Gabion walls that retain the earth

The devastating floods experienced during 2022 in KwaZulu-Natal and other parts of South Africa have highlighted the importance of effective stormwater management. Among the most durable, sustainable and cost-effective are environmentally engineered solutions, says Louis Cheyne, managing director of Gabion Baskets. By Alastair Currie

Initial site preparations for the mass gravity retaining wall. The excavation in the foreground makes provision for the installation of a 1.2 m x 1.2 m stormwater catchment manhole Installation of 400 mm diameter heavy-duty uPVC drainage pipe

Subject to ongoing erosion, even the best compacted earth embankments will progressively lose their structural integrity over time where poor drainage is a factor. Add a severe storm event, and the results can be sudden and catastrophic.

For applications like road embankments and bridge abutments, precast concrete retaining block (CRB) systems are a popular choice for mass gravity walls. Another equally popular and globally applied approach is the specification of gabion systems.

When designed, manufactured and built by industry experts, their steel wire framed and rock filled composition can provide the same or a similar engineered solution to CRB installations, combined with costcomparative advantages.

“The added benefit in the South African context is that gabion systems are labour intensive, creating much-needed jobs in predominantly unemployed communities, as well as fostering a new generation of SMME subcontractors,” says Cheyne.

The latter then have an opportunity to progress to higher levels, and more multifaceted work, within South Africa’s Construction Industry Development Board grading system, allied and backed by the Department of Public Works and Infrastructure’s Expanded Public Works Programme.

The rock-filled gabion baskets are composed of Class A galvanised hexagonal double-twisted wire mesh

Inspection services and project management

Celebrating its 16th year in 2022, Gabion Baskets, headquartered in Johannesburg and operating nationally, as well as into SADC via its dealer network, is a specialist

ASSAGAY ROAD, HILLCREST, KWAZULU-NATAL

A 400 mm diameter heavy-duty uPVC pipe passes through the base of the mass gravity retaining wall, feeding into a brick-built 1.2 m x 1.2 m stormwater catchment manhole (left). On the downstream end, another 400 mm diameter pipe section feeds downhill to a headwall The wall at an advanced stage of completion

manufacturer of gabion systems in all facets. These extend from gabion baskets and gabion mattresses formed using Class A galvanised double-twisted hexagonal woven mesh wire to systems assembled using weld mesh panels. Allied specialist solutions include the supply of integrated geotextiles.

Historically, Gabion Baskets has provided a site inspection service, combined with technical recommendations. This information provides valuable input for the design engineer, where applicable.

“While an engineer’s certificate is not needed for every structure, it is a legal requirement for mass gravity walls above 1.5 m in height,” Cheyne explains.

Gabion Baskets now also supports its multifaceted solutions with on-site project management and construction supervision services, alongside formal training and mentorship programmes for SMME subcontractors.

“We’ve found that offering a project management capability passes on major benefits for the client, engineer and contractor in terms of ensuring a high level of overall quality for the completed installation,” Cheyne explains.

Built to specification

Gabion structures may appear straightforward to construct because of the simplicity of the materials used; however, the widespread evidence of mass gravity gabion wall

FAIRVIEW, KWAZULU-NATAL: LANDSCAPING

A series of terraces have been formed using weld mesh gabions filled with multicoloured stones to create a camouflaged effect The terraced gabion installation provides aesthetic, as well as erosion control, benefits

failures underscores the need for expert design and installation practices. Every step must be adhered to. This includes geotechnical reports to determine the soil characteristics, as well as laboratory tests to confirm that the selected structural fill below and behind the constructed wall has been compacted to 95% MOD-AASHTO.

Particular attention must be paid to drainage, which should always incorporate geotextiles – either woven (permeable) or non-woven (impermeable) – depending on the underlying geology and soil composition.

As Cheyne points out, gabions are naturally permeable because of their rockfilled voids. This has its advantages, but the degree of permeability needs to be controlled. For example, clayey soils will not suffer unduly from fines losses, so a woven geotextile is ideal. The opposite is the case for sandy soils, where a nonwoven option is best. Added to this, and depending again on the height, one or more stormwater pipes may need to be installed behind the wall, exiting at the base to facilitate smooth water run-off.

“Understanding how hydraulic pressures can build up behind a mass gravity retaining wall is a vital part of the stormwater design intervention. Installed correctly, the structure should remain sound and functional, even during extreme storm events,” Cheyne continues.

Assagay Road

Putting these and other techniques into practice, Gabion Baskets is currently involved on a wide range of flood damage reinstatement projects within the eThekwini region. These include design input, product supply and project management on an approximately 9 m high gabion mass gravity wall in Hillcrest completed in October 2022.

“When the storm hit, the deluge of water caused a major embankment slope collapse, sweeping away a section of private road linking a residential dwelling,” says Cheyne. A consulting engineering firm, RJB Projects, was responsible for the design (see Figure 1), with the works constructed by an experienced environmental engineering contractor.

The wall incorporates specialist reinforced soil tie-backs using Gabion Baskets’ Gab-tail product. The latter is a standard rock-filled gabion basket with a mesh tail extended horizontally back into the backfilled embankment.

“For gabion structures higher than 4 m, and built in a fill situation, Gab-tails tend to be more economical and faster to install. The mesh tails improve the lateral shear resistance (tensile strength of the soil), providing improved overall stability,” Cheyne explains.

The selected backfill should be composed of at least a G5 or G6 backfill gravel. To avoid damaging the mesh, individual aggregate sizes should also not exceed 150 mm. Typically, this sized material is compacted in layers of not more than 150 mm, normally to 95% MOD-AASHTO.

Landscaping

While gabions are excellent for engineered applications, they are also proving increasingly popular for landscaping projects, either using woven or weld mesh, the latter providing a far flatter profile with precise edges.

“You could build a mass gravity wall using weld mesh if a specific aesthetic effect was required, but the costs would be higher than for woven mesh gabion systems,” Cheyne continues, adding that the most common weld mesh applications are for architectural cladding, freestanding feature walls and landscape terracing.

“However, each project is unique and it’s increasingly not uncommon for designers to explore hybrid options that combine engineered and architectural gabion elements to maximise the natural appeal of wire and stone,” Cheyne concludes.