” Cheyne concludes.