The Universities of Aberdeen and Bradford have been working together since 1986 to examine how anchorages, used as the main support in rock tunnels, are affected by the drilling and blasting operations used in their construction. Full-scale measurements were conducted during the construction of Penmaenbach and Pen y Clip Tunnels on the A55 North Wales coast road, where rock support was provided by prestressed rock anchorages.
A major finding arising from the research was that the response of the anchorages to the dynamic impulse motion produced by the blast loading, depended on how the anchorage had been constructed and on the nature of the surrounding rock mass. If, for example, the post tension load in the anchorage was changed, or the free length increased, a noticeable change was observed in the response 'signature' as monitored by an accelerometer located at the anchorage head. The conclusion of this was that if a known impulse load could be applied to the anchorage head immediately after construction and the response measured, this would provide a datum response signature for the intact anchorage. If the anchorage was to deteriorate in any way, by say loss of post tension, this should be noticeable on subsequent response signatures gained at a later date.
This approach is the basis of the GRANIT® system. The recorded acceleration signals captured by the GRANIT® system are analysed using various signal processing techniques, and passed onto a trained neural network. The trained network can subsequently be used to test the same anchorage or other anchorages of a similar construction, and in a rock mass of a similar type in order to diagnose the condition of the anchorage.
Additional work on a mathematical model has shown how this can be used to simulate the response of an anchorage to a GRANIT® test. A full investigation into the expected response behaviour of the anchorage can therefore be undertaken by the model, based on the characteristics of the anchorage. Potential faults in the anchorage can be examined in this way in order to improve the diagnostic ability of the GRANIT® system.