Mechanisms of bedrock frost shattering and associated rockfall generation

在日本高山环境中，通过(1)破碎岩样的冻胀实验，(2)冻结引起的岩石节理开度的现场测量，以及(3)环形后墙的落石活动观测，研究了岩壁的霜冻破碎和由此产生的落石活动。在融雪季节，每隔1-10天调查一次落雪石块的分布、轨迹和尺寸。连接到数据记录仪的电子传感器可以连续记录岩石节理和岩石温度两年。在实验室中，对花岗岩破碎块体的冻胀和岩石温度进行了监测，现场测量发现了两次岩石节理张开事件，一次是在仲冬，另一次是在早期融化季节。两者都表明，当充满水的关节在暂时融化后重新冻结时，就会出现开口，这使得水能够渗入关节。实验室研究表明，在岩石温度降至-2ﾟC以下之前，冻结膨胀就完成了。这些结果表明，岩石节理往往在略低于0ﾟC的温度下打开。在冻结阶段经历节理打开的季节性冻结的岩壁融化可能会导致岩石碎屑掉落到积雪覆盖的滑石斜坡上。落石碎石的数量和大小随着融化对岩壁的渗透而增加。平均而言，落石活动的最大值出现在岩壁从雪中暴露出来的大约10天后。热传导理论表明，当融化锋穿透到岩壁深度1~2m时，落石活动最大。

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.