Estimation of Frost-Induced Stress Using Time-Lapse Ultrasonic Testing and their Effect on the Dynamics of an Alpine Rock Pillar

Rock fracturing plays a key role in the formation of mountain landscapes and natural hazards. Freezing is one of the main triggers of erosion and fracturing in the Alps. However, questions remain about the impact of freezing on the resonance frequency of rock pillars and the quantification of mechanical stress generated by ice in natural cliffs. 

To better understand the effect of frost at the centimeter and meter scale, long-term recordings were made using repeatable ultrasonic signals to measure both sound velocity and waveform changes. These observations are combined with the measurement of the pillar's fundamental resonance frequency. The investigated Tête Noire rock pillar consists of micaschist and is instably hanging above the city of Trient in the western Swiss Alps. 

The results show that during freezing, the fundamental resonance frequency increases by 50 %, the P-wave velocity increases by 17 %, and for later arrivals (coda wave) velocity increases by 4 %. After the freezing period, a irreversible drop in P-wave and coda wave velocities is visible, but not in the fundamental resonance frequency which is coming back to its initial value. This decrease in velocity is accompanied by a decorrelation of the ultrasonic waveforms. Reproducing the observed P wave velocity changes on 0.5m thick layer on a finite element COMSOL simulations of the pillar, we determine that the changes in velocity in the rock do not explain the fundamental resonance frequency changes. We therefore propose that the increase in fundamental resonance frequency results from ice filling the rear crack, and we estimate an order of magnitude of about 1.7 m for the ice height, compared with the initial crack size of 10 m. 

To estimate the freezing stress, from the measured velocity changes, we determined the acousto-elastic constant of the Tête Noire micaschist on a representative laboratory sample using uniaxial compression experiments. Those results reveal that the generated freezing induced stress are in the subcritical regime with an order of magnitude of a few tens MPa and are, hence able to damage slightly the rock, irreversibly. The drop in correlation coefficient and in the waves velocity support this conclusion. 

This work was funded by the European Research Council (ERC) under grant No. 101142154 - Crack The Rock project