PP Biaxial Geogrid and Non Woven Geotextile for a Railway Formation in Solapur

Ocean Non Wovens supplied 3,950 sq m of Ocean PP Biaxial Geogrid and 4,000 sq m of Ocean Non Woven Geotextile for a railway project in Solapur, Maharashtra. Pairing a biaxial geogrid with a non woven geotextile is the standard two-layer fix for a weak railway formation: the geotextile keeps subgrade fines out of the track bed, and the geogrid stiffens the granular layer above it. This article covers how biaxial geogrid for railway formation works with the geotextile, and what to check when specifying and laying both around Solapur.

Project snapshot

Why railway formations in Solapur need help

Solapur is a junction on the Mumbai–Chennai main line and the headquarters of Central Railway’s Solapur division, so a lot of new line, doubling and yard work passes through the district.

The ground is the difficulty. Solapur sits on Deccan Trap basalt, and weathering of that basalt has left black cotton soil over much of the district: shallow in the west, medium to deep in the east. Black cotton soil is an expansive clay. It swells when it gets wet, shrinks and cracks when it dries, and loses much of its strength when saturated. The district lies in the rain shadow of the Western Ghats, so rainfall is low and erratic, and the soil cycles between very dry and wet every year.

Under a track, that causes three familiar problems:

• Mud pumping: repeated axle loads on a wet clay subgrade force a slurry of fines up into the blanket and ballast.
• Ballast fouling: once fines reach the ballast, it stops draining and loses its interlock, and the track geometry starts to drift.
• Uneven settlement: soft spots in the formation settle faster than the rest, which means more frequent packing and, in bad cases, speed restrictions.

Formation design puts a blanket of graded granular material between subgrade and ballast; geosynthetics make that blanket work harder.

One layer separates, the other reinforces

The two products supplied do different jobs, and neither replaces the other.

The Ocean Non Woven Geotextile is a needle-punched fabric. Water passes down through it, but clay particles cannot pass up, so the blanket keeps its design thickness and its drainage.

The Ocean PP Biaxial Geogrid is a polypropylene grid with similar strength along and across the roll. Blanket aggregate partly enters the apertures and locks against the ribs. When a wheel passes, the grid stops that aggregate from moving sideways, so the layer stays stiffer and spreads the load over a wider area of subgrade. Lower stress on the clay means less deformation each time a train passes.

Selecting biaxial geogrid for a railway formation

Ocean makes PP biaxial geogrid at 20, 30 and 40 kN/m. The selection logic between them is the same for any formation:

• Look at strength at low strain. A formation works at small strains, so compare tensile strength at 2% and 5% strain rather than the headline kN/m.
• Match aperture to blanket grading. Ocean’s PP grids come in standard and larger-aperture (L) versions so the particles can interlock.
• PP suits treated soils. Black cotton soil is often lime-treated before a formation is built on it. Polyester can lose strength by hydrolysis in strongly alkaline ground. Polypropylene tolerates a much wider pH range, which is one reason PP biaxial grid is the common choice here.

Ocean’s PP biaxial grids come in 100 m rolls, 5.0 m or 5.1 m wide, so 3,950 sq m is roughly eight rolls before overlaps.

Ocean has supplied this material in the same district before: 80,000 sq m of Ocean Biaxial Geogrid 30 kN for railway works at Hotgi, just outside Solapur city.

Specifying the geotextile

Railway formation work in India follows RDSO’s formation specification (RDSO GE: IRS-0004), and EN 13250 lists the geotextile characteristics required for railway use. For a separator on clay, the checks are:

• Retention: the apparent opening size (ASTM D4751) has to be fine enough to hold back the subgrade.
• Flow: permittivity (ASTM D4491) must stay well above that of the soil, so water is never trapped under the fabric.
• Survivability: CBR puncture (ASTM D6241) and tensile strength have to survive blanket stone being tipped and rolled on top.

Ocean geotextiles have gone into other Maharashtra rail work, including the railway formation at Satara.

Laying the two layers

1. Trim and compact the subgrade to profile, fill ruts and remove sharp objects.
2. Unroll the geotextile flat and loose, overlapping adjacent rolls by at least 300 mm, more on soft ground.
3. Lay the geogrid at the level the design gives, pull it taut and overlap as specified. Pin the edges so it does not move during filling.
4. End-tip blanket material and spread it forward. Keep construction plant off the bare grid and maintain at least 150 mm of cover under tracked machines.
5. Compact the blanket to the specified density in layers.

Frequently asked questions

Railway geosynthetics from Ocean Non Wovens

Ocean Non Wovens manufactures and supplies geogrids, non woven geotextiles, geocells and drainage geocomposites for railways, highways, airports and industrial sites across India.

To discuss a railway formation, yard or siding project, contact Ocean Non Wovens with the subgrade data, axle load and quantities.