ABSTRACT:

We developed a new method the air tracer test to investigate the persistence of open cracks and loosening of rock mass slope. In this method, we use smoke, hot air, and/or specific gas as tracers injecting them into open cracks directly or using a specific double packer settled in a borehole. Observation and confirmation of the outflow of the tracer is performed by naked eye for the rock surface, and by a borehole TV, gas sensors, or thermosensors settled in another borehole for rock interior. We applied the method to several rock slopes in which various type of open cracks exist, and demonstrated that the air tracer could move through open cracks from under 1mm to over 1 m in width. The smoke tracer was particularly detectable even after having moved about 100m. The method can clarify not only persistent open cracks and loosened zones but also the loosening mechanism such as toppling, sliding, etc. The air tracer test should become a useful method for selecting appropriate countermeasures for unstable rock slopes.

INTRODUCTION

Most of Japan is mountainous, many landslides have occurred such as rock slide, debris slide, rock fall, and toppling. Big rockslides have recently become a particularly frequent problem in Japan. To prevent such rock slide disasters, it is important to investigate and clarify the distribution and persistence of open cracks in rock mass slope, and loosened zones. However, investigation methods suitable for such needs have not developed until now. For example, though ground penetrating radar is the most high resolution method in geophysical prospectings, it cannot usually detect mm order cracks in width. On the other hand, though borehole TV can detect such a small crack, but it cannot confirm the persistence of two cracks situated at different two boreholes.

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