An Ultrasonic Rock Bolt Sensing Technology (I): Methodology and Laboratory Studies

Rock bolts play a critical role in ground support in mining, tunneling and construction. Being able to obtain timely information on rock bolt load condition and deformation could help immensely in safeguarding workers’ safety, optimization of ground support system, and ensuring stability and longevity of underground structures. The objective of this work was to develop a practical ultrasonic rock bolt sensing technology for monitoring axial load and deformation of a full-bodied rock bolt in both elastic and plastic deformation regimes. To this end, empirical mathematical models involving simultaneous use of times of flight of longitudinal and shear ultrasonic waves propagating along the axial direction of the rock bolt were developed, sensors were designed and fabricated, and laboratory studies were conducted. The results showed that the technology was able to measure load change, provide early detection of yield, and measure both plastic and total elongations of the rock bolt undergoing a pull test. Additionally, sectional load and elongation information could be obtained by applying the technology to rock bolts divided into sections by drilled holes along their shanks, therefore, providing the capability to assess the condition of grouted bolts at different depths within a rock mass. This work has laid the groundwork for the deployment of the technology in the field. Highlights Empirical models based on simultaneous use of times of flight of longitudinal and shear ultrasonic waves were proposed for condition monitoring of full-bodied rock bolts Methods for instrumenting full-bodied rock bolts were developed Calibration methods for determining values of parameters used in the empirical models were established Pull testing showed that the technology could measure load changes within and beyond the elastic limit, detect yield, and measure plastic and total elongations of the rock bolt Sectional load and elongation information could be obtained by applying the technology to rock bolts divided into sections by drilled holes along their shanks