Šimková I., Greif V., Vlčko J. & Ekkertová P., 2014: Physical modelling of wedging mechanism of rock blocks during cyclic thermal load. Acta Geologica Slovaca, 6, 1, 51–59. (in Slovak with English summary)

Fyzikálne modelovanie mechanizmu vkliňovania skalných blokov počas cyklickej teplotnej záťaže

Physical modelling of wedging mechanism of rock blocks during cyclic thermal load

Ivana Šimková, Vladimír Greif, Ján Vlčko & Patricia Ekkertová

Katedra inžinierskej geológie PRIF UK, Mlynská dolina, 842 15 Bratislava; simkova@fns.uniba.sk, greif@fns.uniba.sk, vlčko@fns.uniba.sk, ekkertova@fns.uniba.sk

Abstract

Cyclic diurnal and annual temperature variations acting upon rocks belong to the permanent factors rarely considered as triggering factors of slope movements. The importance of temperature changes is viewed mainly as the precursor of failures where triggers are rainfall, seismic activity of other natural factors. This paper aims to determine the limit conditions where plastic deformation develops in situation where the block or several blocks fallen into open discontinuity create a “wedge-like” behaviour causing non-elastic displacement of block resting on an inclined plane. A physical model was prepared to study this phenomenon. The study was performed in thermal dilatometer where the displacements were measured using LVDT sensors for blocks with different block/wedge ratios, while the temperature cycled in a controlled manner. Together 9 physical models of sandstone blocks were tested within range of ∆T = 35 °C, which corresponds to annual temperature change and permanent displacements of the block was measured in order to confirm the existence of expected type of failure mechanism. Further series of cyclic tests at 9 models were performed in order to determine the threshold temperature change at which the plastic deformation for different block/wedge ratios occurs. The results confirmed the existence of plastic deformation resulting from cyclic wedging mechanism for block/wedge ratio 0.4 and total model size of 50 mm reached permanent displacement of 2.23 × 10^-3 mm for a block resting on an inclined plane of 7°. For the same conditions the temperature change which causes permanent displacement of the block by wedging mechanism was found out to be as low as 6 °C. The results of physical model are in agreement with proposed analytical solution by Pasten (2013) and measurements of Bakun-Mazor et al., (2013) at Masada site in Israel. A thorough testing of the rock material in saturated conditions together with physical models was carried out, which confirmed the existence of the plastic deformations with amplitudes ten times larger compared to the dry conditions.

Key words: thermal wedging, rock stability, failure mechanism, cyclic temperature change

Manuscript received: 2014-01-22

Revised version accepted: 2014-04-04