Civil Engineering July 2022 | Vol 30 No 6

Civil Engineering July 2022 25 existing gabion or concrete block mass gravity retaining walls, or where there was no existing system in place. The damage was compounded in places by inadequate stormwater drainage, plus burst water and sewer lines, which contributed to wall foundation collapses. “Professional design and installation experience is essential when it comes to developing environmentally engineered re- sponses,” says Cheyne. “Where wall failures have occurred, it is important to investigate the underlying cause before reinstating to avoid a repeat of the problem.” River walls are particularly challenging and complex to con- struct. Here gabion retaining walls perform optimally because they are designed to be submerged. Additionally, their degree of permeability – thanks to their rockfill and approximately 35% void composition – and ability to flex under strain, respond well to hydraulic pressures and debris impact. This flexibility also means that gabion retaining walls can be safely founded on expansive clayey soils. The same is not true for a concrete block retaining wall in the same application. Placing a rigid concrete block wall di- rectly onto this flexible soil would spell disaster since ongoing expansion and contraction of the clay will cause any concrete foundation on which it rests to move and eventually crack. One way of countering this is to bench out the flexible soil to a specified depth and then import rockfill material to form the compacted cement stabilised foundation. Another way is to adopt a hybrid approach. Typically, this comprises a gabion foundation in the riverbed resting on a geotextile layer on which the stepped back concrete retaining block wall rests. This provides a robust solution given that a single gabion basket can support 300 tonnes per square metre. FLOOD BARRIERS To meet future flood containment challenges, Gabion Baskets has expanded its product suite to include gabion barrier sys- tems. These are assembled using Class A galvanised weld mesh wire panels (75 mm × 75 mm × 4 mm diameter mesh) and lined with a heavy-duty geotextile. The basic wall height is 1 m, but this can be increased to 2 m by stacking one barrier on top of the other. The major advantage of these barriers is that they’re quick to deploy by hand and can be filled with a variety of materials ranging from earth, sand, and gravel to crushed rock and other granular materials. They are also stronger than sandbags and can be filled with earthmoving plant on site. “It has been said that one of South Africa’s largest exports is soil, and this is a sad reflection on the extent of land degrada- tion in general,” adds Cheyne. “That’s why renewed emphasis has to be placed on sustain- ably engineered solutions that preserve our ecosystems so that human habitation and infrastructure development are in balance. The reality is that the KZN floods have shown that it’s not just about countering erosion, but catastrophic erosion going forward,” Cheyne concludes. Article prepared by Alastair Currie alastair@3smedia.co.za www.gabionbaskets.co.za Gabions and River Mattresses Gabion Rocks Hexagon Wovenmesh Square Weldmesh Geotextiles Gabion Tool Sets Biodegradable Soil Blankets Soil Bioengineering Sandbags Gabion Barriers Gauteng T: 011 882 5788 E: mail@gabionbaskets.co.za KwaZulu Natal T: 031 700 2695 E: salesdbn@gabionbaskets.co.za Western Cape T: 021 911 0890 E: david@gabionbaskets.co.za natural timeless structural