The vital role of retaining wall structures

A collapsed gabion retaining wall system caused by the recent KwaZulu-Natal floods

The vegetated gabion mattress (centre) withstood the brunt of the storm

The 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. 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 vital role of The RETAINING WALL devastating impact of extreme weather events highlights STRUCTURES the key role of environmentally engineered structures as one of the effective countermeasures, says Louis Cheyne, managing director of Gabion Baskets.

By Alastair Currie

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

Concrete block retaining wall supported on a gabion foundation 2 m high gabion benched wall section

Vetiver grass has become popular worldwide as a bioengineered response to soil erosion because of its deep and extensive root system

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 1-2 m² of geotextile per cubic metre 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 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 responses,” says Cheyne. “Where wall failures have occurred, it’s important to investigate the underlying cause before reinstating to avoid a repeat of the problem.”

River walls are particularly challenging and complex to construct. 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 directly on to 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 t/m2 .

A retaining wall failure

Flood barriers

To meet future flood containment challenges, Gabion Baskets has expanded its product suite to include gabion barrier systems. These are assembled using Class A galvanised weld mesh wire panels (75 mm x 75 mm x 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 degradation in general,” adds Cheyne.

“That’s why renewed emphasis has to be placed on sustainably 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.

Severe landslide damage to a housing development in Kloof, KwaZulu-Natal

A gabion barrier system built using welded mesh and lined with a heavy-duty geotextile

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