Mechanisms of bedrock frost shattering and associated rockfall generation

点击翻译按钮获取中文摘要

Frost shattering of rockwalls and resulting rockfall activity were studied in a Japanese high mountain environment by means of (1) a laboratory experiment of freezing expansion of a fractured rock sample, (2) field measurements of rock joint opening due to freezing, and (3) observations of rockfall activity from a cirque backwall. Distribution, trail and dimensions of all rock blocks fallen on snow were investigated at 1-10 day intervals during snow-melting seasons. Electric sensors connected to data loggers permitted continuous recording of separation of rock joints and rock temperatures for two years. In the laboratory, monitoring was made of freezing expansion of a fractured granite block and rock temperatures.Field measurements revealed two events of rock joint opening, one in midwinter and the other in early melting season. Both indicated that opening occurred when a water-filled joint was refrozen following temporary melting which allowed water to percolate into the joint. The laboratory study showed that freezing expansion was completed before the rock temperature was fallen below -2ﾟC.These results suggest that rock joints tend to open at a little lower than 0ﾟC.Thawing of the seasonally frozen rockwall which underwent joint opening during the freezing phase can result in the release of rock debris falling onto snow-covered talus slopes. The number and sizes of the rockfall debris tended to increase with thaw penetration into the rockwalls. On average, the maximum rockfall activity was reached about 10 days behind the exposure of the rockwall from snow. The thermal conduction theory indicated that rockfall activity was maximized when the thawing front penetrated to 1-2 m depth of the rockwalls.