Avalanche Snow Grain Bonds at the Slab Boundaries. Which slab boundary is the most important? Does the strength of the slab hold it in place through bonding around its edges, or is the more important bond underneath the slab between the slab and the bed surface?

Answer: In typical avalanches, the weak layer is the important one, which has been confirmed both theoretically and experimentally, because failure and fracture start within the weak layer first not the slab. That’s why we spend most of our time thinking about, testing, and worrying about the all-important weak layer. We don’t completely ignore the strength of the slab, however. As I discuss in more detail later, the strength and stiffness of the slab help propagate the crack within the weak layer. The bridging effect of slabs is also very important.

Snow Grain at the Slab Boundaries

To understand all of these forces, let’s head to the kitchen and do an experiment. Find three different “slabs” of about the same size but different strengths: a paper towel, a piece of cardboard, and a wooden cutting board. Even though they have about the same bed surface area, it’s easiest to tear the paper towel, harder to tear the cardboard, and impossible to tear the wooden board with your hands.

Thick, stout slabs are more difficult to fracture, and they form much better bonds to the crown, flanks, and stauchwall than thin, flimsy slabs. It’s difficult to trigger stout slabs on a very small avalanche path. These slabs have a relatively large amount of compressive support at the bottom, shear strength at the flanks, and tensile strength at the crown face. Even with little or no bonding on the bottom, the strength of the slab itself can often hold it in place.

On the other end of the spectrum, soft slabs with little strength can slide on nearly any slope, even small slopes and slopes with lots of anchors (trees and rocks). Because they are so soft, their strength plays a relatively insignificant role in keeping them in place. Although the bed surface is by far the most important boundary, the mechanics of an avalanche release depend, like a jazz trio, on the interactions of all three members of the avalanche ensemble: the slab, the weak layer, and the bed surface. Even though one solos while the other two follow along, it still takes all three to make music.

Besides weak layers, slabs also play an important role in avalanche ensembles, because their stiffness helps transfer the shear and collapse forces to the ever-expanding crack within the weak layer. Often the crack quits propagating when the slab becomes too thin or too weak, or it develops a break.

Avalanche pros often talk about slab bridging. To understand bridging, go sit on the edge of your bed. Notice that your weight makes the mattress sink down right under your butt, yet the mattress just a foot away hardly compresses at all. Now go get your biggest cutting board from the kitchen the big one that slides out from under the countertop or perhaps a piece of plywood. Put it on the bed and sit on it. Notice that the mattress under your butt doesn’t sink down as far.

Your weight is bridged, or spread, outward. Now imagine that you are in a spy movie and that someone has planted a pressure-sensitive bomb under your mattress. As soon as the switch registers your full weight, kapow! So what are you going to do? Are you going to sit on the cutting board or not? Maybe you will go get a bigger, stiffer cutting board how about a big piece of plywood? This is why it’s usually easier for a person to trigger soft slabs than hard, stiff slabs.

Good News, Bad News

The good news is that hard slabs are more difficult to trigger, but the bad news is that if you do trigger one, it will propagate farther and make a much larger and more deadly avalanche. And on a planar slope, stiffer slabs tend to break up above and are much harder to escape. Finally, remember that the stiffness and thickness of slabs can vary a lot from place to place, so just because a slab does not trigger in a thick spot, crossing to a thinner area near a ridgeline for instance may trigger the whole shebang, which brings me to the next point.

Trigger Points

The strength of the bond between the slab and its bed surface varies, as does the stiffness of the slab, sometimes dramatically from one place to another. Imagine sprinkling a thin layer of flour on half your countertop. Then rolling out pizza dough over the entire counter. The dough will stick to the unfloured area and slide easily on the floured area. The slab is well bonded in one place but not as well bonded in another. A similar scenario happens in the mountains when surface hoar (frost) forms on top of the snow, then wind destroys the surface hoar in localized areas, and new snow falls, creating a slab a slab that is only sporadically bonded to the underlying snow . Other areas of poor bonding include around bushes and rocks or areas of shallow snowpack.