Mechanisms of bedrock frost shattering and associated rockfall generation

在日本高山环境中，通过(1)裂隙岩样冻结膨胀的室内实验，(2)冻结引起的岩石节理开口的现场测量，以及(3)马戏团后壁岩崩活动的观测，研究了岩壁的霜冻破碎和由此产生的岩崩活动。在融雪季节，每隔1 ~ 10天对所有落在雪上的岩石块的分布、轨迹和尺寸进行调查。连接到数据记录器的电子传感器允许连续记录岩石节理的分离和岩石温度长达两年。在实验室中，监测了断裂花岗岩块的冻结膨胀和岩石温度。现场测量结果显示，岩石节理开启两次，一次发生在隆冬时节，另一次发生在融冰期早期。两者都表明，当一个充满水的接缝在短暂融化后重新冻结时，水就会渗透到接缝中。实验结果表明，在岩石温度低于-2 C之前，冻结膨胀就完成了。这些结果表明，岩石节理在略低于0ºC时倾向于打开。季节性冻结的岩壁在冻结阶段进行联合开放，其解冻会导致岩屑的释放，落在积雪覆盖的距骨坡上。随着融冰对岩壁的渗透，岩屑的数量和大小有增加的趋势。平均而言，最大落石活动是在岩壁被雪暴露后10天左右达到的。热传导理论表明，当解冻锋渗透至岩壁1 ~ 2 m深度时，岩崩活动最大。

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.