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Acoustic emission testing of infrastructure f igure

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Acoustic Emission Testing of Infrastructure F IGURE 24. Schematic section view of the monitoring slope and the arrangements of a variety of transducers. 45 (148) 40 (131) 35 (115) 30 (98) 25 (82) 20 (66) 15 49 Distance, m (ft) Borehole 1 L = 15 m (590 in.) Borehole 2 L = 10 m (393 in.) Borehole 4 L = 10.25 m (403.5 in.) Borehole 3 L = 10 m (393 in.) 7.08 m (278.7 in.) 2.32 m (91.4 in.) Seismometer 3D-1 BS1-2 BS1-1 3D-2 3D-3 3D-4 3D-5 BS3-1 CH1 CH2 CH3 CH4 CH5 8.5 m (335 in.) 6.2 m (244 in.) 3.6 m (142 in.) BS2-1 BS2-2 BS2-3 Legend BS = borehole strain gage CH = acoustic emission transducer L = distance = three-dimensional crack gage (3D) = borehole strain meter (BS) = transducer (CH) = joint observed clearly 8.65 m (340.6 in.)
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activity, the uppermost transducer of channel 5 is placed at a depth of 2.5 m (100 in.). Moreover, to reduce acoustic emission signals from the surface, the portion down to 1.0 m (40 in.) in the borehole was filled with fine sand. Acoustic emission signals generated from the slope were amplified by 40 dB at transducer integrated preamplifiers, and the signals over a 40 dB threshold were acquired in an acoustic emission system. To collect data and to control the system remotely, a remote acoustic emission system with modem communication was applied. A seismometer, crack gages in three dimensions and borehole strain gages were also set on the slope as shown in Fig. 24. There is no relation between precipitation and the acoustic emission activity, even in cases of precipitation over 100 mm (4 in.). This lack of relationship implies that acoustic emission data did not include the acoustic emission activity induced by rain. The technique for eliminating rain signals was successful. Acoustic Emission Analysis Because acoustic emission transducers are linearly placed along the borehole in the waveguide, one-dimensional sources can be identified as shown in Fig. 25. The area of circles reflects the magnitude of the average of the ringdown counts. At the beginning of December, many acoustic emission sources ranging from 4 to 5 m (160 to 200 in.) were intensively observed. Without the period between November and December, the grade in all channels varied over 10. From the criteria of fracture stages in Table 3, the grade over 10 corresponds to the bending type of fracture; this type of fracture could not be of the severe failure level. In the period between November and December in channel 3, however, two decreases down to about 5 are observed. Because the grade around 5 implies the generation of shear fracture, local shear fractures may occur transiently around the location of channel 3. Concentration of acoustic emission sources is shown in Fig. 25 around the place of channel 3. The rock around the channel 3 should have experienced the shear fracture temporally. Borehole Strain and Rock Condition Figure 26 shows borehole strains measured in borehole 2 during the period.
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  • Fall '19
  • The Land, Nondestructive testing, Acoustic Emission

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