Terram Geosynthetics are extensively used in civil engineering in the construction of highways - from preventing intermixing of sub-base and subgrade layers, control erosion on cut slopes, for roadside drainage, as part of SUDS projects for infrastructure access and in the construction of retaining walls, bridge abutments and steep slopes.

Improving granular layer performance

TERRAM Standard geotextiles prevent intermixing of sub-base and subgrade layers. This intermixing causes a loss in bearing strength, as the stone layer becomes progressively contaminated, resulting in failure in the form of deformation. This can be remedied if it’s an unpaved area/road by filling the ruts but even this can be uneconomic or unacceptable if stone has to imported over long distances at
a later date.

If it’s a paved road then the intermixing could manifest itself as deformation at the pavement surface e.g. cracking and/or rutting, by which time the damage at the sub-base/subgrade interface cannot be repaired without re-construction and any repairs are cosmetic rather than structural. Better to pre-empt the problem by including a
suitable geotextile.

Site-damage resistance

The geotextile must first be capable of withstanding the rigours of installation. This is when the textile is most susceptible to damage – not during service life. If it is not sufficiently robust, and is capable of tearing or being punctured, then the product is incapable of performing its design function. For example, the filaments of many low-cost, woven textiles can be easily teased apart, even during physical examination. They have little integrity and it is easy to imagine how they will perform when angular stone is placed on them and compacted. A textile filter/separator must have an apparent pore size which remains unaffected by loading. Any openings which are created or widened, or are caused by tearing/puncture, will allow subgrade particles to be pumped through to the sub-base.

Radial load requires isotropic properties

The loading from a wheel at the sub-base/subgrade interface is radial and this means that the geotextile should have isotropic strength to deal with it. It is not sufficient for the textile to have high strength in one or two directions as with woven products.

Sustained filtration

The geotextile must provide sustained filtration whilst also separating the two layers i.e. the textile must allow the free passage of ground water yet limit the passage of soil particles. This involves matching the pore size with the sub-grade’s smallest particle size. This does not mean that the geotextile should have a pore size smaller than the smallest soil particle. It has been established that a geotextile acts as filter by virtue of a natural filter that is formed against the surface of the textile.

The ultimate objective is to maintain the integrity of the granular layer and thus gain the maximum life from the structure. This can be achieved by allowing its maximum bearing strength to be mobilised throughout the road’s width and depth, and throughout its life.

Typical paved and unpaved applications where geotextiles are used beneath granular layers include:
• Highways
• Car parks
• Hardstandings
• Access and haul roads
• Cycle ways and footpaths

Back to Highways Overview

TERRAM geotextile filter / separators prevent intermixing of granular layers
TERRAM standard geotextiles are used in highway projects
Terram geotextiles prevent sub-base and subgrade deformation


TERRAM Highways & Roads
TERRAM Product Brochure
TERRAM Railways
B1 Drainage Geocomposite
1BZ Drainage Geocomposite
GEOCELL Slope stabilisation (M25 Widening)
GEOCELL & T1000 Erosion control for canal slope (Monmouth and Brecon Canal)
BODPAVE 85 Gravel-filled paver access / cycle route (Blackpool)
RG7 - Landfill Basal-cell-liner & side-slope protection/drainage (Beacon Hill LFS)
RG4, RG11 RG-SD19 Protection to Landfill basal & capping liners (Lean Quarry LFS)
RG25 & RG31 Liner protection for a deep landfill cell (Clifton Marsh LFS)
RG3 Landfill capping liner protection (Erin LFS)
RG4 & RG-SD19 Landfill cell side-wall drainage (Bryn Pica LFS)
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