Benefits of Using Geosynthetics in Railway Lines - Mud Pumping

Whole of life cost on rail track has become increasingly important as rail asset owners and operators look for ways to maximise their maintenance budgets whilst maintaining safety and serviceability.

An independent life cycle cost analysis conducted by industry leading cost engineers, WT Partnership, has shown that geosynthetics can significantly reduce whole of life costs when compared to traditional track formations and construction methods.

WT Partnership’s report, Track Formation Rehabilitation Options (Geofabrics) – Cost Options and Whole of Life Comparisons (June 2016), was carried out comparing a range of different scenarios using Geosynthetics and comparing them to conventional track formation practices which do not utilise Geosynthetic solutions.

The study investigated a 200 metre long section of track renewal over a 40 year life span and made conservative assumptions around total replacement costs. The study investigated three capital works life span replacement intervals: 15, 20 and 25 years. These intervals can be considered conservative as geosynthetics have proven to contribute to the performance of rail lines for intervals in excess of these time frames.

An independent report on “mud pumping” in a rail line was carried out in February 2017 at a trial site location at Bradley Junction, UK. This report considered a 6 year performance of Tracktex Anti-Pumping Geocomposite to prevent “mud pumping” and compared it to the performance of an adjacent control section. Photos below clearly highlight the performance of Tracktex to prevent “mud pumping”.

 

As Geosynthetics have also proven to increase the maintenance cycles of rail lines a further assumption has been made in respect to tamping operations. The study considered tamping on a yearly basis for the control sections and at three yearly intervals for the Geosynthetic sections. This is also considered conservative as Fig 3 Maintenance at Coppull Moor, shows a greater than three times improvement to tamping cycles.

Ballast stabilisation further increases tamping cycles by using a purpose made ballast geogrid Tensar TX190L directly on top of the Tracktex. The Tensar TX190L effectively locks the ballast preventing it from movement and breakdown. Further to the Coppull Moor trial, the use of ballast geogrids are further enhanced by a study conducted at Penn State University by the inclusion of a Smartrock into a test bed. Fig 4 shows the translational movement of ballast in a track bed with and without Tensar TX190L geogrid.

A combination of both Tracktex and Tensar TX190L will provide Rail Authorities with a robust solution to the never-ending problem of “Mud Holes” in a rail track as was shown in a study carried out by Hudson et al from the University of Southampton, UK. This study described measurements at a mud pumping site before and after a complete track renewal where previous, localised gravel packing interventions had been shown to be ineffective. The renewal solution selected to span the wet bed area, was to include a micro-porous filter (TRACKTEX) and a large aperture “Ballast” Tensar Geogrid placed immediately beneath the ballast layer. This is a type of renewal that while relatively extensive falls short of a complete replacement of the track bed and omits the placement of a sub-ballast or capping layer. Trackside measurements by a Track Recording Car (TRC) were taken prior to, immediately after and 5 months after renewal to determine the efficiency of the renewal in eliminating the recurrence of the wet bed.

Deflections from the TRC taken on the top right rail are shown in Fig. 5 and vertical sleeper deflections Fig 6 across the wet bed area both pre and post renewal show the much improved performance and highlighted that repeated digging out and re-packing with gravel layers had been ineffective. The renewal of combining Tracktex and Tensar Geogrid has resolved the problems at this site.

WT Partnership’s report, shows that over a 40 year period the use of a Tracktex and Tensar TX190L combination for a track renewal could potentially save $20.69 million per km of track in ongoing maintenance costs.

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Mud Pumping Tech Note - August 2018