Experimental and numerical evaluation on debonding of fully grouted rockbolt under pull-out loading

Abstract

The axial loading in rockbolts changes due to stress redistribution and rheology in the country rock mass. Such a change may lead to debonding at rockbolt to grout interface or rupture of the rockbolt. In this study, based on laboratory experiments, ultrasonic guided wave propagation in fully grouted rockbolt under different pull-out loads was investigated in order to examine the resultant debonding of rockbolt. The signals obtained from the ultrasonic monitoring during the pull-out test were processed using wavelet multi-scale analysis and frequency spectrum analysis, the signal amplitude and the amplitude ratio (Q) of low frequency to high frequency were defined to quantify the debonding of rockbolt. In addition to the laboratory test, numerical simulation on the effect of the embedment lengths on ultrasonic guided wave propagation in rockbolt was conducted by using a damage-based model, and the debonding between rockbolt and cement mortar was numerically examined. It was confirmed that the ultrasonic guided wave propagation in rockbolt was very sensitive to the debonding because of pull-out load, therefore, the critical bond length could be calculated based on the propagation of guided wave in the grouted rockbolt. In time domain, the signal amplitude in rockbolt increased with pull-out load from 0 to 100 kN until the completely debonding, thus quantifying the debonding under the different pull-out loads. In the frequency domain, as the Q value increased, the debonding length of rockbolt decreased exponentially. The numerical results confirmed that the guided wave propagation in the fully grouted rockbolt was effective in detecting and quantifying the debonding of rockbolt under pull-out load.

Propagation Characteristics of Ultrasonic Guided Waves in Grouted Rockbolt Systems with Bond Defects under Different Confining Conditions

“…The ultrasonic guided wave was excited at the A-end, and the wave was received at the F-end. The comparison between the numerical and experimental results is shown in Figure 4 [15]. The signal received at the other end was in good agreement with the experimental result, which indicated that the numerical model could simulate the guided wave propagation in grouted rockbolt systems.…”

“…Based on test results in the reference [15], the rockbolt experiences elastic ascent, yielding, hardening and complete slip without breaking. Therefore, the rockbolt is set as an elastoplastic material in ABAQUS.…”

“…The ultrasonic guided wave is used for detection in the numerical simulation, and the cut-off frequency of the guided wave in the grouted rockbolt free of loading is 22 kHz. Therefore, the input waveform of the ultrasonic guided wave was the sine wave with 10 cycles and 22 kHz frequency obtained by the Hanning window (Figure 3) [15]. The ultrasonic guided wave was excited at the A-end, and the wave was received at the F-end.…”

“…Liu et al [14] analysed the influence of tensile stress on the propagation characteristics of ultrasonic guided waves in steel cables based on a fast Fourier transform, and their results showed that the energy transfer of ultrasonic guided waves increased with increasing tensile force. Yu et al [15] used a rockbolt pull-out and stress wave detection test device to pull out the rockbolt and detect its bond quality under different loads and then used the wavelet transform to carry out wavelet multiscale and frequency spectrum analysis on the detection signal, determined the debonding length of the rockbolt under the pull-out load and evaluated the bond quality of the rockbolt. Ivanovic and Neilson [16] used a magnetostrictive nondestructive testing method to evaluate the effective length of a rockbolt under a pull-out load without considering the defects in the grouted rockbolt system.…”

Investigation on the dynamic characteristics of anchor rods considering size effect and pull-out loads based on IIR-FDM

“…To mitigate the influence of environmental and equipment noise on the detection results, several signal processing methods have been proposed for anchor rods NDT. These methods include Fourier transform (FFT) [3], wavelet filtering [4,5], variational mode decomposition-Hilbert Huang transform (VMD-HHT) [6], empirical mode decompositionspectral kurtosis denoising [7], HHT [8], AILPSA [9], grey cloud model [10], etc. Among them, FFT and wavelet filtering are earlier and widely applied signal processing methods.…”