Permanent rock bolts used to support rock tunnels are required to perform for the design life of the structure as a minimum. Quality of installation is critical to the performance over the life of the rock bolt. This paper will present the findings from an in investigation into the integrity of rock bolts installed in an underground hydroelectric power station cavern and access tunnel. The 25 mm diameter rock bolts were installed 50 years ago, tensioned and post cement grouted. Due to a major machine upgrade the whole facility was subject to an asset survey. The owners are responsible for managing the risks associated with the failure of these rock bolts and ensuring the structural integrity of the tunnels and caverns is maintained to continue operations, protect machinery, assure production and more importantly maintain a safe working environment for all employees and visitors. To help manage risk a proactive approach is desired. To this end a test schedule using a nondestructive testing (NDT) system was carried out by Independent Integrity Testing Pty Ltd (i2t). The system used in this application was RokTel~(TM). The system operates by applying a small axial impact to the rock bolt end and measuring the resultant acceleration response. The rock bolt characteristics affect the acceleration response. The data output from the system is analysed principally using Fourier Analysis (FFT) and sinusoidal waves. The energy generated in the bar, the rate of damping and changes in frequency are all assessed. Testing was carried out on 70 rock bolts within the tunnel and cavern. A selection were then over cored, photographed and documented. The data gathered from the testing is then correlated with physical evidence to establish a calibration for the remainder of the rock bolts and assess if this is a viable method for identifying defective elements, and what degree of accuracy can be achieved. The findings will assist with the future assessment of similar hard rock underground facilities.
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