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The other type of waveguide is deformation related

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The other type of waveguide is deformation related, designed to generate emission caused by guide deformation corresponding to the rock motion as shown in Fig. 17b. Pipes are used for hollow waveguides containing filler such as sand, rosin and fiberglass. In a hollow waveguide, acoustic emission waves are generated by the fracture of the filler. It is noted that acoustic emission waves detected with these waveguides are not associated with actual failure mechanisms but result from macroscopic deformation of the soil mass. Because they are devised for relatively ductile materials such as soil, these waveguides would be difficult to apply to a brittle material such as rock. Rock Materials Compared to other geotechnical materials, rock is relatively brittle and the deformation is expected to be slight. Therefore, it is difficult to apply waveguides in rock. There are three desirable conditions for waveguides in rock. 1. Acoustic emission waves can propagate up to the acoustic emission transducer, avoiding strong attenuation influenced by rock itself or by joint conditions. 2. Acoustic emission waves detected should reflect actual failure mechanisms within the slope. 3. Stability of the rock slope can be reasonably evaluated by analysis of acoustic emission signals. Waveguide for Rock Slope Monitoring To meet the above conditions, a waveguide was developed for rock slope monitoring. 22 The waveguide consists of cement based materials and reinforcement as well as acoustic emission transducers with 60 kHz peak resonance. To use the waveguide, a borehole is excavated to install the acoustic emission transducers and to obtain the mechanical properties of the rock. A variety of physical tests are performed on core samples and the correct proportions of the cement based material can be inferred from the mechanical properties of the core. Comparing the mechanical properties of the filler to those of the surrounding rock shows that acoustic emission waves are comparable to those caused by the actual rock slope. Figure 18 shows the schematic behavior of the waveguide along with acoustic emission sources expected during rock deformation. As shown in Fig. 18, the intact state can provide a good environment for acoustic emission propagation because no cracks are generated. When acoustic emission sources are generated by the deformation of surrounding rock, acoustic emission signals can travel to the transducers efficiently. As a result, information on the fracture source still remains in the detected acoustic emission signal. Consequently, in the waveguide application, characteristics of the acoustic emission waves would be comparable to those from rock deformation. As the fracture continues, the crack condition of the filler becomes the same as that of the surrounding rock. Acoustic emission signals are so strongly attenuated at that point that it becomes difficult to detect acoustic emission signals efficiently anymore. This condition is like the final 318 Acoustic Emission Testing F IGURE 17.
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  • Fall '19
  • The Land, Nondestructive testing, Acoustic Emission

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