| OCR Text |
Show At Berthoud Pass the studies are focused on an attempt to isolate the characteristics which indicate a snowslab that will fracture and become an avalanche. The principal test area is The Roll, a slope prone to the formation of slab in snow transported from the surface by wind. From observations to date it appears that the critical wind velocity is in the neighborhood of 15 mph. Below this level no appreciable transport of snow takes place. In the absence of snowfall, 20 hours of wind at 15 mph deposits 6 inches of surface transported snow on the test area; 15- 20 mph wind deposits 12 to 18 inches; 20- 30 mph wind deposits 24 to 30 inches. If snowfall is also taking place, 20 hours of 15 mph wind deposits from 12 to 18 inches of snow in the test area; 15- 20 mph wind deposits 18 to 30 inches; 20- 30 mph wind deposits 30 to 48 inches. During one observed 36- hour period, wind averaging 27 mph deposited 6 feet of snow on The Roll. These deposition figures indicate that a delayed action slab avalanche condition can develop in areas similar to the test area in a very short time. In fact, the development is so rapid that stabilization by constant use could not be effective without day and night shifts. Incidentally, the test area is an area of concentrated public use. The following profiles with their interpretations illustrate the methods used to reach certain preliminary conclusions regarding the stability of slab as revealed by the penetrometer. The profiles were all taken directly in the slidepath either just before or just after the occurrence of an avalanche. In two cases the avalanches were triggered by stabilization operations with explosives. In one case blasting produced extensive fracturing with stabilization in place. In two cases avalanches were produced by protective skiing. Interpretation Chart No. 3 ( Figure 39) Profile taken in the Berttfoud Pass test area. The sections of particular interest are marked A, B, C, and D. MA" was a slab deposited during a storm when there was wind action on snowfall and on snow transported from the surface. " B" was a weak layer of older snow. MC" was an old comparatively thin slab of high resistance. " D" was a very weak layer of cup crystals. Under explosives, the low resistance slab layer " A" fractured. The break penetrated weak layer " B". Significantly, high resistance layer " C" withstood the shock of the explosion and supported the vibration and weight of the avalanche until it had attained full momentum. " C" eventually collapsed and the fracture penetrated weak layer " D11 to the ground. This profile illustrates several important points. The snowpack contained two slab layers, each lying above a zone of minimum support. The upper slab layer had low resistance, the other high resistance. The low resistance layer was 33 cm thick, the high resistance layer only 13 cm thick. Layer nC" was subjected to a blast and a moving load which amounted to 108 pounds per square foot, dead weight. Nevertheless it was able to maintain its position on a 40 degree slope until the avalanche from layer " A" attained full speed. - 108 - |
