Large rock falls in scale such as sliding, toppling and buckling have occurred frequently in the slope of columnar joint structure in welded tuff at Sounkyo Gorge in Hokkaido. In this work, the fracture mechanism of sliding was elucidated by considering theoretically the crack propagation problem along a columnar joint and the crack propagating behaviors had been monitored over two years by the acoustic emission method.
Practically, the reaction on an outside column was determined on the basis of the observational results of the internal temperature gradient.Moreover, the stress intensity factor caused by the reaction was calculated by applying the formula on a double cantilever beam. From this calculated results, it was seen that the crack propagated along a columnar joint corresponding to a decrease in the fracture toughness caused by stress corrosion or an increase in the temperature gradient.The sliding of column is generated by a decrease in the cohesive area due to the crack propargation.
The acoustic emission activity due to the crack propagation fluctuated in response of the seasonal changes of rock temperature and reached the peak in June when the temperature gradient became steepest. This seasonal fluctuation of the acoustic emission activity proved the validity of the theoretical consideration on the crack propagation along a columnar joint.