Geogrid-reinforced embankment for GO Transit rail service expansion

Overview

In 2006, GO Transit, the Greater Toronto Area’s commuter rail and bus system operator, underwent a significant expansion of its services. As part of this expansion, the rail service between Hamilton and Toronto, approximately 50 km to the east, was improved. An additional track was needed to meet the growing demand.

In West Burlington, just south of the intersection of Highways 403 and 6, an existing railway embankment supported CN Rail’s twin tracks as they passed adjacent to Sunfish Pond. This historic, non-engineered rail embankment, originally constructed around 1900, supported the main CN Rail line running from Halifax to Chicago, along with GO Transit services and other passenger and freight traffic. To accommodate the increasing traffic demand, a third track was required along the embankment alignment, necessitating a widening of the embankment crest.

Challenge

Sunfish Pond was part of an environmentally sensitive watershed managed by the Royal Botanical Gardens authority. As a result, widening the existing rail embankment was prohibited from impacting the pond. A conventional, widened 1V:2H embankment slope would have caused the slope toe to encroach significantly into the pond, making it unfeasible. Therefore, an alternative solution that satisfied both track alignment and environmental requirements was implemented.

After evaluating several options, the chosen solution combined a steel sheet pile wall at the toe of the slope with a vegetated geogrid-reinforced steepened slope above it. The sheet pile section of the steepened embankment slope was tied back using either earth anchors or battered piles, depending on the location. A 5-meter-high sheet pile wall was constructed immediately adjacent to Sunfish Pond. As the sheet pile wall was installed, an earthworks contractor followed closely behind, placing, and compacting specified granular fill behind the wall.

Solution

Above the sheet pile wall, a 1V:1.4H geogrid-reinforced fill slope was constructed and vegetated. The slope comprised compacted granular fill reinforced with layers of MIRAGRID® 7XT geogrid as the primary reinforcement, placed at 1.0 m vertical intervals and extending 6 m into the slope. MIRAGRID 2XT geogrid served as the secondary reinforcement to ensure surficial slope face stability, installed at 1.0 m vertical intervals between the primary geogrid layers and extending 2.0 m into the slope face. MIRAGRID 7XT and 2XT uniaxial geogrids were composed of high-strength, high-tenacity polyester yarns encased in a durable PVC polymer coating, with ultimate tensile strengths of 90 kN/m and 35 kN/m, respectively.

To construct the new reinforced fill slope and achieve the required geogrid reinforcement embedment lengths, excavation into the existing embankment slope was necessary. This slope was then stabilized temporarily using nail reinforcement. The reinforced fill slope was integrated with the excavated, nailed embankment slope, with the primary MIRAGRID 7XT geogrid extending across the full width of the new slope.

Upon completion of the structural portion of the reinforced fill slope, the surface was covered with 100 mm of topsoil and hydro-seeded with a grass mixture. A coconut/straw fiber blend erosion control blanket (ECB) was installed to protect against unwanted erosion and safeguard the hydro-seeded slope until vegetation became established. The degradable ECB provided essential protection against surface soil loss, enabling the vegetation’s root systems to stabilize the slope effectively. To ensure stability, the degradable ECB was stapled to the slope face at 1 m intervals and was trenched into the toe of the slope. Full vegetation coverage of the slope was achieved within approximately three weeks—a remarkably fast timeframe. Following this, the third rail track was constructed atop the reinforced fill slope.

This project has demonstrated exceptional long-term performance. The geogrid-reinforced soil slope has performed as intended for 18 years with virtually no maintenance. This durability highlights the effectiveness of combining structural geosynthetic reinforcement with vegetative stabilization to achieve both engineering and environmental objectives.