Civil Engineering July 2022 | Vol 30 No 6

24 July 2022  Civil Engineering T he importance of soil retention and erosion control was un- derscored by the extreme floods in KwaZulu-Natal (KZN) and other parts of South Africa during April and May 2022. Landslides, raging stormwater and structural collapses were wide- spread, with properties bordering rivers among the worst affected. From an environmental perspective, the devastation high- lights 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 mat- tresses, other product solutions include woven and non-woven geotextiles, and biodegradable blankets. 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 protec- tion 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. IMPORTANCE OF GEOTEXTILES For any retaining wall structure, the design starts with a soil analysis. This will determine which geotextile is best-suited. 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 geotextile is specified to allow water to pass through the gabion system and so prevent hydraulic pressure build-up.” Depending on the system design, Gabion Baskets normally recommends the installation of approximately 1 m² to 2 m² of geotextile per m³ of gabion. KZN DAMAGE ASSESSMENT In response to the KZN floods, Gabion Baskets personnel from the Johannesburg factory, together with members from its Durban office, were on site in April to respond to enquiries from property owners and contractors for repair solutions. In many cases, banks and slopes had collapsed due to the failure of Sponsored content The vital role of retaining wall structures The recent 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. Biodegradable blankets used on steep slopes or newly constructed fills as erosion protection to reduce water run-off velocities