Railways
20Pages

Geosynthetics in railways and related applications TERRAM’s PW range of geosynthetics perform up to four functions when used in trackbed 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 The use of geosynthetics to reduce or replace traditional layers is now an accepted part of trackbed construction and renewals around the world. When correctly specified and installed, geotextiles and geogrids are proven...

Extending trackbed life using geosynthetics Sub-grade erosion pumping over a clay or silty sub-grade. With ballast placed directly over a clay or silty sub-grade there is the possibility of a slurry being formed at the ballast / sub-grade interface; particularly if there are depressions or pockets at formation level. Regular traffic causes the ballast to oscillate at the interface which disturbs the clay / silt, and the presence of water in the pockets causes the particles to form a slurry. As the ballast dilates, the slurry moves into the void. The slurry is pumped upwards as the ballast...

Sand does not contain the gradedgravel fraction of a sub-ballast and is thus prone to intermixing with the ballast. TERRAM PW1 also acts to prevent this. Unlike sand, TERRAM PW1 is compact to transport and is rapidly laid ready for placement of the ballast. It has factory-controlled properties which do not rely on the need, unlike sand, for the correct thickness to be laid consistently across and along the track. In addition, excavation and the attendant disposal of fill is reduced when a geosynthetic is used to reduce the sand-blanket depth. TERRAM PW1’s ability to act as a filter over the...

Preventing Clay Pumping Placement of a sand blanket is time consuming. TERRAM can help you meet demanding schedules. Replacing a sand blanket entirely TERRAM has continued to develop and evaluate likely candidates for a sand-blanket replacement knowing that the ideal product would have to be: • Quicker to install • Lower in (installed) cost • More readily available • More environmentally friendly TERRAM recognised that its own development needed to be focused on the filter function as TERRAM PW1 alone is not capable of preventing the passage of the smallest clay / silt particles. TERRAM’s...

7 Filter / protector Fine filter Filter / protector TERRAM Hydrotex composites allow pore-water pressure to dissipate whilst preventing fine clay and silt particles migrating to contaminate the ballast. In addition, an Hydrotex composite conforms to the formation profile to minimise the potential for ponding. TERRAM Hydrotex Sand Blanket Replacing the whole of the sand blanket reduces the excavation that would normally be required to accommodate the sand and the attendant cost for disposing of the spoil. TERRAM Hydrotex see page 19

Preventing sub-grade erosion Preventing sub-grade erosion with fine soils, coarse soils and granular soils i.e. sand and gravel. Preventing sub-grade erosion with fine soils Fine soils are mixed soils, containing possibly all the grading sizes including clay, silt, sand and gravel. The clay and silt fractions dominate its engineering properties / behaviour. Sub-grade erosion over fine soils can be prevented by the inclusion of a Hydrotex geocomposite at the ballast / sub-grade interface. Preventing sub-grade erosion with coarse soils Ballast TERRAM Hydrotex Blanketing sand Fine soil...

If some contamination can be accepted, the use of PW1 directly on top of the coarse soil, without a sand blanket, may be considered appropriate. It is recommended that this solution is only adopted if the percentage of silt and clay fraction in the coarse soil is less than 15%. Preventing sub-grade erosion with granular soils i.e. sand and gravel If ballast is to be placed directly on a loose, slightly silty, fine to medium sand with occasional gravel, and there is a high water table, then TERRAM PW1 will aid the construction process over what would be difficult ground. In addition, TERRAM...

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

Low load-bearing capacity can exist with most soil types including organic soils. Susceptibility of the sub-grade 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 sub-grade type.

Controlling erosion on railway slopes TERRAM GEOCELLS are three-dimensional blankets of interconnected cells which are placed on slopes, secured in position using pins, and filled with friable topsoil. Controlling erosion on railway slopes Once installed, a TERRAM GEOCELL provides immediate stability by confining the fill and greatly improves resistance to wind and run-off. With a topsoil fill, the stability is further enhanced as vegetation becomes established and a geocell can be used to establish vegetation on slopes where establishing cover would otherwise have been a problem. The...

Product pages

TERRAM PW1Trackbed SeparatorTERRAM PW1 trackbed separator is designed to maintain separation between the adjacent sand / ballast layers within the trackbed construction, preventing the upward movement of fine sub-grade particles. Features: • Needle-punched geotextile • Suitable for good granular sub-grade soil • Can be used as separation between ballast and a sand blanket • High puncture and abrasion resistance • High permeability properties Network Rail TERRAM PW1 is suitable for where sub-grade soils are good, have sufficient strength and the particles are of even size. TERRAM...