Geosynthetics in Road Construction

Geosynthetics are polymer products placed in or on soil to make a structure perform better. The name says it: “geo” for the earth and soil, “synthetics” for the polymer (usually polypropylene, polyester or polyethylene) they are made from.

Each type of geosynthetic does one or more jobs: reinforcement, separation, filtration, drainage or acting as a fluid barrier. They are used wherever soil is part of the structure: roads, railways, dams, canals, marine works, foundations, embankments and retaining walls.

Polymers do not rot in the ground the way natural materials do, which is why geosynthetics suit buried, long-term use. In most projects the aim is the same: make weak soil carry more load, or keep good material working longer, for less cost than digging out and replacing the soil.

Functions of Geosynthetics

The main functions of geosynthetics are:

• Soil reinforcement
• Separation
• Filtration
• Drainage
• Moisture or fluid barrier

Soil Reinforcement

Soil is strong in compression but has almost no tensile strength. A geogrid or high-strength geotextile carries tension and spreads load over a wider area, so the soil beneath sees lower shear stress.

That is what allows steeper slopes, or a taller embankment on soft ground, than the soil could support on its own.

Separation

A separator keeps two different soils apart. In a road, a geotextile between soft subgrade and granular base stops the subgrade fines pumping up into the aggregate under traffic, and stops the aggregate punching down into the mud.

The same function is needed in pavements, graded filters, clay covers and landfill liner systems.

Filtration

Filter-grade geotextiles let water pass across the fabric while holding the soil back on the upstream side.

To do that, the fabric needs two properties that pull against each other: enough permeability, which needs an open structure, and enough soil retention, which needs small openings. Designers match the fabric’s apparent opening size (ASTM D4751) and permittivity (ASTM D4491) to the soil being filtered.

The filter also has to stay compatible with the soil over its life, so it does not clog and lose flow.

Drainage

In drainage, water flows along the plane of the material rather than through it. Thick non-woven geotextiles can do this at low flows; where more capacity is needed, geonets and drainage geocomposites are used.

Barrier

Geomembranes and bitumen-impregnated geotextiles act as barriers that cut fluid and vapour flow. Typical uses are canal linings, liquid impoundments, dams and municipal waste landfills.

4 ADVANTAGES OF GEOSYNTHETICS IN ROAD CONSTRUCTION

Road construction is one of the most common uses of geosynthetics, especially for unpaved and temporary roads such as construction haul roads, access roads and forest roads. For permanent pavements in India, IRC:SP:59 (Guidelines for Use of Geosynthetics in Road Pavements and Associated Works) sets out where and how they are used. These are the main benefits.

Bearing capacity

Roads and parking areas need a stable subgrade with enough bearing capacity. A geogrid placed between the subgrade and the base course raises it: the aggregate interlocks with the grid apertures, the grid takes up horizontal forces, and the load spreads over a wider area. In many cases this lets the designer reduce base course thickness and avoid costly soil replacement.

Rutting

Unpaved roads on soft subgrade fail by rutting and by the cover material mixing into the soil below. Better load spreading reduces both. The grade of geogrid needed depends on the subgrade strength, traffic and cover thickness of the specific project.

Force-extension

Reinforcement works at small strains, so what matters is how much tensile force a geosynthetic develops at low elongation, not only its ultimate strength. Geogrid datasheets therefore quote tensile strength at 2 percent and 5 percent strain (ASTM D6637 or ISO 10319). For more demanding uses, stiffer products with extension at break of around 8 percent are available.

Resistance to installation damage

Placing and compacting cover soil and base course puts high dynamic stresses on the reinforcement. A geosynthetic for roads has to come through installation with enough strength left to do its job. Installation damage is assessed by exposing and retrieving samples after field placement (ASTM D5818) or by laboratory testing (ISO 10722), and the result is applied as a reduction factor in design.

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Ocean Global supplies geotextiles, geogrids, geomembranes and other geosynthetics for road and soil projects. Tell us your subgrade conditions and loading and we will suggest a product. Contact Us Now