From volume 102, winter 2012-13

By Rod Gee

My 25 year glide slab education began on an early morning in January 1989. A snowplow operator on Highway 16 west of Terrace reported witnessing a Size 3.5 airborne wet avalanche cross the railroad and highway corridors.

The deposit pushed sections of concrete guardrail into the Skeena River. Fortunately, no one was involved. I arrived at the site shortly after hearing the plow operator’s report. “Argh! It’s the glide slab I’ve been monitoring for the last week," I thought; "Why this morning? It’s notraining, and it’s not warm. Why did it run now? Were there indicators I’d missed?”

I came to the north coast of British Columbia to work in CN Rail’s Skeena avalanche program. I brought seven years of work experience in the Rockies, and ITP training in the Selkirk Mountains and the Coast Ranges. However, I had minimal knowledge of glide slab behaviour.

Glide slab prediction is a challenge, compared to the relative predictability of most maritime snowpack avalanche activity. They are classic poster children for the discussion surrounding why “Hazard Level 2” is perhaps a better descriptor than “Stability Good, with the occasional size 4.” Without start zone instrumentation monitoring glide rates, the CN Skeena program offsets uncertainty to some degree with frequent explosives control, and, where effective, runout zone earthworks.

Glide slab explosives control results, 50 mile path. Skeena River corridor, west of Terrace, BC // Rod Gee

These are some of the observations on formation and natural initiation I now use to evaluate glide slab stability:

• A low-friction ground surface is important for slab formation, but the degree of support from terrain features immediately below the slab is at least as important for slab failure.
• Rapid, early season snowpack accumulation associated with relatively warm air temperatures increases the likelihood of early- and mid-season glide slab formation.
• Lack of an effective ground freeze prior to snowpack accumulation results in increased mid-winter glide rates.
• Rainfall and meltwater percolation in an isothermal starting zone snowpack may accelerate glide rate by decreasing friction at the slab/ground interface. Free water may also decrease the strength of the supporting snow downslope of the glide slab as well as the slab itself. Rain falling into the glide crack above the slab, likely has a similar net effect. However, rain does not guarantee slab failure; it is only part of the equation.
• Glide slab failure does not require an isothermal snowpack. Failure may occur before the snowpack becomes isothermal or during the overnight cooling phase of the diurnal cycle, and without free water being present at the snow/ground interface.

Explosives Initiation
The ideal condition for explosives control occurs when the slab itself maintains a degree of strength greater than that of the snowpack below the toe and along the flanks of the slab. In an ideal scenario, a combination of terrain and weather factors unbalances the downslope snowpack stress/strength relationship to a greater degree than within the slab itself. The toe and flanks are now barely able to support the loading of the gliding slab. Explosives applied at this time cause slab initiation by triggering a failure of the snowpack at the toe of the slab.

Technicians Herb Bleuer and Mike Zylicz began experimenting with charge quantity and placement in the Skeena corridor in the early 1980s. They realized that conventional charge quantity was usually insufficient for glide slab initiation, and that charge placement was extremely critical. They also realized that placing the explosives charge into the glide crack above the slab was ineffective because that was not where the stress/strength relationship was deteriorating. Effective glide slab control is about “kicking the knees out” from under the slab, and not adding load to the slab itself. Their testing produced reasonable results using 100-150kg ANFO charges placed at the toe of the slab.

The best charge placement is a very specific point where the gliding slab is having the greatest effect on the non-gliding downslope snowpack. Current Skeena corridor glide slab control strategy includes the use of charges of 150 and 500kg on 200-500cm deep slabs. Large charges are used
because they increase the likelihood of triggering, which reduces hazard at the runout zone transportation corridor and minimizes the likelihood of natural events disrupting rail operations.

That said, control is not always successful. A complex, ever-changing interplay of factors affects glide slab stability, and the puzzle is not completely understood. Some factors I consider in evaluating explosives control effectiveness include:

• Control is more likely to be successful on glide slabs poorly supported by the terrain below the slab. For example, a poorly-supported glide slab can be initiated with explosives so it will then trigger a better supported glide slab lower in the starting zone that does not respond to explosives.
• Rain or melt-water at the ground/snow interface is not essential for initiation to occur, but it does increase the likelihood.
• Initiating sections of glide slabs is useful both by reducing the deposit volume of a single occurrence, but also because it exposes the ground surface to solar radiation, which then potentially aids in increasing glide rate by introducing more heat into the slab’s basal layers. 
• The strength of the snowpack below and alongside the slab allows the slab to glide a significant distance downslope without failing. Increasing glide rate may indicate decreasing snowpack strength.
• A 300-600cm slab can easily glide 50-100m without initiating if the downslope snowpack and terrain accommodates the glide’s loading effect. Increasing glide rate and/or deteriorating strength of the snowpack supporting the slab are two critical initiation factors.
• Three reliable nearby indicator paths I use to assess an east aspect path prone to glide slab formation have a northwest aspect, but the starting zones are at the same elevation. This suggests ambient air temperature affects glide slab behaviour to a lesser, but still relevant, degree.

Prediction and control have improved since the 1980s, but we still include a healthy dose of “art” to the “science” of our craft. Explosives control in January 2012 put a size 4 deposit within 2m of the rail roadbed. Is our understanding of glide slab management improving, or were we just lucky on that mission?