A new two-sensor non-destructive testing method of grouted rock bolts

Abstract

Poor anchorage quality of rock bolts is easily caused by inappropriate control of the grouting process, leakage of grouting due to broken rock, and insufficient length of bolts. Wave propagation-based methods were widely used to non-destructive test of rock bolts. Accurate and automatic identification of arrival time of effective reflected waves is significant in the non-destructive test of rock bolts. However, the direct waves and multiple reflections from the start of the bolt body mask weak reflection from the grouted defect and bolt end using only one sensor recording. In this paper, we developed a new stress wave reflection non-destructive test method of rock bolts using a two-sensor acquisition. The sensors are fixed at the free steel bar with clips and the signals are recorded by a two-channel prototype. We define the integrated instantaneous phase difference (IIPD) as an index that characterizes any estimated instantaneous phase difference at the two sensors. The instantaneous phase difference of effective reflected waves between two sensors can be calculated by the two sensors spacing, wave velocity, and dominant frequency of test signals. Higher IIPD values can signify the arrival of a reflection. Thus, IIPD peaks can identify the geometric parameters of rock bolts, the size of the single or multiple grouted defects. Both numerical simulations and experimental model tests were used to demonstrate the effectiveness of the two sensor IIPD method. The two-sensor non-destructive bolt test method and prototype can be widely applied in slope, dam and underground engineering.