Non-Woven Geotextile for a Powergrid Substation in Koppal, Karnataka

A power substation is a large, heavy, permanent installation that has to stay level, well drained and safe to walk on for decades. Much of that depends on the ground beneath it and on the thick blanket of crushed stone that covers the switchyard. A geotextile keeps that stone layer clean and stable over a weak or variable subgrade.

On this Powergrid project at Koppal, Karnataka, Ocean Non Wovens supplied 110,000 sq m of 2 mm non-woven polypropylene geotextile. It went in mainly for separation and filtration beneath the switchyard surfacing, internal roads and drainage works.

Project snapshot

Why substations use geotextiles

Substations and transmission works place specific demands on the ground:

• Large graded areas must stay level and free-draining for years.
• Gravel surfacing across the switchyard must stay clean for safety and drainage.
• Weak or variable subgrades cannot be dug out and replaced economically over such a large area.
• Imported aggregate is costly, so it has to be protected from contamination.

A non-woven geotextile handles all four by separating, filtering and draining in one layer. It keeps the expensive stone working, keeps water moving, and gives a stable platform without deep excavation.

What the geotextile does under the switchyard

The switchyard is covered with a thick layer of crushed stone. It gives a safe, free-draining working surface, and its high resistivity is part of the yard’s electrical safety design, limiting step and touch voltages during a fault. That stone layer has to stay clean and intact. The 2 mm non-woven geotextile protects it in three ways:

If the subgrade and the stone intermix, the gravel blanket loses depth and cleanliness, the yard holds water, and the surface goes uneven. The geotextile stops that for the life of the installation.

The full 110,000 sq m was supplied to one specification. On a yard this size, every roll has to perform the same way, so mass per unit area and thickness are worth checking on delivery.

Geotextile vs thicker stone or excavate-and-replace

Without a geotextile, the usual answer to a weak subgrade beneath a stone blanket is to dig it out and replace it, or to lay a much thicker stone layer to allow for contamination. Both waste material and time.

Across 110,000 sq m, a thin, clean stone layer instead of a thick, contaminated one saves a large amount of quarried aggregate, haulage and cost.

Properties to specify and check

Ocean Non-Woven Geotextile (PP), 2 mm

Site conditions at Koppal

The site and the size of the yard shaped the design:

• Large area: one consistent separation layer was needed across the whole switchyard and road network.
• Weak or variable subgrade: the natural ground could not support a clean stone blanket on its own.
• Gravel migration: without separation, crushed stone sinks into soft soil and the blanket thins.
• Drainage: the yard must shed monsoon rain quickly and not hold water around equipment foundations.
• Dust and contamination: fines pumping up into the stone compromise both safety and drainage.
• Programme pressure: a large power project runs to a tight schedule, so fast installation matters.

Matching the fabric to the subgrade

Filtration balances two properties that pull against each other. The opening size, measured to ASTM D4751, must be small enough to retain the soil. The permittivity, measured to ASTM D4491, must keep the fabric more permeable than the soil so water drains without pressure building up. Matching the fabric to the subgrade is the core of the design, and a 2 mm non-woven gives a good balance of strength and flow for this duty. Internal access roads built over the same separation layer also resist rutting better, because the base course stays clean and keeps its load-spreading thickness.

Installation sequence

A separation layer only performs if it is installed correctly. The main steps:

1. Prepare the subgrade: trim and level the formation and remove sharp debris.
2. Lay the geotextile: roll out the fabric across the formation with the specified overlaps, increasing the overlap on softer ground.
3. Place the stone: push the crushed stone forward over the fabric instead of driving directly on it.
4. Compact in layers: place and compact the surfacing and road layers in controlled lifts.
5. Protect from sun: cover the fabric promptly, since polypropylene loses strength under prolonged ultraviolet exposure.

Common mistakes to avoid:

• Too little overlap between panels on soft ground
• Driving plant directly on the uncovered geotextile
• Using the wrong opening size for the subgrade soil
• Leaving the fabric exposed to sunlight for too long

Ocean Non Wovens helps customers match the geotextile grade to the subgrade and the loading so the installed layer performs as designed.

Long-term performance

Once buried, the geotextile is protected from sunlight and works for the life of the platform:

• Chemical and biological resistance: polypropylene is unaffected by the salts and mild acidity of most soils.
• Cyclic durability: it keeps separating and filtering under repeated wetting, drying and loading.
• Clog resistance: with the opening size matched to the soil, it keeps draining without clogging.
• UV protection: stabilised for construction exposure, and protected once buried.
• Low maintenance: a permanent buried layer with no moving parts needs only routine inspection.
• Lifecycle savings: it avoids the recurring cost of topping up a contaminated, pumping stone blanket, and the fewer truckloads of stone on a thinner blanket cut haulage across the yard.

Where else this geotextile is used

The same separation, filtration and drainage functions apply across many works:

• Substation and switchyard surfacing
• Solar and wind farm internal roads and platforms
• Highways, rural roads and haul roads
• Railway formations
• Behind retaining walls and in subsurface and edge drains
• Beneath industrial yards and hardstanding

Frequently asked questions

Why is a geotextile laid under substation gravel?

It separates the crushed stone from the subgrade soil. Without it, the stone sinks into soft ground and soil pumps up into the stone, so the gravel blanket thins, holds water and stops working as a clean, free-draining surface.

How much geotextile was used at the Koppal Powergrid project?

110,000 sq m of 2 mm non-woven polypropylene geotextile, beneath the switchyard surfacing, internal roads and drainage works.

Does a geotextile reduce the amount of crushed stone needed?

Yes. A separated stone layer stays clean, so it does not have to be laid extra thick to allow for contamination. Over a large yard that saves quarried stone, haulage and earthwork.

Why must the geotextile be covered quickly after laying?

Polypropylene loses strength under prolonged ultraviolet exposure. The fabric is stabilised for construction exposure, but it should be covered with stone promptly.

Working with Ocean Non Wovens

Ocean Non Wovens supplied 110,000 sq m of geotextile to this Powergrid project at Koppal. The company manufactures non-woven and woven geotextiles, geocells, geogrids, geobags and HDPE liners in India, tested to Indian and international specifications, for projects of this scale. Its materials go into power and energy infrastructure, highways and railways, water resources and environmental works, airports, industrial developments and defence projects across India.

For separation, filtration, drainage, reinforcement or containment, the Ocean team can recommend a geosynthetic grade for your soil and loading. See more case studies or contact the team about your project.