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Introduction
The Canadian National Railway (CN Rail) wanted to build an additional six-track extension at its Transcona Yards in Winnipeg, Manitoba. Heavy railcars were expected to pass over this extension area and CN Rail was looking for an economical, efficient, environmentally friendly, and technically sound solution for its track extension.
Challenges
The construction site had swampy conditions with wet and soft clay soils. In fact, someone standing on the ground would easily sink a minimum of 6in. due to the low shear strength of the subgrade soils in wet, saturated conditions.
Because it was not possible to place a capping layer on these soils, CN Rail was forced to delay construction of the track extension, which was further complicated by continuous heavy rainfall. Additionally, a railway track constructed over soft and plastic subgrade soils may experience progressive shear failure and excessive plastic deformation. This would require frequent maintenance while disrupting operations, reducing efficiency, and increasing costs.
The challenge was to design and construct a stable formation on the weak subgrades that could serve as a satisfactory foundation for the rail tracks trafficked by heavy rail cars.
Conventional solutions
Conventional solutions for soft soil stabilization include techniques such as excavation and replacement, lime or lime-cement stabilization, preloading, sand/sand-lime piles, or stone columns, among others.
These can be cumbersome and expensive jobs. In some cases a working platform may be needed to allow the construction plant and equipment to move around.
Because of these constraints and the costs involved, these solutions were not considered appropriate for this particular site.
Geosynthetics solution
A proposal was submitted that specified geosynthetic materials as the most suitable for this project. Based on the nature of the subgrade soils and the type of loading, the geosynthetics needed to do three things: separate, filter, and reinforce.
The geosynthetic materials in this case included a biaxial geogrid bonded to a 6-oz continuous filament, needle-punched nonwoven geotextile. This composite was installed directly over the weak and saturated subgrade and a 600mm-thick granular (< 50mm limestone) capping/subballast layer was placed and compacted above it.
The apertures of the geogrid provided a high degree of interlock with the granular material. The positive mechanical interlock of the geogrid component with the fill particles created a laterally and flexurally stiff platform.
Consequently, the geogrid composite layer with the first lift of fill acted as a firm working platform and facilitated speedy construction. The nonwoven component acted as a separator, preventing loss of granular material into the soft subgrade during placement and compaction of the fill. This confinement allowed the capping layer to be compacted to high density.
The reinforced granular foundation distributed the heavy dynamic loads evenly and widely and provided a high degree of lateral restraint. This enabled the weak subgrade to resist the repeated loads of the railcars without experiencing progressive shear failure and undergoing excessive plastic strains. The nonwoven geotextile functioned as a filter to minimize the pumping of fines from the subgrade into the granular layer and prevent buildup of excess pore pressures in the subgrade from repeated traffic loads.
The geogrid composite was accepted as the optimal solution for the site and was approved by the project’s design engineer. The construction work was executed successfully and, as expected, the geosynthetic materials performed well. It provided a simple, easy to construct, effective, environment friendly, and economic solution to a challenging problem.
Benefits of geosynthetic materials
- Eliminated excavation and replacement with imported fill
- Reduced elastic deflections with heavy rail car traffic while allowing consistent high speeds to be achieved
- Reduced maintenance
- Offered significant saving in fill thicknesses
- Minimized differential settlement
- Reduced rate of permanent settlement