Tensar International

A grid for subgrade stability

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With research to back up quantifiable benefits and recognized design standards, the use of geogrids may be ready for wider acceptance among North American railroads.

A product with a 20-year plus track record is rarely referred to as 'new.' However, this is a label those involved with geogrids find themselves tagged with when it comes to the prod¬uct's use on North American railroads. While experts give various reasons for the product's lack of recognition, geogrids may be on the cusp of becoming mainstream, or rather, mainline.

Matthew Brough, director of pavements, trackbed and materials consultancy at URS Scott Wilson in the United Kingdom, points to the time needed for proper evaluation of a technology as one reasons why adoption of geogrids could be perceived as slow.

'For most of the initial experience of geogrids in roadbed, the performance, although generally good has been anecdotal. It was not until in recent years that full-scale monitored work has been carried out and engineers are able to quantify the benefits,' said Brough. 'Within North American railroads, undercutting is often a preferred method of ballast renewal as it is a relatively quick, production based technique that maximizes the re-use of material back in track in short working windows (i.e. with minimal disruption to normal service). However, this makes the installation of geogrids and geocomposites more difficult operationally and as such, their use in ballast renewals has not been maximized.'

Jim Penman, vice president of sales for Checkmate Geosynthetics (U.S.). believes more promotion is needed to increase awareness of geogrid benefits.

'I started the [promotion] process about six years ago and there has been a significant increase in geogrid use during that time,' said Penman. 'The other thing that has happened fairly recently is that geogrids are now recognized by AREMA. A new chapter was officially published in the 2010 edition of the AREMA Manual for Railway Engineering. AREMA recognition was a huge step forward, as before that, there was no official guidance in North America on geogrid use in rail applications.'

Brough agrees that AREMA recognition could act as a catalyst, citing the U.K. as an example where the use of geogrids on soft ground was written into Network Rail standards more than eight years ago, resulting in an increased specification in ballasted track renewals.

'North American railroads have readily accepted the use of geogrids in sub-ballast applications hut have heen reluctant to add a new material to the ballast section before they have evaluated possible effects on ballast maintenance,' said John Bolton, director of marketing at Tensar International Corporation. 'The addition of a geogrid within a railroad design section can assist with ballast life extension, roadbed thickness reduction or a combination of both of these applications. Railroad sections stabilized with a geogrid have also been shown to reduce the vertical deflection of rails under load, which can provide a benefit by reducing fatigue in rail joints. Geogrids can also be used to stabilize heavy-duty pavement sections found at intermodal facilities where pavement life extension or thickness reduction can be achieved.'

Penman estimates that when a geogrid is placed in the sub-ballast for better load distribution onto the sub-grade, the resulting thickness reduction typically saves $30,000 to $50,000 per mile of track.

'In the UK, geogrids are also being used to minimize differential settlement at transitions with ballasted structures (e.g. on approach embankments). Current specifications are also being revised to include geogrids as part of the roadbed design under all switches and crossing renewals,' said Brough.

However, geogrids are not a universal remedy and many factors play into whether their use would be a good addition to the ballast or sub-ballast structure.

'The popular misconception is that geogrids only provide value in a rail structure when particularly soft sub-grades are encountered. In fact, though geogrids provide a more economic solution in most cases where the subgrade strengths up to those typical for a stiff clay. However, it is true to say that the performance benefits (relative to standard unrcinforced sections) arc more quickly observed where the strength of the subgrade is lower and/or when the track loads are higher and/or where track speeds are greater. Some engineers unfamiliar with geogrid use worry about their performance on heavy Class I railroads in particular. The reality is that the loads applied during trafficking are very small relative to the tensile strength of the geogrid,' said Penman, 'The main parameters used for the design of geogrid-reinforced roadbeds are the magnitude of the wheel loads, the maximum speed of the train and the stiffness of the subgrade. There are also some minor factors such as rail type, crosstie type and spacing, etc.'

Brough says design alternatives that utilize geogrids can be considered for all conventional track support systems.

'The designer must confirm that any geogrid that would be considered against a conventional design alternative has full-scale performance testing to ensure the integrity of the geogrid design. Geogrids have greatest effect where ballast strains arc highest; i.e., where there are heavy-axle-loads or soft subgrade. The key to correct application is ensuring decisions are based upon sound engineering design principles combined with an understanding of the implications to the contractor on operational installation,' said Brough.

'Other factors that are important when designing with geogrids are the strength of the existing subgrade, the rail tie spacing and the track loading conditions. If used in existing track, the existing roadbed and drainage condition are also of huge importance. For example, a geogrid used over a highly slurried formation layer with choked lineside drainage will fail early, as the overriding failure mechanism will not have been addressed. Alternatively, if applied on very stiff sub-ballast layers {e.g. bedrock) there will be lack of penetration of ballast into the geogrid with minimal observed benefits,' he said.

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