Cleft Kinematics

The mechanisms that control climate-dependent rock fall activity from permafrost mountain slopes are currently poorly understood. The results from the rock slope monitoring at Matterhorn Hoernligrat are the following: A negative dependency of cleft expansion on temperature prevails at all observed clefts for most of the year. It is interrupted at most clefts by a period with enhanced opening and shearing in summer, which lasts from sustained melt to initial refreeze within the respective cleft.

rock_move_ov
Evolution of the rock and cleft temperatures. The melting period (Tcleft ≥ 0 C) lasts from beginning of June to mid October (yellow bars). The seasonal patterns of the cleft dilatations are anti-cyclic to the temperatures (b). In case of the crackmeter mh01 the diurnal temperature variations dominates the effect on the cleft opening. These anti-cyclic dilatations are interrupted when positive temperatures in the respective cleft are reached (number 1 and 3 in figure below). For mh02 the progressive opening is stopped with initial freezing (number 2). During an intense temperature drop in autumn an increased opening occurs (number 4). The shearing of the clefts (c) is more active if the measured cleft temperatures are above zero degree (number 5 to 6).

 

Two distinct process regimes may cause these temporal patterns. Based on current theories and laboratory evidences on rock slope stability we hypothesize that:

  • The negative temperature dependency is caused by thermo-mechanical forcing and is reinforced by cryogenic processes during the freezing period.
  • The enhanced movement originates from hydro-thermally induced strength reduction in rock fractures containing perennial ice.

The combination of these hypotheses points to an understanding of their interaction in the field: The thermo-mechanical and cryogenic processes slowly modify geometric settings and cleft characteristics of permafrost rock slopes on the long-term, while thaw-related processes immediately act on  stability. Such short-term stability minima would activate rock masses subject to the slow changes. According to our measurements, response to the surface thermal conditions of the creeping motion related to this weakening is within hours to weeks, depending on the type of heat transport.

mh02_meltinit
The snow patterns within the installation site (mh02 and mh20) show that thawing activity correlates with the initiation of the enhanced summer opening. That supports the hypothesis on the hydro-thermally induced strength reduction.