Reinforcement of the base course with biaxial geogrid: how much aggregate thickness do you save?

Solutions & Applications · Geosynthetics

On soil with low bearing capacity, there are two approaches. The first is to add more aggregate. The second is to make the same aggregates act as a single slab rather than a collection of unconnected grains. Geogrids provide a geotechnical solution to an economic problem.

The mechanism: anchoring, not tension

This is the point that is most frequently misunderstood. The biaxial stabilisation geogrid does not function under tension. It functions through mechanical interlocking:

1. The aggregates penetrate the openings in the mesh during laying and compaction.
2. They are mechanically anchored and cannot move laterally under load — the lateral displacement of the grains is precisely the mechanism by which an unreinforced base course fails.
3. The layer behaves as a slab rather than a pile of granules; its stiffness increases.
4. The load is distributed over a larger area of the underlying ground, resulting in lower contact pressure.
5. The failure criterion is shifted: a smaller thickness is required for the same traffic load, or a longer service life is achieved for the same thickness.

In practice, two mechanisms operate simultaneously, at different stages of the road’s life cycle. Mechanical anchoring predominates from the outset, under very small deformation. However, once a rutting pattern develops, the behaviour of a tensioned membrane also comes into play: the mesh absorbs tensile stresses and restrains the rutting.

The distinction from soil reinforcement remains essential. Uniaxial reinforcement in a reinforced earth wall is designed from the outset for tension in one direction and is installed under tension. Biaxial reinforcement in the sub-base layer is not pre-tensioned during installation — it is laid flat and its strength must be symmetrical, as traffic does not have a preferred direction.

Where does the profit go?

Reducing thickness does not simply mean using fewer cubic metres of aggregate. In a Greek project, the bill has four lines, and the last two often exceed the first:

And one line that does not appear in the quotation but determines profitability: the ability of the construction site to operate. On soft ground, the first layer of aggregate is often ‘swallowed up’ and vehicles sink before construction has even begun. The geogrid creates a working platform from day one.

The geogrid does not separate

A second common misconception, with real-world consequences. In soft, fine-grained subsoil — clay, silt — the mesh openings are left open. The subgrade mixes with the base course under repeated traffic, the aggregates become contaminated and the course loses its function.

Geogrids provide reinforcement; separation is a different function and requires a different material. There are two approaches:

• Two layers: a separating geotextile on top of the sub-base, with a geogrid on top. Maximum flexibility in the choice of each, but two installation methods.
• Multi-component: reinforcement, separation and drainage in a single material and a single installation. Reduced site time and zero risk of errors in the sequence of layers.

The second option becomes more advantageous as the site size increases, because the installation time increases proportionally with the number of stremmata.

Three MacGrid variants

Within the same range, there are three different constructions, and the choice is determined by the required level of strength:

The triangular opening of the EG-T is worth noting: in a square loop, the restraint is strong in two directions and weaker along the diagonals. The triangular mesh has no ‘weak’ direction — and on a platform or in a car park, where vehicles turn and loads do not follow a straight line, this matters.

Biaxial or uniaxial

Uniaxial compaction works even on embankments with low-quality fill material — which is crucial when good-quality borrow pit material is not readily available.

Installation: five site rules

1. Without tension. The biaxial membrane is laid flat and loosely, without creases or tension. Its performance does not derive from prestressing.
2. Overlaps in the direction of laying. The next roll should overlap the previous one in the direction in which the equipment is moving, so that the edge does not lift.
3. No foot traffic directly on the mesh. Aggregates are dumped at the edge and pushed forwards over the layer already laid. A wheel or track running directly over the mesh displaces and damages it.
4. Minimum thickness of the first layer before any vehicle is allowed to pass over it — determined by the design and the size of the equipment.
5. Layered compaction. The anchoring of the grains in the voids is completed by compaction; without this, the mechanism is not activated.

Design series

1. Soil classification: bearing capacity and grain size distribution — these determine whether segregation is required.
2. Traffic loads: vehicle type and number of passages over the service life, not just the maximum axle load.
3. Grain size distribution of aggregates: this determines the mesh size.
4. Calculation of thickness reduction using a recognised design method — and a cost comparison with the unreinforced solution.
5. Choice of architectural layers: geogrid and geotextile separately, or a composite system in a single installation.
6. Drainage: the reinforced layer does not remove water — if the subsoil retains water, a drainage system must be provided.

Geogrids

  <span class="c">Reinforcement + partitioning + drainage</span>
  <span class="n">Multicomposite · Multifunctional Geosynthetics</span><span class="k">&rarr;</span>

  <span class="c">Uniaxial · 30–1,600 kN/m</span>
  <span class="n">Paraproduct · Soil Reinforcement</span><span class="k">&rarr;</span>
<a class="av-card" href="/en/category/geogrids/">
  <span class="c">Category · 3 series</span>
  <span class="n">Geogrids</span><span class="k">&rarr;</span></a>

Subsoil Separation

Drainage & Categories

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