Environmental noise, acquired duringshort term monitoring, can be manuallyexcluded by reviewing the observationsmade during waveform recording. Forlong term monitoring, manual filteringbecomes difficult because of the quantityof raw data.Therefore, before transducers areinstalled on a slope, factors influencingthe evaluation of long term acousticemission monitoring should be studied.Environmental noise is the biggestconcern for slope monitoring and is oftencaused by precipitation. Each form ofprecipitation can generate continuousemission but the behavior of eachprovides a unique acoustic signature tohelp identify it.In general, raindrops larger than100 μm (0.004 in.) in diameter are calledwater dropletswhereas smaller ones arecalled cloud particlesbecause the droppingvelocity becomes smaller than that of theupward current of air. A water dropletlarger than 0.5 mm (0.02 in.) forms anellipse because of the interaction betweenair friction and surface tension. Araindrop that becomes larger than 3.5 mm(0.14 in.) in diameter divides into twodroplets — no raindrop is more than3.5 mm (0.14 in.) in diameter. A typicalraindrop has a diameter of 1 mm(0.04 in.) and a terminal velocity from4 to 6 m·s–1(9 to 13 mi·h–1); a largerraindrop has a diameter of 3 mm(0.12 in.) and a terminal velocity of9 m·s–1(20 mi·h–1).Experimental ProcedureFigure 14 shows an experimentalconfiguration used for the raindrop tests.Three prism specimens consisting ofordinary portland cement and aggregatewere prepared, 150 mm (6 in.) deep×150 mm (6 in.) wide ×500 mm (20 in.)tall. The ratio of sand to total aggregatewas 60 percent and the maximum sizeGmaxof the aggregate was 15 mm (0.6 in.).The specimen was cured for one day intap water and subsequently twospecimens were cut into several pieceswith lengths of 100, 200, 300 and400 mm (4, 8, 12 and 16 in.). In this way,five lengths of specimen at 10, 20, 30, 40and 50 mm (0.4, 0.8, 1.2 and 1.6 in.) wereprepared for the test.The specimen was subjected to theimpact of large water droplets from asyringe at a height of 2 m (80 in.) abovethe top surface of the test object. Thediameter of one water droplet wasstandardized to 2.75 mm (0.11 in.),315Acoustic Emission Testing of InfrastructurePART3.Evaluation of Slope Stability by AcousticEmission Testing
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measured by averaging the weight of tenwater droplets.The test was performed after 1 day,3 days and 15 days. Table 1 shows Young’smoduli and the compression strengthcorresponding to those ages.On the bottom surface of thespecimen, two types of acoustic emissiontransducers (15 and 60 kHz resonance)were placed with a constant contactpressure applied by springs. Acousticemission waves generated by waterdroplets impacting on the surface traveledto the bottom of specimen and weredetected by both types of transducers. Theacoustic emission signals detected weresubsequently amplified at 40 dB bypreamplifiers and were processed andacquired by the acoustic emissioninstrument.
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