New evidence of thermal response of a monitored rock block in the west cliff of Masada Mountain is presented, accompanied by a conceptual model of weather-induced “wedging” failure mechanism. In order to measure, in high precision, rock block response to daily and seasonal environmental oscillations, a monitoring system was installed on a finite keyblock, separated from the rock mass by several intersecting rock joints. The monitoring system, comprised of four joint meters, temperature, and relative humidity transducers, has been collecting output data since July, 2009 until present.
An inverse relationship between joint opening and temperature level is indicated, suggesting that the rock block material contracts and expands as a response to environmental cooling and warming, respectively. While the rock mass into which one end of each joint meter is fixed remains static, the contraction and expansion of the rock block materials into which the other end of the joint meter is attached manifests in joint opening or closure, as indicated by joint meter output. A similar behavior was monitored in the East slope of the mountain along the Snake Path cliff. Comparison between displacement results suggests that the amplitude of displacement fluctuation is proportional to block width, supporting our working assumption that the monitored displacements are thermo-elastic in origin.
In order to account for temperature changes inside the entire rock block thermal conductivity of dolomite samples was measured in the lab, and a temperature profile was calculated. Since seasonal temperature fluctuation in the field decay at a distance of several meters into the rock mass, it is suggested that the thermo-elastic response is limited by individual block size. Ongoing seasonal displacement fluctuations may result in creep along discontinuities, a process that may cause some cohesion loss and shear strength degradation across the mobilized discontinuities. An initial development of a weather-induced wedging failure mechanism is presented that helps explain field monitoring results of block creep.
It should be remembered that while the suggested thermo-elastic based creeping mechanism leads to minute block displacements, strong and sudden rainstorm episodes may control the behavior in the short term and in some cases may even lead to total block collapse, as documented by a series of high quality photographs taken before and immediately after a strong rainstorm in the study area.