The NSTP was recognised from the outset as a very tightly programmed project requiring high speed tunnelling of the storage tunnels. It was also recognised that, unlike normal sewer tunnels, these tunnels would be accessible for maintenance for substantial periods of time due to the system only operating in significant rain events.
This meant that a support system compatible with fast TBM drivage and one that could be designed knowing regular maintenance was available, would be needed. A one-shot chemically anchored and encapsulated system met this requirement.
The chemically anchored rockbolt was also the preferred system in the ground through which these tunnels were driven due to the prevalence of siltstone and shale bands that often cause conventional mechanical end-anchors to fail. The drawback of chemical anchorage and encapsulation is that full encapsulation is not always certain and therefore a normal steel bolt would be particularly prone to corrosion.
The alliance team looked at a variety of solutions to the corrosion issue from stainless steel bolts, epoxy coating, metal coating, galvanising, composite stainless and high tensile steel bolts and the use of impressed current corrosion protection. It came rapidly to the conclusion that a fibreglass bolt with similar capacity to a steel bolt would provide the required support and not be so prone to corrosion since the load carrying elements, the glass fibres, were effectively inert. Fortunately, 30 tonne capacity GRP bolts were readily available in the market. These are either moulded or pultruded and are much used in the coal mining industry where support has often needed to be cut through during the mining process.
Finite element analysis demonstrated that there was little to distinguish between steel and GRP bolts even where shear strength was concerned. The system was used throughout the 20km of the tunnels.
One final issue remained to be resolved in terms of the GRP bolt. The bolter systems on the TBMs and the design requirements necessitated some of the bolts to be installed at quite steep angles to the radial line through the collar of the bolt hole. This tilt had to be accommodated in the faceplate so that the bolt was not bent at the bolt hole. In conjunction with one of the GRP bolt manufacturers, Cutabolt Pty Ltd, a high capacity domed plastic faceplate was specially developed for this project to overcome this problem andwas used in substantial lengths of the tunnels.