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NPA geocell for railway line repair in permafrost region

Case Studies, Feature | June 1, 2021 | By: Sanat Pokharel and Marc Breault

FIGURE 1 Exposed degraded permafrost on the damaged track after the flood event in 2017

Churchill is a port city located on Hudson Bay in northern Manitoba, Canada. The nearest roadhead village of Gillam is 156 miles (252 km) south. Hudson Bay Railway (HBR), the only mode of land transport to Churchill, was completed in 1929. This single-line railway track has served the northern port of Manitoba uninterrupted; however, due to the lack of timely maintenance and upgrades, it gets damaged, causing service interruptions and ultimately leading to an extra burden on the maintenance budget. The railway was built on extremely soft ground that crosses many stretches of permafrost. In recent years, permafrost degradation caused by rising temperatures and other forms of human intervention have impeded the smoothness of the rail ride and reliability. The flood in spring 2017 severely damaged the rail line at all stream crossings. This rendered the only mode of land transport to the town unusable, and all basic supplies were airlifted, causing further hardships where the daily temperature remains subzero for more than eight months a year.

In May 2016, the authors visited the site to assess the condition of the railway track and recommended high-strength geocell reinforcement to control the lateral spreading of ballast/subballast material and protect the frozen subgrade. However, after the 2017 flood, the intent was a quick-and-affordable solution to repair the damage caused by recent track washouts and reestablish access to the town at the earliest possible time. An innovative design was required to complete the job in five weeks at a total cost in the range of 20%–25% of the more than $100 million initially anticipated for the rehabilitation work.

For the railway embankment structure, a robust structure to withstand the railway traffic while optimizing the construction costs and using the available resources in that area was proposed. Considering the general rule recommended by Pokharel et al. (2017) for other pavement structures, the railway track was designed with high-strength novel-polymeric alloy (NPA) geocell reinforcement.

This article discusses the design and construction methodology applied using NPA geocell in fall 2018. The findings are supported by design basis and innovative construction practice, photographs and construction reports. Attempts made to protect the permafrost under the rail line have also been explained. Based on more than two years of operation, this article also recommends a viable future of transportation infrastructure development method in the permafrost region.

Geocells on railway application

Pokharel (2010) and Han et al. (2010) identified three key mechanisms of geocell reinforcement as lateral and vertical confinement, wider stress distribution, and the beam or slab effect. The higher stiffness of the geocell system reduces the stress applied to the subgrade due to the bending stiffness of the mattress composite, similar to a slab (Pokharel et al. 2011). On road pavement applications, Norouzi et al. (2019) recommended geocell reinforcement as the reliable options under repeated loading condition. Railway embankments also undergo extensive cyclic loading and must be able to withstand the applied repeated load occurrences and, at the same time, be economical and sustainable in the long term. The need for a more robust embankment structure poses a huge challenge to the supply of granular material that is of adequate quality and is affordable (Pokharel et al. 2017). Where good-quality material is not easily available, it requires a reliable 3D geosynthetic reinforcement. The ability of geocells to use recycled, marginal or poorly graded granular material helps reduce the burden on the environment and adds value to the design. In railway applications for the reinforcement of structural layers, geocells can be used to reinforce the ballast or subballast to improve the reinforced layer’s modulus and reduce the stress transferred to the soft subgrades (Kief 2016). The NPA geocells that were used in this project enable the use of inferior-quality, locally available granular material while improving the modulus ratio over the underlying surface by up to 7.6 times; the higher strength and stiffness of the geocell produces a higher improvement factor (Pokharel 2010). This eliminates the need for hauling high-quality granular material and reduces associated CO2 emissions (Pokharel et al. 2016), making NPA geocell an attractive option from a sustainable-development perspective.

Leshchinsky and Ling (2013) reported greatly reduced lateral spreading and vertical deformations of the ballast when reinforced with geocells. The benefit was more pronounced when the railway substructure overlies softer subgrades and weaker ballast material was used. Palese et al. (2017) conducted performance tests on Amtrak’s Northeast Corridor near Havre de Grace, Md., in the U.S. with NPA geocell reinforcement in the track substructure. The material was selected due to its strength and its creep resistance properties for the Federal Railway Administration (FRA) Class 7 track that sees traffic from both high-speed trains (125 mph [201 kph]) and regional trains (110 mph [177 kph]). Palese et al. (2017) reported significant reduction in pressure at the ballast/subgrade interface of 50% and a corresponding reduction in the rate of degradation of a factor of 6. Overall, the test based on the pressure data from load cells placed above the subbase directly beneath the rails and track geometry data showed significant benefits using NPA geocell reinforcement above and beyond that seen by more traditional rebuild and drainage improvements.

FIGURE 2 The condition of the railway line in May 2016

HBR track rehabilitation and permafrost considerations

Designing sustainable infrastructure with limited engineering data is always a challenge and more so when it comes to where minimal interference to the environment is desired. Rehabilitation of the HBR line had similar challenges, as it passes through several stretches of permafrost. The rail line was damaged by a flood that was triggered by melting snow and rain, which was aggravated by the degradation of the underlying permafrost. This, compounded with the harsh winter, shorter construction season and linear construction sequence, demanded a solution that could address all these issues and complete the work within a reasonable time and budget. A design with NPA geocell reinforcement for the railway embankment addressed several issues like:

  • Providing necessary structural strength for the railway traffic
  • Protecting the underlying permafrost from further degradation
  • Controlling erosion at the embankment slopes against snowmelt and creek flows
  • Facilitating a linear construction schedule 
  • Completing the project within the available time and a significantly lower budget 

The postflood condition of the track in spring of 2018 and preflood condition in May 2016 are shown in Figures 1, 2 and 3.

For infrastructure development in the permafrost region, innovative design, sustainable construction methods, and the protection of the permafrost are important both for the structural stability and to protect the permafrost. Surface melt due to warm temperatures causes permafrost degradation, and, at the same time, the subsurface is a frozen, impermeable medium, meaning the entire melt becomes surface flow, causing flood damages. Any design work in this region, therefore, needs to avoid creating further problems in the creek crossings as well.

The HBR track in Manitoba was facing severe problems caused by the lateral spreading of the ballast and subballast material, erosion of the embankment sides and permafrost degradation. Under the heavy dynamic loading coming from train movement, a reliable reinforcement mechanism that comes through a high-strength and highly creep-resistant geocell was needed to reduce the stress transferred to the subgrade and keep the cover intact to protect the underlying permafrost.

Twelve of twenty-six damaged locations were identified along the 34-mile (55-km) stretch of the track for immediate repair and were designed with geocell reinforcement. All twelve locations had degraded permafrost and needed culvert pipe installation, as there were defined streamflow lines. The length of the washouts ranged from 230 to 689 feet (70 to 210 m). The repair work included reinforced embankment construction to support the structure from dynamic train loading, installing culverts and erosion control at the slopes.

FIGURE 3 Washed out railway line after the 2017 flood event (2018)

The design concepts

Several techniques have been implemented in Tibet to construct sustainable transportation infrastructure in the permafrost region. Wu et al. (1998) indicated that a height of the embankment of more than 2.6 feet (0.8 m) maintains a steady state on the permafrost and suggested that a 5.2-foot (1.6-m) embankment for the gravel road is optimal. 

The HBR track was not in good serviceability condition because of permafrost degradation causing vertical deformations and the lateral spreading of embankment material. The aim this time was to bring the track to its condition just before the flood events. The mode of failure was washout, which had exposed the permafrost and caused erosion at almost all the stream crossings. The design for rehabilitation needed to address issues such as structural strength, erosion control, protecting the permafrost from further degradation and, most importantly, utilizing the washed-out material deposited at the washout locations. If the material was not enough, a low-quality 3-inch (75-mm) pit run gravel would have to be hauled from Gillam (97 miles [156 km south]) of the first repair site. The only access to the site was the railway line itself. Given the time line of five weeks, options other than geocells seemed impossible, because there were more than 26 damage locations along a 93-mile (150-km) stretch.

American Railway Engineering and Maintenance-of-Way Association (AREMA) and Canadian National (CN) railway design principles were checked to confirm the structural requirements of the geocell-reinforced structure. The mechanism of load distribution via the slab effect to protect the weaker region within the permafrost was anticipated by transferring the load to a wider and more competent area. The design had attempted to make only 5% of the applied stress to be transferred to the subgrade. The geocell reinforcement was designed to increase the modulus of the unreinforced gravel by 3.5 to 4 times. In the design, the thickness of geocell plus 1 inch (25 mm) was assumed as the reinforced thickness for stress transfer calculations, as recommended by Pokharel (2010). 

Due to the construction constraints and available tamping equipment at the site, only the subballast layer was reinforced, and the layer of reinforcement required was determined by the fill height. To avoid further damage to the existing surface and to protect the permafrost, nothing was removed, and construction started right on top of the existing subgrade.

At some locations, the culvert pipes were found to have settled, rendering them useless. The new design raised the culvert beds to the existing streambed level and provided a semirigid NPA geocell-reinforced mat at the culverts to avoid a similar situation in the future. For erosion control at the toe of the embankment at and near the streambank, riprap was also provided.

TABLE 1 Properties of NPA geocell used in the design

Materials used

The geosynthetic materials used in this project were woven and nonwoven geotextiles and high-strength polymeric geocells. Salvaged, washed-away granular material was used wherever possible. Granular infill material with less than 12% fines was desired; the washed-out granular material deposited at the downstream side was used to save construction time and cost in bringing similar material from Gillam. Sand and 3-inch (75-mm) pit run gravel were the approved materials for infill in the geocell. Riprap used was 6–12 inches (150–300 mm) in size.

Two types of NPA geocells were chosen for the design, as NPA geocells have higher tensile strength, modulus and creep resistance than other available geocell material. The top layer of subballast just below the ballast was reinforced with Type D geocell, and layers below were reinforced with Type C geocell. Both NPA geocells were perforated, 6-inches (150-mm) high and had 13 inches (330 mm) between seams. The properties of the geocells used are given in Table 1.

The nonwoven geotextiles had a grab tensile strength of 205 pounds-force (911 N) and 299 pounds-force (1,330 N) with 50% elongation on grab. The woven geotextile had a grab tensile strength of 200 pounds-force (890 N). Nonwoven geotextiles were used on top of subgrade.

FIGURE 4 The track removal and geosynthetics installation for the repair work

Construction methods

Photographs and videos of the damaged area were made available but not the survey of other engineering data available for the design. An aerial survey was conducted to assess the extent of the damage and required repair work at the washout locations to resume rail traffic. It was assessed that the project would be a linear construction, and access to the next location would be available only after completing work at the preceding location. The designers reached the first damaged site by rail and walked to the next locations to access the damage and check the suitability of the design and construction methods. As the access was limited, the work needed to be completed with a limited amount of construction equipment—whatever could be transported to the site by available means. Even the daily commute for the construction crew from the nearest camp in Gillam on road-rail vehicle was six hours round-trip.

FIGURE 5 Geocell installation above the existing subgrade

There was limited space to work in the repair locations, as the construction team was told to work within the railway right-of-way. Transporting heavy equipment that could be accommodated within the width of the railway line was out of the question. The construction team followed the geosynthetic installation procedures per the manufacturer’s installation guide, and experience-based compaction methodology was applied for compacting the granular fill at the subballast and layers below. The lower layer of the granular fill was compacted to the possible extent to protect the permafrost layer. As there was no facility to do compaction testing, ruts developed by the 4-ton (3.6-tonne) compactor were used as an indicator of the required degree of compaction. A maximum rut of 0.5 inch (12.5 mm) under the roller wheel was considered as equivalent to 95% compaction for upper layers. Twelve passes of the compactor were made to meet the minimum compaction criteria at each location. Figures 4–8 show the construction sequence of the repair work: track cutting and removal, geocell installation, gravel compaction and reinstalled rail track.

FIGURE 6 Compaction after granular aggregate infill in geocell

Geocell was stretched and installed at the site with guide stakes. The requirement of more than one layer of the geocell was decided on-site wherever there was more than 29.5 inches (750 mm) of fill required between two layers. The railway track was cut and set aside to allow the installation of the geocell and to construct the embankment. When the embankment was ready, it was lifted back into position and connected.

FIGURE 7 Geocell installation for subballast reinforcement


Sustainable construction can be achieved by appropriately choosing the technology and using innovative ideas. Reusing the washed-out material can reduce the burden on environment and save money and time. As pointed out in the preceding sections, the design in this project not only considered the strength requirement but also gave due consideration to the environment. The design had to balance protecting the permafrost, controlling the lateral spreading and embankment side erosion, and strengthening the subballast structure to bear the railway traffic load. The use of available aggregate from the washed-out locations reduced about 50% of the virgin aggregate extraction and lessened the need for hauling and stockpiling burdens. The innovative geocell design, unlike the traditional design approach, provided the required structural strength by confinement of poorly graded granular material, and reused the washed-out material. It saved a huge amount of virgin aggregate extraction and carbon emission that would have happened due to hauling and mining virgin aggregate from a far-off location. This was made possible by using geocells as reinforcement.

FIGURE 8 The rails are connected, and the section is ready for ballast fill.

This project also showed that the knowledge and experience of the construction crew to work in a harsh climate under difficult limitations play a very important role. The project was completed in 33 working days at a cost of about 20% of what was initially anticipated for conventional rehabilitation work. The savings were primarily because of the reduced construction time, use of the washout material and working right on top of the existing subgrade without removing the degraded subgrade material. The construction equipment available was limited, so a coordinated approach between the geocell installers, culvert pipe installers and earthwork crew were of utmost importance to getting the job done in a timely fashion.

Unless a ballast-tamping method that does not damage the reinforcing geocell at the ballast layer is developed, the ballast layer cannot be reinforced, so the loss of ballast material by lateral spreading in this railway line is expected. So, there is a need to find a different technique in the remote area to do the tamping that allows ballast reinforcement and controls the loss of ballast material by lateral spreading.

All possible precautions were made to cause minimal or no damage to the unspoiled surrounding ground. Better preplanning on construction activities involving all the subcontractors can improve overall productivity of a project. At times, the primary contractor and the client’s engineers encountered misunderstandings due to their different knowledge of geocell technology. However, as the work progressed, the work went smoothly.

FIGURE 9 Completed rail reinstallation near switch at Milepost 414

The damage to this railway infrastructure was caused in part by the melting permafrost, unmanaged cross-drainage structures and erosion-prone embankment slopes. The permafrost degradation along the railway track is evident by the uneven settlement along the railway length and subsidence of the existing culvert pipes. Lateral spreading of ballast was another problem. The railway line is in operation, but a 93-mile (150-km) stretch of the railway line north of Gillam is still in need of repair. Once those areas are also repaired, the rail line can be run at 31 mph (50 kph) as designed. There are many locations where the ballast and subballast material is scattered and spreading laterally. A confinement technique such as high-strength geocell reinforcement can control that and reduce the maintenance costs in the long run.

This rehabilitation work was completed just when daytime temperatures started falling to subzero in that region (Figure 9). Rail access to Churchill was reestablished on the last day of October 2018 after the track was inspected and approved for operation by the Transportation Safety Board of Canada. The railway has been in operation for two full seasons after the rehabilitation without any major concerns. Figure 10 shows the first train arriving in Churchill after the rehabilitation work.

FIGURE 10 First train arrival, Oct. 31, 2018, in Churchill in almost two years


The NPA geocell-reinforced design of the railway embankment proved to be a successful solution for maintaining and strengthening the railway structure. In this project, it allowed the railway that was nonoperational for almost two years to start up operations again within a short time frame after the rehabilitation work started. The time required for construction was enough to repair the washout location, because half of the granular infill material was taken from the washout location itself, a fact made possible by the geocell reinforcement. The geocell reinforcement controlled lateral movement of subballast, which was a poorly graded, sandy pit run gravel, and controlled the settlement and damage to the permafrost. It also controlled the erosion of the embankment slopes. The design saved construction time and money. Based on more than two years of operation of the railway line, this article also recommends this design as a viable method for future transportation infrastructure development in the permafrost region and other similar areas.

Sanat Pokharel, Ph.D., P.Eng., is a principal engineer at Stratum Logics in Acheson, Alb., Canada.
Marc Breault is president of Paradox Access Solutions in Acheson, Alb., Canada.
All figures courtesy of the authors.


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Project Highlights

NPA geocells and geotextiles for railway

OWNER: Arctic Gateway Group LP

LOCATION: Manitoba, Canada

CONTRACTOR: Paradox Access Solutions Inc.

DESIGN ENGINEER: Sanat Pokharel, Stratum Logics Inc.

GEOSYNTHETICS PRODUCTS: NPA geocells, woven and nonwoven geotextiles

GEOSYNTHETICS MANUFACTURER: PRS Geo Technologies (NPA geocells), Paradox Access Solutions Inc. (geotextiles)

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