It is important to identify unstable rock blocks and take countermeasures to
prevent sudden rock fall disasters. However, identifying such blocks
visually is extremely difficult, so an identification method using peculiar
features of unstable blocks must be developed. The method reported here uses
a vibrometer, which is inexpensive and easy to operate.
In order to assess the feasibility of the method, a field experiment was
carried out on rock cliffs in three regions of Japan where unstable blocks
are likely to exist. Vibrometers were set up on the cliffs to capture two
types of vibration waves in three dimensions, i.e., micro-tremor and
reactive vibration. The former type naturally exists all the time, while the
latter is generated only by applying stimulation waves. At least one of the
vibrometers was installed on stable baserock to compare the results with the
wave patterns of unstable rock blocks.
In addition to conventional items (amplitude, frequency spectrum, vibration
particle trace), trace accumulation length, that is the accumulation of the
trace length of a vibrating particle for ten seconds, was introduced to
analyze the patterns for both types of wave.
As a result, unstable rock blocks were found to generate higher amplitudes
of vibration waves than stable rock blocks, and different patterns of
frequency spectrum, direction of vibration particle trace, and trace
accumulation length. Hence, vibrators were shown to be useful for
identifying unstable rock blocks. In particular, by using trace accumulation
length as an indicator, the stability of a block can be evaluated without
generating stimulative waves, providing a direction for developing a
cost-effective simple method for identifying unstable blocks in future. |