A geosynthetic provides one or more of four functions when used in track-bed construction: Separation to maintain the integrity of adjacent soil types i.e. prevent intermixing. Filtration to prevent leaching of soil particles. Drainage to allow the free passage of water. Reinforcement to provide additional strength.

Ballast over weak subgrades

It’s accepted that a geogrid stiffens the ballast by providing reinforcement at its base, and that this is highly desirable when track is being constructed over a soft subgrade.

Ballast over weak subgrades
TERRAM PW4-LA is a composite of PW1 and a geogrid which speeds up the installation time when both reinforcement and a filter/separator are both required.

An extensive, full-scale, independent research programme was carried out by British Rail and this clearly showed that the use of a geogrid beneath ballast over a soft sub-grade:

• Helps to extend maintenance intervals by minimising settlement

• Enables the rate of settlement to approach that of tracks on firm foundations

• Has a stiffening effect and will reduce the elastic deflections

• Can limit the lateral creep of ballast, reducing settlement and, therefore, the rate of deterioration of the vertical track geometry

Other benefits of using TERRAM PW4-LA are:

• provides a genuine alternative to increasing ballast depth or chemical stabilisation

• improves ballast performance and makes it more consistent

• allows consistent high speeds to be achieved

• avoids subgrade excavation and replacement with thick layers of imported fill.

Low load-bearing capacity can exist with most soil types including organic soils. 

Susceptibility of the subgrade to erosion pumping may or may not be an issue. If it is then a suitable grade of Hydrotex or a sand blanket plus PW1 should be used in conjunction with the geogrid as required by the subgrade type.

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TERRAM PW4 for supporting ballast over weak subgrades
PW4LA geocomposite of PW1 and a TENSAR geogrid
TERRAM Pw4-LA for weak subgrades, soft ground / peat and boggy conditions