NUMERICAL AND EXPERIMENTAL STUDY OF GROUTED ROCK BOLTS AND THEIR DEFECTS USING ULTRASONIC GUIDED WAVES

Abstract

A rock bolt installed in field has only one short exposed end on the rock surface. This condition has posed challenges in field instrumentation and made it difficult to use the ultrasonic guided wave method for rock bolt monitoring. In rock bolt laboratory tests using ultrasonic guided waves, the input and receiving transducers are typically installed at the two exposed ends of a bolt. This is suitable to laboratory conditions but not practical in the field because one of the ends of a rock bolt is embedded in the rock mass. A method needs to be found to install the receiving transducer at a suitable location in the field for receiving valid wave data.

In this thesis, a practical approach is proposed for conducting field tests with the installation of the receiving transducer on the grout surface near the exposed end of the bolt. The effects of the installation location of the receiving transducer are studied with numerical modeling. Experiments are conducted to verify the numerical modeling results. The results indicate that the data obtained from the receiving transducer installed on the grout surface at a proper location are representative and can be analyzed through the established correlations to determine the required parameters.

Previous researches have mostly focused on the feasibility of the ultrasonic guided wave method for rock bolt tests and on the behavior of ultrasonic guided waves of fully grouted rock bolts in laboratory conditions. No further study has been performed to identify the grout defects in grouted rock bolts. Adequate understanding of the behaviour of ultrasonic guided waves in rock bolts with defects is therefore prerequisite for this method to be applied in practice. This thesis investigates the effects of some typical defects (e.g., an insufficient rebar length, a missing grout at the ground end, and a void in grout) in grouted rock bolts using the developed field measurement method and numerical modeling. The results are verified by laboratory tests using the equipment set-up established in this research. The results indicate that it is practically possible to identify those grout defects using ultrasonic guided waves.