Geogrid in Railway Construction: Blanket, Ballast and Formation

A geogrid in railway construction does one job: it stiffens a granular layer by locking its particles into a grid of ribs, so the layer spreads wheel loads further and deforms less under repeated passes. On Indian Railways track that layer is usually the blanket between the ballast and the subgrade, and sometimes the ballast itself. This post covers where a railway geogrid goes in a ballasted track, what it can and cannot do, and how to specify one for an Indian formation.

How a ballasted track carries load

A broad gauge train load passes down through four layers:

1. Rail and sleeper. The sleeper spreads each axle load across a few hundred millimetres of ballast.
2. Ballast. Angular crushed stone that holds the sleeper in line and level and lets water drain away. Indian Railways uses 65 mm machine-crushed ballast, with most of each lot sized between 40 mm and 65 mm. RDSO’s own tender documents describe cushions of 300–350 mm under the sleeper on important routes.
3. Blanket. A layer of well-graded sand, gravel or moorum placed on top of the subgrade. It cuts the stress that reaches the soil, stops ballast being pushed into the subgrade, and keeps subgrade fines from pumping up into the ballast.
4. Subgrade. The embankment or cutting soil. On black cotton soil, soft alluvium or silty clay this is the weak link.

Most track geometry problems on weak formations start at the bottom: the subgrade softens in the monsoon, the blanket rides down into it, fines pump up, the ballast fouls, and the track settles unevenly between tamping cycles. A geogrid works on the granular layers above the soil to slow that chain down.

Where the geogrid goes, and why

### Inside or at the base of the blanket

This is the most common position on Indian projects. RDSO’s *Guidelines and Specifications for Design of Formation for Heavy Axle Load* (RDSO/2007/GE: 0014, November 2009) carries an annexure on geosynthetics in track bed applications, and RDSO’s later *Comprehensive Guidelines and Specifications for Railway Formation* (RDSO/2020/GE: IRS-0004, September 2020) consolidates its formation guidance. Both are on the RDSO website.

Blanket material compacted onto a geogrid interlocks with the apertures. The grid takes up the lateral spreading that would otherwise loosen the layer under each passing bogie, so the blanket stays denser and stiffer. The practical results designers look for are lower stress on the subgrade, less rutting of the blanket surface, and a case for a thinner blanket where the design and the railway’s approval allow it.

Grid aperture has to suit the blanket gradation. A biaxial polypropylene geogrid with apertures in the 30–40 mm range interlocks well with a sand-gravel blanket. Ocean’s biaxial geogrid range is made to this pattern, rated by peak tensile strength in both directions and tested to ASTM D6637 / EN ISO 10319.

### At the subgrade surface, with a geotextile

Where the subgrade is fine-grained and wet, a geogrid alone does not stop fines migrating upwards: its apertures are far too open. That job belongs to a nonwoven geotextile laid on the subgrade, which separates and filters while water still passes. Many formation designs use both: geotextile on the subgrade, geogrid within or at the base of the blanket. A bonded geocomposite can do both jobs in one layer.

### Under the ballast

A geogrid placed directly under or within the ballast is aimed at lateral spreading of the stone, which is what makes the track lose top level between tampings. This needs a grid whose apertures match 40–65 mm ballast and whose ribs survive sharp stone and tamping. It also has to sit deep enough that tamping tines never reach it. This is a specialised design; treat it as a separate specification from the blanket grid, not the same roll laid higher up.

What a geogrid will not fix

• Poor drainage. If water stands in the formation because side drains are blocked or the cess is too high, the subgrade will soften with or without a grid. Fix drainage first.
• Swelling soil. Black cotton soil heaves and shrinks with moisture. A grid in the blanket reduces the damage that differential movement does to the track, but it does not stop the soil moving. That needs soil treatment, blanket design or replacement.
• Deep-seated settlement or slope failure. An embankment on soft clay that is consolidating, or a slope that is sliding, needs geotechnical design: staged construction, basal reinforcement, slope reinforcement or drainage. Our basal reinforcement page covers the embankment case.
• Fouled ballast. A grid does not clean ballast. Deep screening still has to happen; a grid can help the cleaned layer stay in shape afterwards.

Specifying a railway geogrid

Ask for these on the specification and check them against the test certificate:

Strength rating alone is a poor way to compare grids for track work. Two 30 kN/m grids can behave quite differently at 2% strain, and the low-strain number is closer to what the blanket sees in service.

A worked example (illustrative)

The numbers below are made up to show the arithmetic. Your own design will come from the formation design and the railway’s approval.

Say a new broad gauge siding of 3 km crosses silty clay, and the formation design calls for a 1.0 m blanket. The designer considers a geotextile on the subgrade plus one biaxial geogrid layer in the blanket, and a field trial shows that a 0.7 m blanket with the grid matches the performance target.

• Average blanket width assumed: 7 m.
• Blanket volume saved: 3,000 m × 7 m × 0.3 m = 6,300 m³.
• Geogrid needed: 3,000 m × 7 m = 21,000 sq m, plus lap allowance (say 8%) = about 22,700 sq m.
• Geotextile needed: similar area, plus its own laps.

Whether the trade makes sense depends on the local cost of blanket material and its haulage against the supply cost of the two geosynthetic layers. Where good blanket material is far away, haulage savings usually decide it; where it is close by, the case rests on lower maintenance.

Installation on the formation

• Prepare and compact the subgrade to the specified density and camber before anything is laid.
• Lay the geotextile first, if specified, with laps as the design requires.
• Roll the geogrid out along the track, pulled taut by hand so it lies flat with no waves, and pin it at the start of each roll.
• Lap adjoining rolls as the design specifies. Do not let rolls bunch at the edges of the formation.
• Tip blanket material on the layer already placed and push it forward over the grid. Do not drive tracked or wheeled plant directly on an uncovered grid.
• Spread and compact the blanket in layers to the specified density.
• Keep rolls covered or shaded in storage.

Ocean on railway projects

Ocean has supplied geosynthetics to several railway formations in India. Examples on this site include 80,000 sq m of Ocean biaxial geogrid 30 kN at Hotgi, Maharashtra, geotextile and geogrid for a railway project in Maharashtra, and Ocean geocell on the NWR doubling project KTWS–NNL. For ballast-side measures, see our post on geosynthetics for railway ballast stabilisation, and the railways sector page for the full product list.

Frequently asked questions