Snowpack Supporting Structures and RACS – A New Era of Avalanche Mitigation in Canada

Location and Site Conditions

Cougar Corner is located approximately 9 km west of Rogers Pass (Figure 1). The three avalanche paths that were designated for the installation of snow nets are Cougar Corner 6, 7, and 8. The starting zone elevation of the three avalanche paths range between 1400 m and 1700 m. The approximate areal extent of all three starting zones is 24,726 m2, as illustrated in Figure 2.

Figure 1. Overview of snow net location

Figure 2. Cougar Corner (CC) paths

The Cougar Corner site is mainly steep, rugged, and rocky with limited tree cover or vegetation except in Cougar Corner 8 (Figure 3). Complex rock features create several interruptions in the terrain, making it difficult to layout longer rows of barrier structure.

Figure 3. Cougar Corner site with snow nets installed in 2017. Paths outlined in red

Background on Snow Nets

The idea of inhibiting the formation of avalanches in avalanche starting zones goes back more than a century (FAO 1985). Wooden stakes, stone walls and other landscape modifications were built to protect settlements, often after a historic avalanche event had occurred. Since the 1960s there have been significant advancements with snow-supporting structural systems, and in addition to protecting towns and villages, they have been used to protect ski areas, industrial sites, and transportation corridors.

Generally speaking, there are two types of structural systems:

1. Rigid steel snow-bridges or snow rakes that consist of steel piles and steel profiles bolted together.
2. Snow nets with swivel posts, and high-tensile strength mesh (Figure 4).

Figure 4. Rigid steel snow–bridges and flexible snow nets side by side

Snow nets have been increasingly applied in areas where rockfall is a problem, or where there is challenging terrain or ground conditions (Figure 5).

Figure 5. Complex site geometry and conditions above the Trans-Canada Highway near Rogers Pass, BC (right)

The basic components in a snow net consist of swivel posts, high-tensile steel specialized mesh, anchors and foundation elements (Figure 6).

Snow nets are sized according to thickness of the snowpack, and the design thickness is designated as ‘Dk’. Normally the design thickness for a snow net system is determined by considering the extreme snow depth, or ‘Hk’. The common range of design Dk values for snow nets are from 2.5 to 4.5 m.

There are several advantages to the application of snow nets:

• Snow net components are light-weight and installation with a helicopter is possible in areas that are otherwise inaccessible.
• Snow nets are visually less obtrusive compared to rigid structures, which is relevant for tourism and parks.
• In areas subject to rockfall, the nets can be effectively adapted to the terrain.

The flexible snow-net system adapts to areas in permafrost terrain and creeping unconsolidated rock (e.g. free-standing plates are able to react together with the slope (no stiff, concrete foundation), and seasonal adjustments to the snow net floating base plates (Figure 7) and net structure can be made if there is movement under the net structure itself).

 

Figure 6. General components and layout of snow nets

Figure 7. Geobrugg SPIDER snow net floating base plates.

Currently, there are two other snow net installations in Canada:

1. Kicking Horse Canyon – the second installation in Canada. This installation has four rows and protects a segment of the TCH approximately 10 km east of Golden, BC.
2. 35 Mile Bluffs. This installation was completed in October 2014 and consists of three rows. It protects a short segment of Highway 16 approximately 50 km west of Terrace, BC.

In contrast to these smaller installations, the Cougar Corner project consists of 98 row segments.

Initial Desktop Design

The “Technical Guidelines for Avalanche Defense Structures in Avalanche Starting Zones” issued in 1990 and revised in 2007 (Margreth 2007) serve as the basis for the development and approval in Switzerland. The Cougar Corner snow nets were designed according to these ‘Swiss Guidelines’, which have become the international standard for snow supporting structures in avalanche starting zones.

These guidelines specify:

• Fundamental principles for general arrangement and coverage in order to effectively stop large avalanches from releasing, including minimum fence height and maximum separation between adjacent fence segments in a row.
• Maximum distance between rows of fences down a slope, in order to withstand static snow creep and glide forces.

The initial desktop design included terrain analysis using a high resolution (1 m) Digital Elevation Model (DEM) acquired through LiDAR survey of the site when it was snow free. This was used to determine slope shape and incline, identify constraining terrain features, and plan an initial layout (Figure 8).

Gumbel extreme value analysis of historical snow depth data from nearby weather stations was used in conjunction with analysis of snow drifting patterns in historical photographs to determine minimum fence heights. Avalanche starting zone boundaries were delineated from initial outlines provided by Parks Canada and detailed terrain analysis using the 1 m DEM and terrain photographs.

Field Investigations and Layout

Initial field investigations involved analysis of both the large features that were noticed during the desktop phase, and micro-features such as small steps, overhangs, and grooves that generally were not recognizable. These micro-features perhaps posed the most significant challenge on the project, as snow net systems require a certain degree of terrain uniformity and consistent (and competent) rock for the high strength anchors required to sustain the high snow creep and glide forces.

In addition to providing a challenge for the design and layout of the snow nets, the steepness and ruggedness of the terrain presented significant safety risk for personnel on site due to both falling, and rockfall from above. As a result, fall protection (primarily rope access) and rockfall management systems, including extensive scaling, were employed anywhere these hazards exist, which included approximately 75 % of the project site. Although these safety systems resulted in reduced speed and increased time to complete the project, they ensured a safe worksite for the many drillers, installers, and engineers that were on site on a daily basis.

As is typical with snow nets, layout was completed using a combination of cable jigs provided by Geobrugg (the manufacturer), measuring tape, inclinometers, and rangefinders. A Global Positioning System (GPS) with base station was used regularly to position the layout crew in locations designated on the initial desktop design. Dimensions for layout require accuracy for the snow nets to be supported properly in the terrain (Figure 9). Final layout relied extensively on judgement and experience of snow and avalanche engineers, and Geobrugg’s technical representatives.

Figure 8. Screenshot of the initial desktop snow fence layout drawing for Cougar Corner 7 illustrating the avalanche starting zone boundary (red line), post type and locations (green, yellow and pink squares), uphill toeedge of the Spider nets (black hashed line), and proposed debris flow net locations (orange and green lines)

Quality Control

Ongoing quality control occurred during the field layout phase to ensure the layout conformed to the Swiss Guidelines and Geobrugg specifications. A GPS system with GNSS correction was used to survey anchor and post locations that were laid out in the field. These coordinates were then analysed using a Geographical Information System (GIS) to determine spacing between points, distance between rows of fences, separation between adjacent fence segments and inflection angles at post locations (Figure 10).

Figure 10. Screenshot of the layout drawing for the lower section of the Cougar Corner 7 starting zone (red outline) showing post (green and pink squares) and anchor (red and blue circles) locations with spacing distances between points (black numbers (m)) and post inflection angles (red numbers (°))

These parameters were then compared to the Swiss Guidelines and Geobrugg specifications and areas that needed adjustment were highlighted. This was an iterative process that provided feedback to the field layout crew on a daily basis.

 

Discontinuous Terrain

Initial field investigations indicated extensive discontinuous terrain, sloping benches, and other terrain discontinuity that would not allow for a continuous snow net system (Figure 11). Although the Spider net system can accommodate continuous fence lengths of up to 60 m, in order to adapt to the discontinuous terrain, as well as wildlife permeability and practical installation purposes, shorter fence segments, typically involving at least two full panels, and 7 to 30 m in length, were used. Essentially, there were several isolated terrain segments that would only accommodate a single central panel of net with 2 triangle end panels, which was referred to as a ‘2-post system’ (Figure 12). Although these 2-post systems had isolated use previous to this project, Cougar Corner incorporated 23 2-post systems which accounted for 24 % of the rows.

Figure 11. Continuous snow net system

Figure 12. 2-post system

Incised Gully

The central section of Cougar Corner 7 path is a narrow converging gully that is over 5 m deep and 10 to 15 m wide at its narrowest point. Terrain investigations determined that standard snow nets were not suitable for this location. After discussing with the manufacturer’s technical representatives, it was realized that a debris flow barrier system could effectively be employed. The system is designed for significant loading from debris flow mass, which is much denser and heavier than even the densest snow pack. And the debris flow nets could act as a catchment for any loose snow sloughs that occur between rows of nets. The final installation is illustrated in Figure 13.

Figure 13. Use of Debris Flow net barrier in CC7 gully

 

Location and Site Conditions

The Cutbank avalanche area is located east of Rogers Pass on the eastern boundary of Glacier National Park approximately 40 km northwest of Golden, BC. Although the avalanche paths only affect the railway alignment, they were previously controlled with artillery that was fired from the highway, which required highway closures. Furthermore, mobilization time was higher due to location being far away from the primary artillery program. Therefore, the installation of RACS at this site is expected to reduce highway closure times and increase efficiency of avalanche control.

The RACS installations are located in a remote location near treeline, which required helicopter access. Every day, work crews, materials and equipment were flown in from either one or both staging areas as shown in Figure 14.

Figure 14. Overview map of the Cutbank project area showing the worksite as well as the two helicopter staging areas: Sorcerer and East Gate Rd

Background on RACS

Remote Avalanche Control Systems (RACS) refers to any avalanche explosive control system that is installed in and around starting zones and uses telemetry and electronics to relay and transmit a coded triggering sequence from a computer (office or mobile device) to a fixed installation on the mountain. RACS may incorporate cast explosives (tethered or mortar based) (Figures 15 & 16) or gas mixture that is detonated by a timed ignition sequence (Figure 17). The Cutbank avalanche paths incorporates the Wyssen® Tower system (Figure 15), while the Fortitude and Fidelity/Park One avalanche paths employ the Avalanche Guard® system (Figure 16). Parks Canada also has several Gazex® installations along the TCH as well as the Sunshine ski area road, since 1996.

Figure 15. Wyssen Tower explosives-based RACS. This Wyssen Tower system was installed at Cutbank during summer 2017

Figure 16. Avalanche Guard RACS system. As of 2016, there are six of these systems installed at Fortitude and Fidelity/Park One avalanche paths near Rogers Pass (photo courtesy of BC Ministry of Transportation and Infrastructure)

Figure 17. Gazex ® gas-based RACS. As of 2017, there are six Gazex exploder installations in paths affecting the TCH, approximately 10 km west of Lake Louise

The Wyssen Towers used for the Cutbank project is a selfcontained unit consisting of a portable explosives magazine deployment box that is seasonally installed on top of a remotely installed inclined mast (Figure 15). Inside the portable deployment box are 12 tethered charges preloaded into a mechanically rotating tray (dispenser). The initiation sequence involves a release of a suspended charge on a tether up to 14 m in length above the start zone.

The operational unit is removed and replaced remotely by helicopter long-line transport system, which allows for maintenance and reloading of explosives in a comfortable location in the valley. The magazine has a capacity of 12 charges of up to 5 kg each.

The main advantages of the Wyssen Tower include the portability of the deployment box (for worker safety, ease of reloading and maintenance, and summer storage), and the significant effective range of the air blast from the suspended charge (up to a 130 m fracture radius).

Design Methods

The initial requirement for the project was to ensure that all artillery targets could be impacted by the new RACS. This was accomplished through detailed field studies as well as a GIS-based viewshed analysis. The final result was five Wyssen Towers as shown in Figure 17.

Figure 17. Photo of the Cutbank avalanche paths from the Trans-Canada Highway. Approximate Wyssen Tower locations are shown (red crosses). The two railway alignments can be seen lower in the avalanche path

Fire Hazard and Smoke

The foundation construction and tower installation took place over a period of three months over the summer 2017. This was a period of intense forest fire activity in southern BC, including two large fires in proximity to the project site. As such, there were challenges for flying work crews and especially long-lining materials and equipment with the reduced visibility caused by the smoke.

Furthermore, due to the fire hazard, fuel could not be left unattended at the worksite. This meant that all fuel had to be flown off site at the end of the work day and back on site every morning, which significantly increased the amount of helicopter long-lining required.

Subsurface Conditions

The subsurface conditions encountered after drilling commenced were significantly different than expected. Poor quality rock was encountered at most foundation locations, which required anchoring methods to be adjusted. Solutions included significantly deeper anchors and hole casings, which required different drilling equipment and considerably more grout than originally planned.