Construction of the Pringle Hill Tunnel

McConnell Dowell, as part of the Northern Network Alliance (NNA), was awarded the contract for the Northern Pipeline Interconnector – Stage 2 (NPI) by LinkWater. The NPI – Stage 2 is a vital piece of infrastructure designed to help secure the water supply for South East Queensland, including the Sunshine Coast. The works include planning, design, construction and commissioning of approximately 48 km of underground, reverse-flow pipeline which will connect the existing NPI – Stage 1 at Eudlo to the Noosa Water Treatment Plant at Cooroy. The works also include the construction of a balance tank, pump stations, a water quality facility and augmentation to
existing facilities.

The pipeline combines both trenching and microtunnelling technology to traverse the 48 km. The key component of the tunnelling scope of works was the design and construction of the Pringle Hill Tunnel. This tunnel was by far the most challenging and was one of the key milestones on the project’s critical path. McConnell Dowell’s tunnelling team was responsible for the delivery of the Pringle Hill Tunnel.

Initial planning and construction

Initial planning considerations included the following:

The tunnel alignment and diameter would have to accommodate a 1.29 m outside diameter carrier pipe throughout the tunnel length.

The tunnelling system had to be capable of achieving a single drive length of 1,032 m through the vertical curve.

The tunnel had to negotiate the mountainous sandstone outcrop and a gully formation, resulting in a vertical difference of 63 m between the launch and receival ends of the tunnel.

The site investigations identified significantly varying ground conditions, ranging from residual soil to high-strength, cemented sandstone. The geological investigation provided the required information for optimum machine selection and slurry system.

Considering this, microtunnelling technology was the preferred solution for the Pringle Hill Tunnel, using a 2.57 m outside diameter slurry tunnel boring machine (TBM), which was selected due to its ability to tunnel the distance and to negotiate the considerable vertical geometry. Reinforced concrete jacking pipe was selected to line the tunnel (each 3 m long and 2.1 m internal diameter).

Pringle Hill launch shaft

The shaft design for the Pringle Hill launch site presented a challenge as it had to accommodate the installation of a 13.5 m long carrier pipe, and it also had to have the capacity to resist a thrust reaction force of 1,400 tonnes from the pipe jacking system.

The optimum shape for such a shaft was an ellipse, which due to its circumferential load distribution proved most effective in terms of ground support requirements.

The shaft was originally designed with a thrust wall to the rear of the shaft capable of resisting the 1,400 tonne thrust force. However, later it was discovered that the ground at the rear of the shaft was unsuitable for resisting this design thrust reaction force. The solution was to transfer the loading into the base of the shaft, as opposed to the conventional method of transferring the load through the thrust wall to the rear of the shaft.

Tunnel planning and design

The tunnel alignment has a length of 1,032 m in a straight line on the horizontal plane, while in the vertical plane the first 40 m was level, followed by a large vertical curve of 8,000 m radius with a maximum grade of 9.7 per cent towards the end of the drive. A series of intermediate jacking stations were implemented at 100 m intervals throughout the length of the tunnel to reduce the risk associated with rising jacking forces while tunnelling through the vertical curve.

Other considerations which had to be addressed were mechanisms to control the return of the slurry from the slurry lines during the pipe change operations, the logistics of accessing the length of the tunnel for cutter tool changes and maintenance, and ventilation requirements.

Tunnel construction

The TBM achieved an average productivity rate of 7.5 m per shift. This productivity varied depending on the geology and the distance from the launch shaft. The maximum distance achieved in any one shift was 17 m. At the peak of the production advance rate was approximately 9 m per shift. The installation of the interjack systems slowed down production, with each interjack taking on average 4.5 hours to install.

In areas where sandstone was predominant, the pressures and excavated materials were monitored to ensure that the cutter tools were performing as expected and inspections could be carried out and changes performed when necessary. Breakthrough was achieved in November 2010 with high accuracy; the drive was less than 50 mm off the target point in both the horizontal and vertical planes.

Carrier pipe installation

The carrier pipe operations also required careful planning and design to overcome the issues that existed due to the distinctive tunnel characteristics. Considerations included:

Support mechanism for the 1.29 m external diameter mild steel cement mortar-lined (MSCL) pipe within the 2.1 m internal diameter enveloper pipe.

Logistics for the fitout of the tunnel over the 1,032 m distance.

Creating two complete and intact fibre optic conduits over the length of the tunnel.

Pushing the pipe through the 8,000 m radius vertical curve and the additional forces required to push the carrier pipe up the considerable grade.

Ensuring the MSCL pipe itself was capable of negotiating the vertical curve.

A mechanism to provide a secondary means of pipe restraint to mitigate the pipe string’s tendency to move downgrade during the retraction of the push frame.

The pipes used were 13.5 m long, 1.29 m in external diameter with a 10 mm wall, 2.3 mm Sintakote coating and 19 mm cement mortar lining. The support mechanism for the carrier pipe combined the use of nylon rollers and steel head restraints. The rollers were spaced at 2 m centres and the head restraints at 6 m centres. Carrier pipe installation was conducted on a ten shift per week basis.

Grouting

The key considerations in the planning of the grouting operations were:

Eliminating the effects of the grout pressure on the exterior of the carrier pipe

Design and construction of a restraint to hold the carrier pipe when full of water during the grouting operations

Construction of an annulus bulkhead of sufficient strength to restrain the expected grout pressure

Logistics of completing the pour from the launch shaft under static head pressure

Managing bleed water from the grout pour.

The total grout pour consisted of 2,338 cubic metres. With 4.8 cubic metres loads, this equated to 487 trucks. The grout pour took place in four stages.

The first pour was 1,000 cubic metres, the second pour was 930 cubic metres, the third pour was 380 cubic metres and the fourth and final pour was a top-up pour from the reception end and was only 28 cubic metres.

Conclusion

The Pringle Hill tunnel is an integral component of the NPI project, as it connects the pipeline through a mountainous sandstone outcrop impassable to surface trenching due its challenging topography. The 2.1 m internal diameter tunnel is distinctive on this project, due to the length (1,032 m), the elevation difference between the ends (63 m), and the large volume of grout required to fill the annulus around the 1.29 m external diameter carrier pipe.

Despite the challenges encountered, the experience and capability of McConnell Dowell’s team addressed each of the challenges and adopted solutions to achieve the safe and productive delivery of the Southern Hemisphere’s longest microtunnel. The successful completion of the Pringle Hill Tunnel was an important milestone for the project, and ensured that the critical path was achieved to tie in with the pipelaying.