Abstract
Two experiments, with differing equipment setups, were used to test rock bolts, with differing structures and grouting qualities, using low frequency (20–200 kHz) and high frequency (700 kHz-3 MHz) guided waves to determine the effect of grouting quality on the propagating velocity of the guided waves. The results indicate that grouting quality has a significant effect on the velocity at which waves of low frequencies propagate through embedded rock bolts. As guided wave frequency increases, the sensitivity of the propagating velocity of guided waves to grouting quality decreases. Furthermore, the influence of grouting quality on propagating velocity becomes negligible once the frequency of the guided wave is greater than or equal to 1.0 MHz. An investigation was conducted to ascertain the feasibility of utilizing high frequency guided waves to determine the grouting quality of embedded rock bolts. Moreover, this study discusses a method of evaluating the grouting quality of embedded rock bolts using the peak ratios and average amplitude ratios of the high frequency guided waves. Through an analysis of the results of the abovementioned method, it was discovered that the optimal guided wave frequency is 2.65 MHz for the evaluation of 20 mm fully-embedded rock bolts because waves with this frequency have the largest average amplitude ratios.
Highlights
It can be seen from the dispersion equation of the free [19] end of the rock bolt that the propagation velocity and attenuation characteristics of guided waves in a rock bolt are related to the product of frequency of the guided wave and the diameter of anchor rod [5]. erefore, the broad range of frequencies enables the testing of rock bolts with a broad range of diameters
E difference in propagation velocity between the fullyembedded rock bolt and the free rock bolt is quite large, and the basic trend, in Figure 3, is that the velocity difference decreases as the frequency increases. us, it can be inferred that the propagation velocity of low frequency guided waves is sensitive to the grouting quality, and the influence of the grouting quality on the propagation velocity of guided waves decreases as the frequency increases. erefore, the grouting quality can be measured using low frequency guided waves by calculating the difference between the two velocities
The results indicate that high frequency guided waves have greater propagation velocities than low frequency guided waves
Summary
The guided wave frequency ranges from 20 kHz to 3 MHz, and two experimental systems, one for low frequencies (20 kHz–200 kHz) and one for high frequencies (0.7 MHz–3 MHz) were used to complete the test. e low frequency experiment uses the equipment setup shown in Figure 1. e desired waveform generated from the arbitrary waveform generator is input into the transmitter that is fixed at the free end of the rock bolt, the waveform travels through the rock bolt and is received with a sensor that is fixed at the other side of the rock bolt. e sensors used for these tests are acoustic emission sensors with high sensitivity. e characteristic of this system is that the frequency and waveform of the input wave can be altered according to the rock bolt structure, and the input and output do not affect each other. e drawback of this system is that the experimental conditions deviate from the practical conditions or field conditions in which the rock bolts are used; it can still adequately study the propagation characteristics of guided waves in a rock bolt. E wave frequency range generated by the sensor is broad, but the width of the range makes signal recognition difficult. It can be seen from the dispersion equation of the free [19] end of the rock bolt that the propagation velocity and attenuation characteristics of guided waves in a rock bolt are related to the product of frequency of the guided wave and the diameter of anchor rod [5] (the range of the rock bolt diameter used in the field is 16–24 mm). E material used as the anchoring medium for this experiment is cement. #425 cement and coarse sand were chosen for the anchoring medium according to the strength
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