ROCK NOISE IN LANDSLIDES AND SLOPE FAILURES

Abstract

THE POSSIBLE APPLICATION OF ROCK NOISE MONITORING TO LANDSLIDE AND SLOPE STABILITY PROBLEMS IS DISCUSSED. WHEN A ROCK SPECIMEN IS PLACED UNDER LOAD IN THE LABORATORY, ACOUSTIC DISTURBANCES, KNOWN AS SUBAUDIBLE ROCK NOISE /SARN/ ARE EMITTED. WITH A SENSITIVE PICKUP ON THE SPECIMEN AND A HIGH GRAIN AMPLIFIER, THESE NOISES CAN BE DETECTED. AFTER CONSTRUCTING AND FIELD TESTING A SUITABLE ROCK NOISE DETECTOR, A NUMBER OF RECENTLY ACTIVE LANDSLIDES AND HIGHWAY CUT SLOPE FAILURES IN NORTHERN CALIFORNIA WERE MONITORED TO DETERMINE WHETHER OR NOT SLIDES EMIT DETECTABLE ROCK NOISES. THERE WAS A DEFINITE CORRELATION BETWEEN THE ESTIMATED STATE OF ACTIVITY AND THE ROCK NOISE RATE IN THESE SLIDE AREAS. DURING THE TWO YEARS OF THIS INVESTIGATION, ABOUT 50 HR OF ROCK NOISE RECORDINGS WERE MADE AND SOME 15,000 NOISE EVENTS WERE EXAMINED IN THE LABORATORY. ABOUT 40 SEPARATE SLIDES OR CUTS WERE MONITORED. THE FOLLOWING CONCLUSIONS CONCERNING THE APPLICABILITY OF THE ROCK NOISE METHOD TO LANDSLIDES AND CUT SLOPES WERE REACHED' /1/ THERE IS NO QUESTION THAT ACTIVELY CREEPING LANDSLIDES GENERATE DETECTABLE AUDIO FREQUENCY DISTURBANCES, /2/ BECAUSE OF THE NECESSITY TO DISTINGUISH ROCK NOISE FROM EXTRANEOUS SOURCES OF NOISE A MULTICHANNEL SURVEY SHOULD BE PERFORMED, /3/ MOST ROCK NOISES RECEIVED BY A PROBE IN A LANDSLIDE ORIGINATE WITHIN A DISTANCE OF 100 FT, /4/ THE RATE OF ATTENUATION OF HIGH-FREQUENCY SIGNALS AND THE VARIATION OF WAVE VELOCITIES OVER SHORT DISTANCES IN SOFT LANDSLIDE MATERIALS MAKES IT DIFFICULT TO DETERMINE THE FOCUS OF A ROCK NOISE EVENT. /5/ NO ROCK NOISES HAVE BEEN DETECTED IN ROCK CLIFFS AND STEEP ROCK CUTS SUBJECT TO FREQUENT ROCK FALLS, AND /6/ ROCK NOISE MONITORING DOES FOREWARN OF ACCELERATED MOVEMENTS IN LANDSLIDES AND ROCKSLIDES.