The Northside Tunnel Project
The Northside Storage Tunnel Project involved building 22km of tunnels using four TBMs and excavating an additional 250,000m3 of tunnels with roadheaders.
The project, valued at A$460 million, began in February 1998 and was completed within 24 months under an alliance between Sydney Water, Transfield, Connell Wagner, and Montgomery Watson.
The tunnel was commissioned to collect storm overflows from the sewerage system to reduce contamination from raw sewage entering Sydney's iconic harbor.
Project Challenges
Highly corrosion environment
Due to the potentially corrosive nature of the chemicals present in the stormwater and sewage overflow, it was determined that stainless steel reinforcement could not offer the required lifespan and that a more corrosion-resistant material would be required.
GFRP (Glass Fiber Reinforced Polymer) composites are well-suited for corrosive tunnel environments due to their inherent corrosion resistance. The composite's glass fibers, are highly resistant to chemical attack and do not corrode like metals. This makes GFRP composites ideal for protecting structural components in tunnels where exposure to aggressive substances such as saltwater or chemicals are common. GFRP composites offer excellent durability, high strength-to-weight ratio, and low maintenance requirements. These properties further enhance their suitability for corrosive tunnel environments, providing long-lasting structural integrity and reducing the risk of costly corrosion-related damage.
Tunnel speed and geology considerations
From the beginning, the Eastland Tunnel project was seen as a highly time-sensitive project that required fast tunneling, especially for the storage tunnels. It was also understood that, unlike typical sewage facilities, the storage tunnels would be accessible for maintenance purposes for extended periods of time. This unique aspect provided an opportunity to develop a cost-effective roof support system, considering that a higher level of support system maintenance would be feasible.
Considering the need for TBM speed, a support system compatible with this requirement was necessary. A chemically anchored and encapsulated system, designed for one-shot installation, fulfilled this need. The chemically anchored rockbolt was the preferred choice in the ground conditions encountered during the tunneling, as siltstone and shale bands were prevalent, posing a risk of mechanical end-anchor failures. However, there was a drawback to the chemical anchorage and encapsulation method in that complete encapsulation was not guaranteed. This implied that a standard steel bolt would be especially susceptible to corrosion at points where the encapsulation provided incomplete protection, as well as at cracks that may develop in the resin when the bolts are subjected to load as the surrounding ground settles. These cracks would occur precisely at the most vulnerable locations of the bolts, where they experience the highest levels of stress.
Engineers selected GFRP bolts with 30-tonne capacity
Investigation was conducted to explore various options for support elements, including stainless steel bolts, bolts with epoxy coating or metal coating, galvanized bolts, composite stainless and high tensile steel bolts, and the use of impressed current corrosion protection.
After evaluating these options, it was concluded that a fibreglass bolt with a comparable capacity to a steel bolt would offer the necessary support while being less susceptible to corrosion. This is due to the inert nature of the load carrying elements, the glass fibres, in the fibreglass bolts. In the market, there were readily available GRP (Glass Reinforced Plastic) bolts with a capacity of 30 tonnes, commonly used in the coal mining industry where cuttable support is required during mining operations. It was determined that these GRP bolts could reasonably be expected to have a working life of 50 years.
The UDEC (Universal Distinct Element Code) modeling of the TBM (Tunnel Boring Machine) drive support demonstrated that both steel and fibreglass bolts provided comparable support. Although fibreglass bolts had inherently lower shear strength, their flexibility sometimes compensated for this weakness. Based on this finding, the decision was made to utilize 30 tonne fibreglass bolts in the TBM drives.
Mateenbar™ mechanical properties met criteria
We worked with the tunnel design engineers to develop a specialized rockbolt based on the Mateenbar™ technology which fulfilled the mechanical properties and offered extreme corrosion-resistant requirements for this difficult application.