Geosynthetic solution offers both separation and reinforcement for this Canadian rail project.
Canadian Pacific Railway (CPR) is renowned for building rail lines through some of North America’s toughest terrain. Just west of Kenora, Ontario, near the Manitoba/Ontario border, the main CPR track crosses the Telford Marsh, an area of weak organic soil. A 250 m (285 ft.) section of track located at mile 45.2 (Photo 1) required frequent tamping of the ballast to maintain proper cross-level alignment. Typically a slow order is issued if there is a horizontal differential of 19 to 32 mm (0.75 to 1.25 in.) between rails.
In March 2005, CPR contracted with KGS Engineering Group, a Manitoba-based geotechnical consultant with extensive railway experience, to solve this ongoing cross-level defect. To further complicate matters, access to the area for both site investigation and construction had to be by rail from a level crossing 1.6 km (1 mile) away. In addition, the engineered solution would have to be constructed during a series of 7-hour segments to reduce the disruption of rail traffic.
KGS engineers determined that a berm constructed on the north side of the track would prevent further movement of the existing rail embankment. The stratigraphy at the marsh consists of 2.5 to 3.5 m (8 to 11.5 ft.) of soft, saturated, organic soil with a shear strength of 6 kPa (125 psf) over 1.5 to 2.0 m (5 to 6.5 ft.) of loose fine-grained sand overlying more than 1m (39 in.) of soft, intermediate plasticity clay with a shear strength of 15 kPa (312 psf). Photos 2 & 3.
CPR also requested that KGS examine the possibility of relocating the track onto the new berm in the future. A major consideration was the capacity of the weak foundation soils under the increased loads if the railway track was moved. A detailed stability analysis was performed to determine the safety factor for the berm with and without geosynthetic reinforcement. It was determined that the berm would need to be 1m (39 in.) thick by 250m (820 ft.) long by 10m (33 ft.) wide, with a geosynthetic separation layer and tensile reinforcement at the base.
A geosynthetic reinforcement material, having a minimum tensile strength of 300 kNm (20,556 lb/ft) was proposed, to provide an acceptable factor of safety. Another constraint was that the berm be constructed with 13 mm (0.5 in.) diameter granite crusher fines, a waste product of the railway’s ballast production operation.
A conceptual berm design was generated incorporating a biaxial polypropylene geogrid for reinforcement with a nonwoven geotextile to provide separation between the granular and in-situ organic soils. However, there was concern regarding the strength provided by this geogrid/nonwoven combination.
KGS turned to Armtec, TenCate’s Canadian distributor, in a search for potentially better geosynthetic solutions. Armtec’s recommendation was Mirafi® PP300. This geotextile provides the benefits of both separation and reinforcement, with high tensile strength at low strains (5%). High permeability was also required to reduce pore pressures in the foundation soils. This requirement was important because the designers were relying on increased shear strength of the foundation soils through the consolidation process. This geotextile also has low construction damage value, an important plus when used in contact with the railway ballast reject (the crusher fines).
The geotextile was factory-sewn into 6 panels of 10 x 50 m (33 x 164 ft). Each panel weighed approximately 400 kg (880 lbs.) and was folded in a concertina fashion. These folds would later simplify the installation process. A butterfly seam using a type 401 double-thread “lock stitch” with approximately 4 stitches per 25 mm (1 in.) was used to sew the geotextile pieces together. Seam strength was 25 km.
Construction was scheduled for July 2005. Before deploying the geotextile panels, contractor Hugh Munro Construction dug a 600 x 1000 mm (24 x 39 in.) anchor trench along the length of the existing railway embankment (Photo 4).
A front-end loader was used to unfold each panel on top of the existing tracks and then dragged over undisturbed ground (Photo 5), assisted by an excavator and five men. The panels were placed with the seams running perpendicular to the existing embankment, extending into the anchor trench (Photos 6 & 7).
Adjacent panels had a minimum overlap of 1.5m (5 ft). Before placing any fill on the geotextile, the anchor trench was backfilled and nominally compacted to lock in the material (Photo 10). Once locked in place, backfilling could commence. Side-dump rail cars were used to bring in the granular material (Photos 8 & 9).
Working from a pad constructed on the geotextile, a large excavator was used to place the granular fill to a designed thickness. This desired thickness reduced the potential of a specific area of the berm sinking into the peat and then being further loaded to bring it to a designed elevation. Building to a designed elevation, without consideration of the berm thickness has contributed to the failure of similar geogrid-reinforced berms in other areas of organic soil.
Compaction for this project was achieved by walking the excavator back and forth over the granular as the berm was constructed (Photo 11).
As with most railway projects, the construction had to be completed without disrupting rail traffic. At the Telford Marsh site, that meant a series of 7-hour time blocks from 4 a.m. to 11 a.m. The entire construction sequence took three weeks to complete, working 3 10-hour days each week.
The fabricated geotextile panels used in this project helped to provide a successful completion of the berm on time and as designed.