A collapsed gabion retaining wall system caused by the recent KwaZulu-Natal floods
The vital role of
RETAINING WALL
The devastating impact of extreme weather events highlights the key role of environmentally engineered structures as one of the effective countermeasures, says Louis Cheyne, managing director of Gabion Baskets. By Alastair Currie
T
he importance of soil retention and erosion control was underscored by the extreme floods in KwaZulu-Natal (KZN) and other parts of South Africa in April 2022. Landslides, raging stormwater and structural collapses were widespread, with properties bordering rivers among the worst affected. From an environmental perspective, the devastation highlights the impact of urban expansion, at times at the expense of natural forests and vegetated areas. “With more hard surfaced areas and greater run-off velocities, soil erosion is an ongoing issue that can slowly but surely undermine key infrastructure like buildings, walls, roads and bridges,” Cheyne explains. A specialist manufacturer and supplier of gabion systems, Gabion Baskets has extensive experience in providing expert advice and design recommendations for environmentally engineered systems. In addition to standard Class A galvanised double-twisted hexagonal mesh gabion baskets and gabion mattresses, other product solutions include woven and non-woven geotextiles, and biodegradable blankets.
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IMIESA May 2022
STRUCTURES
Forming part of a bio-engineered solution, biodegradable blankets are laid out and pegged in place – commonly on steep slopes or newly constructed fills – as an affordable erosion protection system, and to reduce water run-off velocities. These blankets, which are often combined with gabion systems, can be vegetated with resilient plants that help to stabilise slopes and embankments. As these blankets degrade over time, they release nitrogen into the soil, which promotes the initial vegetation growth phase. One plant receiving increasing attention worldwide for natural soil retention is vetiver grass, which has a dense, interlocking root system that extends vertically downwards to around 3 m within two years of being planted. Fire-resistant, with exceptional tensile strength, vetiver also absorbs large amounts of water, which contributes to an overall ‘negative pore pressure’. “Another intervention is the installation of gabion mattresses to counter slope erosion, and these can also be blanketed and covered with seeded topsoil to naturally blend in,” says Cheyne.
Importance of geotextiles For any retaining wall structure, the design starts with a soil analysis. This will determine which geotextile is best suited.
The vegetated gabion mattress (centre) withstood the brunt of the storm
These are installed as an interface behind and below the gabion structure and the soil being retained. As Cheyne points out, soils are high in compressive strength, but very low in tensile strength. Add water and the pore pressure goes to 100%, so controlled drainage is crucial for structural integrity. “Where geotextiles are not employed, the migration of fine soil can cause subsidence problems, especially in sandy soils. Here a non-woven fabric is required,” Cheyne explains. “However, clayey soils do not generally require a geotextile filter fabric as these materials seldom leach through the basket section. Where installed, a woven
