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GABIONS AND GEOTEXTILES WORK TOGETHER
Where there are soft soil conditions, geotextiles are an essential component
of gabion installations; but with clay, the opposite is true. Then there’s the
geotextile overlap. By Alastair Currie
Gabion walls constructed as freestanding structures are intended to serve as architectural features and not retaining systems. However, these walls still need a central steel support system and must be founded on concrete. “It illustrates the fact that Gabions have a very diverse range of architectural and construction applications, and that each installation needs to be specifically designed and built to serve its intended purpose,” says Louis Cheyne, managing director of Gabion Baskets.
If built well, Gabion installations will last for decades. A more recent example that ticks all the right boxes is an approximately 80 m long contoured Gabion retaining wall commissioned by the City of Cape Town. What makes this installation distinctive is that tubular Gabions were installed, which require an extra level of proficiency from both the designer and contractor. This installation was erected in Cape Town’s northern suburb of Tygerberg.
The 4 m high retaining wall is founded on a gabion mattress foundation. The wall runs alongside a river channel section that borders a multi-storey building on the one side and a paved road on the other. The primary purpose of the wall is to protect the building’s exposed pile foundations against erosion, while ensuring embankment stabilisation in case of flood events.
Says Cheyne, “The tubular gabion effect has a strong aesthetic appeal since it reduces visible joins, which can sometimes look untidy, depending on the proficiency of the installation technique.”
Rounding the curves
Essentially, tubular Gabions are meshed front faced, elongated and diaphragm-segmented structures that have the flexibility to round curves in a way that cannot be matched by conventional square baskets. Gabion mesh rolls come in 50 m lengths. The mesh is
then traditionally cut and shaped into 1 m3 rock-filled baskets.
This traditional, modular approach differs when it comes to tubular Gabions, where there’s extensive design flexibility to space the diaphragm panel intervals, especially when going around a corner. (The diaphragms provide structural stability.) In the Tygerberg example, the intervals were at 800 mm to create a more attractive front face. Another advantage of Gabions is that the width can also be varied according to the design, which is advantageous for the top baskets in particular.
The speed of construction is also improved. For traditional square gabions, the rate is around one man per 1 m3 per day. However, with tubular gabions, the work rate can be up to 3 m3
per day because there’s less joining required.
Permeability and soil retention
In most mass-gravity gabion wall installations, non-woven needlepunched
geotextiles are an integral part of the design and are placed behind and below the structure to hold the soil back.
Gabions are intended to be porous, free-draining structures, essentially composed of 65% rock and 35% voids. For this reason, and in most instances, a soil-stabilising mechanism (a geotextile) needs to be present to allow the water to pass through, while preventing the passage of fine sands or silts.
Progressive loss of soil materials will eventually undermine the structure. An exception is where there are cohesive soils present, like clayey materials. Cheyne says the recommendation then is not to use geotextiles (especially non-woven varieties), as these typically become clogged. The subsequent hydrostatic pressure build-up can lead to catastrophic wall failures. “The critical requirement for gabion retaining walls is to achieve the correct geotextile fit,” says Cheyne. The two main approaches for joining geotextiles are the overlap technique and the blanket-stitch method. In the first case, the overlap should be between 140 mm and 300 mm. Geotextile thickness is also important. A minimum 200 g/m2 is recommended behind the wall, as well as below the mattresses. For angular rockfills though – for example, where backfill materials are sourced from blasted rock – the geotextile needs to be thicker to minimise the risk of puncturing.
“Those who think building gabions simply requires assembling a basket and throwing rocks into it could be in for an unhappy experience down the track,” adds Cheyne. “Gabions are engineered structures, which, if correctly packed and braced, are very stable and amazingly strong. For example, when designed as crash barriers or seawalls, they’ve proved to withstand tremendous forces. “But perhaps the best benefit for Africa is that gabion structures are affordable to construct, can use in situ materials, and are labour-intensive,” he concludes.
