STAB ASSESSMENT:
WET SLAB HAZARD REPORT
A quick few notes on the reporting standard we are using for this stability hazard.
Assumptions:
You know how to locate and access terrain safely, conduct this assessment, and understand the basic terms and assessment techniques (CT / ECT - Snowform Identification) consistent with your minimum AST2 (Or equivalent) training.
Given the prevailing conditions we are looking for signs of WET SLAB instability. The two things that we are looking for are initiation ( a weak layer) and propagation (enough cohesion to create a fracture). Clues are harder to garner, look for remote triggered slides, and observed natural slides. If you get these kinds of warnings then we probably already have enough rough intel (Red Flags) about the hazard to raise a RED ALERT.
If conditions are safe enough to permit and / or 'Red flags' have been noted and responded to appropriately, in the process of running this patrol then conduct some pit tests in a suitable (<22˚ incline) aspect. Ideally at an altitude of 1700m or more, although this is not essential.
STEP 1:
Snow Profile & Compression test (CT)
Starting with a simple column test, you will go through the process of isolating any weak layers. If you get quick, easy initiation results at any depth... don't nerd out, jump to Step 2. If not make routine records in your book (CT Scores and Depths), recording the shear quality ( fast/ resistant / stubborn), noting the interface of the weak layer(s) (Crystal forms).
STEP 2:
Extended Column Test (ECT).
Here in Australia we have a variable and temperate climate, the rate of metamorphosis is relatively high. Our snowpack readily stabilises, establishing its ability to propagate is very important. In the case of wet slab, if the you don't get propagation on the extended column, but consistent easy CT score, the problem is a Wet Slide (Loose Wet) hazard.
The pit we need for this is very specific for consistent reporting. The task is to isolate a column that is 90cm wide by 30cm from the front to back, and isolated to the depth of the weakness (if on crust), or ideally deeper (on rounds / facets etc).
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TECHNIQUE
Having isolated the block as shown in FIG 1, using the same tapping / loading steps as your Compression Test, tap the loading area and observe closely any propagation across the column. If the column propagation extends the width of the column we report this as ECT:P, or if not report ECT:X.
You can record the loading step as (10), for example ECT:P(10)
STEP 3: Propagation Saw Test (PST)
If you got an ECT:P, or ECT:X but easy fast CT results then it will be worth running this quick additional study. Using the side of the pit you have dug, isolate a column that is 30cm wide by 100cm
TECHNIQUE
Having isolated the block as shown in FIG 2
push the saw along the weakness in an uphill direction. Again we are waiting to see if the weakness propagates. We want to record three things here. The distance of the cut (X) along the total column (Y) when propagation occurred, and what pulled out (Z) (FIG 3) and the depth of the weak layer (A). This is recorded as PST:X/Y(Z)@Acm
Example:
PST:35/100(END)@35cm
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Had enough yet? Why not drop a Pooling Test Step 4:
Wet Slab / Slide Pooling Test .
Ok, this is far from an exact science, but we consider it an important part of establishing or eliminating this hazard. Over a 100m area you can get huge variation, so all we need to know is wether the moisture you have detected is pooling.
TECHNIQUE
Find a slope, any slope steep enough really, as we are looking at a coverall instability. Avoid standing within of upslope of your test area. You will need a liquid dye, in quite large quantities (2 litres?), we use organic beetroot juice as the most environmentally friendly solution. Pour grid at 30cm intervals (a sauce bottle is a good dispenser). The flow of liquid needs to be generous enough to penetrate to the depth of your crust. An art leant through trial and error. Go big so it looks like a geometric 'murder scene'. Then let it settle for 1 hour.
Once the time is up, excavate the block. Then, cut along the upslope lines to expose the transported dye within the snowpack. You will find either continuous vertical lines (percolating), or a culmination along your crust which means pooling, the hazardous outcome. Either way, take some photos and flip them to us via email: advisory@mountainsportscollective.org.
