Field observations and laboratory experiments concerning the creep process of rock blocks in an arid environment

Data on soil erosion by runoff at the Sde Boqer experimental site, northern Negev, Israel, indicate that runoff erosion is unlikely to transport coarse material over the treads of bedrock steps. Detailed field observations led to the idea that, despite the arid conditions, such material could be transported by a mass movement process, namely creep. To test this hypothesis a laboratory experiment, inspired by the situation in the field, was designed. A limestone block was laid upon a gently inclined earth pedestal derived from the Sde Boqer area. Both the pedestal and the block were submitted to the following tests which are assumed to contribute to the creep process in the field: wet-dry cycles; wet-frost-thaw cycles; variable conditions of air humidity; artificial structural disturbance of the earth pedestal. Data obtained indicate that a continuous "dry block-creep" is active for long periods when the loam pedestal is dry (1 % - 3 %) and submitted to a low relative air humidity and high air temperature. The cumulative movement over a period of 445 days amounted to 1200 microns. The dry movement was accelerated when air relative humidity was increased up to 100%. A faster downslope movement - 0.29 mm/cycle - was recorded on wet-frost-thaw tests; and a more acclerated one was obtained when structural disturbance were applied by perforating the earth pedestal, a process similar to that performed by burrowing animals. Surprisingly, once the earth pedestal stabilized, no net gain in downslope movement was recorded on wetting-drying tests. Results obtained in the laboratory, as well as their possible application to field problems are discussed.