Corrosion protection for slopes
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Solutions & Applications · Corrosion Protection
After a fire, the critical factor is not the growth of vegetation — it is the first heavy rainfall. Anything that has not been protected by then is washed away. And it is not just soil that is washed away: the seed bank, organic matter and the very ability of the slope to regenerate itself are all lost.
Why does a burnt slope not behave like an exposed slope?
Fire does not merely remove vegetation. It alters the soil itself, and the four changes act cumulatively:
| Change | Corrosion resistance |
|---|---|
| Loss of vegetation cover | The drop strikes the ground directly — grain detachment and surface waterproofing |
| Loss of leaflet layer | The retention and slowing down of runoff ceases |
| Soil hydrophobicity | Organic compounds evaporate and condense a few centimetres below the surface, forming a water-repellent layer. Filtration breaks down and runoff increases |
| Loss of cohesion due to roots | Dead roots gradually decompose — the mechanical cohesion of the soil is reduced for years following the event |
The third point is the most surprising: a burnt slope can produce many times more runoff than a naturally bare slope of the same gradient, because water does not penetrate it at all. This is why post-fire flooding occurs even with rainfall that would have been insignificant prior to the fire.
The contractor does not buy soil protection — they buy time. The role of the geosynthetic is to hold the soil and seed in place until the root system takes over. From then on, vegetation is the permanent solution.
This has a direct implication for planning: the project is not judged by its handover date but by whether it was completed before the first autumn rains. Work carried out in November on a slope that burnt in July has already missed the most critical window.
At ARGYNET, we draw up specifications according to slope and soil type, and calculate materials and fixings per stremma.
The three technologies
| Material | Mechanism | When to choose |
|---|---|---|
| MacMat · Corrosion-control geomat | Three-dimensional mesh made of synthetic monofilament, heat-sealed at the intersections; porosity greater than 90 per cent | Slopes where vegetation will re-establish itself — the matrix is filled with soil and seed, and the roots grow through it |
| MacMat R · Reinforced geotextile | The three-dimensional matrix encased in a metal mesh; BBA certification, design life of the mesh 120 years | Steep slopes where surface stabilisation is insufficient — mechanical reinforcement is also required |
| MacWeb · Geocells | Honeycomb system made from continuous polyethylene sheets; three-dimensional encapsulation of aggregates or soil; 100 years’ certified service life | Very steep slopes or unstable ground, where the backfill material must be mechanically secured to prevent it from sliding down |
The selection criterion is twofold and simple: how steep the slope is and how realistic natural regrowth is. Where vegetation will return, a geotextile is sufficient and becomes visually imperceptible within two growing seasons. Where the soil is shallow, stony or the gradient is marginal, cell-based stabilisation is the only solution that keeps the fill material in place.
Porosity is the defining characteristic, not strength
In corrosion-control materials, the key parameter is not strength but porosity in excess of 90 per cent. The reason for this is functional:
- The soil fills the matrix rather than sitting on top of it. A dense material acts as a barrier; a porous one becomes integrated.
- The seed finds a substrate and is protected from being washed away, whilst receiving light and water.
- The roots penetrate and anchor the whole — the substrate, soil and vegetation become a single entity.
- Surface runoff slows down within the pores, loses energy and does not reach the velocity required to carry away grains.
The permanence of the synthetic material does not preclude vegetation: the matrix remains within the ground as a permanent reinforced zone, even when it is no longer visible. In areas with a high frequency of fires, this is the key argument — after the next incident, the protection is already in place.
Anchoring is crucial to the project
- Crown trench: the material is embedded in a trench at the top of the slope. If the edge is left exposed, water enters from above and causes the entire sheet to come away.
- Number and type of anchors: these are calculated based on the gradient and soil type. This is where cost-cutting takes place on site, and this is where the project fails.
- Overlaps in the direction of flow: the upstream sheet overlaps the downstream sheet, so that water flows over the joint rather than through it.
- Full contact with the ground: any gap between the formwork and the embankment acts as a pathway for undermining. The surface must be levelled before installation.
- Slope toe: the downstream end must be terminated and anchored — it must not be left to ‘hang’.
The runoff must go somewhere
This is the most common oversight in restoration projects. The anti-erosion layer retains the soil but does not reduce the volume of water — and in a burnt-out basin, this volume is many times greater than usual due to hydrophobicity.
Without organised drainage, water accumulates at the base of the embankment, undermines the toe and causes the entire structure to fail from the bottom up — whilst the surface appears undamaged. The design provides for drainage ditches, perforated pipes with filters and relief points, together with the selection of the geotextile – and not as an afterthought.
What the tender requires
In public sector projects — forestry authorities, regional authorities, funded restoration programmes — the technical tender is assessed on the basis of the following documents:
- Certified service life: declared design life, not a marketing claim. MacMat R specifies 120 years for the mesh, whilst MacWeb specifies 100 years.
- Independent certification: BBA approval is recognised by assessment committees and is rarely contested.
- Declarations of performance and CE marking for each item to be supplied.
- Quantity calculations: square metres, anchors, overlaps and trenches — quantifiable and verifiable.
- Method documentation: a technical description of the installation process that the supervisor can follow.
Design series
- Risk mapping: gradients, slope lengths, soil depth, severity of burning per zone — the burnt area is not treated as a single, uniform whole.
- Prioritisation: priority should be given to slopes upstream of infrastructure, settlements and watercourses.
- Timeframe: target date before the first rains — this also determines the method.
- Selection of technology by zone: geomat, geomat with reinforcement or geocells.
- Anchorage design and material calculations.
- Drainage management: trenches, drainage, relief points.
- Seeding or planting and a monitoring programme for the first two growing seasons.
Corrosion Protection
<span class="c">Category · 3 series</span>
<span class="n">Corrosion Protection</span><span class="k">→</span>
Soil Reinforcement & Separation
<span class="c">Uniaxial · 30–1,600 kN/m</span>
<span class="n">Paraproduct</span><span class="k">→</span>
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<span class="c">Separation & filtration</span>
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<span class="c">Category · 3 series</span>
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Runoff Management
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