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Show Snow stabilization by means of explosives was in the experimental stage in the United States at the time " The Alta Avalanche Studies" was written. The techniques have since been standardized and are routine in areas of heavy use administered by the Forest Service. Fundamentally, stabilization by means of explosives and stabilization by use are the same thing. The objective is to expose the hazard and then reduce it. In the case of test and protective skiing, this is done by applying to the snow the avalanche triggers of weight and shearing. Where explosives are employed the triggers are vibration and shearing. One situation has already been described where protective skiing might yield a deceptive answer. There are also slopes too big and steep to be controlled by protective skiing with an acceptable margin of safety. Finally there are slopes inaccessible for stabilization by use but which overhang principal ski runs or important installations such as highways and buildings. In these cases explosives are the solution to safety problems which would otherwise lead to excessive restrictions. Equally important, a properly executed blasting operation yields a definite answer. That slope is safe until the next radical change in snow conditions. There are several ways of utilizing explosives for the control of avalanche hazard. The most common is a charge, put in position by hand and detonated on the surface. We have found this to be the most effective application for slope avalanche conditions of either loose or slab- type snow. The absorptive power of snow is very great and it has been our experience that much of the shock of detonation is wasted when the charges are buried. These hand- placed charges are set off as close to the natural avalanche release points as possible ( Figure 20). The charge can be lowered into place from above or tossed out so that the operator can work from a protected position. If the slope is very wide, two or more charges can be detonated at once. A rule of thumb is that a standard charge of 16 to 20 pounds of military demolition or similar explosive controls 100 feet of slope width. However, stabilization with explosives is a tailor- made operation for each slope. Many slidepaths have more than one release point, all of which must be tested. The only hard and fast rule in explosives operations is to use plenty. The location of the charges depends upon the characteristics of the individual slidepaths; these in turn are revealed by observation and experiment. Even in unfamiliar territory, there are clues to guide the avalanche buster. Cornice and drift formations indicate the prevailing wind direction, the lee slopes and the probable location of slabs. On a convex slope, the sharpest point on the curve is the logical place for a charge. The natural release point of a concave slope is less obvious. An explosion anywhere on the steeper part will break the top and side anchorages. Since the anchorage at the toe of a concave slidepath is generally the strongest, a charge near the transition is often effective. - 64 - |