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 kN 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.

Experimental and Numerical Studies on Bond Quality of Fully Grouted Rockbolt under Confining Pressure and Pull-Out Load

“…erefore, in numerical simulations, the con ning pressures of 0, 2, 5, 10, and 15 MPa are considered, which are used to analyze the in uence of con ning pressure on the ultrasonic guided wave propagation law and determine the bond quality of rockbolts under con ning pressure. In Figure 6, the input waveform of the ultrasonic guided wave is the sine wave with 10 cycles and 22 kHz frequency obtained by the Hanning window [33].…”

Study on Bond Defect Detection in Grouted Rock Bolt Systems under Pullout Loads

“…A pullout testing machine (PTM) was designed and manufactured to conduct the rock bolt pullout test with monitoring of the stress wave propagation in the rock bolt [25,26]. e pullout load was applied to the rock bolt by a hollow Jack with a 300 kN loading capacity, and the load was measured by a load transducer.…”

“…e cut-off frequency of the guided wave in the grouted rock bolt free of loading was 22 kHz. erefore, 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 7) [26]. e comparison between the numerical and experimental results is shown in Figure 8.…”