The Upper Blue Mountains Sewerage Scheme is part of Stage 1 of the Priority Sewerage Program (PSP) and is delivered through an alliance involving Sydney Water Corporation, MWH, John Holland, United Group and Manidis Roberts.

The Upper Blue Mountains area is located approximately 110 km west of Sydney. The three townships included in the Scheme are Medlow Bath, Blackheath and Mount Victoria.

The scheme comprises a 23 km pressure sewerage and gravity transfer main from Mount Victoria to North Katoomba linking the Upper Blue Mountains sewerage system to the existing Blue Mountains Sewerage Tunnel at North Katoomba. It also involves the construction of 45 km of reticulation network, both pressure sewerage systems (PSS) and gravity, to service properties in the three townships and the construction of five underground sewage pumping stations. Decommissioning of the existing sewage pumping stations and sewage treatment plants at Mount Victoria and Blackheath was also involved.

Construction commenced in December 2006 and presently the key infrastructure is completed and ready for commissioning. Minor reticulation is due for completion by late 2008, well ahead of the committed completion date of mid 2009. The total cost of the scheme is approximately $120 million.

Utilised trenchless technologies

The Scheme presented the PSP team with numerous challenges including the approvals process, environment, safety, quality, community and external stakeholder communications, technical constraints, time and cost imperatives. To meet these limitations, Trenchless Technology was used on critical elements throughout the sewage scheme.

AJ Lucas was engaged as the sub-alliance partner to deliver the main horizontal directional drilling (HDD) for the scheme at the Cascade Dam and Mount Boyce. Pezzimenti Laser Bore constructed the transfer main micro bores, and Codmah worked on HDD for pressure and on-property reticulation systems. CLM Trenchless was involved with sewer pipe amplifications pipe bursting techniques – see article on page 50 – and Construction Infrastructure constructed the HDD rising mains.

Approvals

The use of trenchless technology allowed the concerns of stakeholders to be successfully addressed to meet various approvals. As a result the works program incurred no delays due to the approval process.

Environment

Trenchless methods were adopted to minimise restoration impacts across the scheme and to assist with the management of traffic. The proximity of the works to the Blue Mountains National Park and World Heritage Area water catchments and areas of heritage and cultural significance also lead to the selection of Trenchless Technologies. These techniques also allowed the avoidance of regionally significant communities and a unique ecological community protected by Commonwealth Legislation. During the works, noise, vibration and the management of ground water, spoil, drilling fluids was critical.

Community and external stakeholder communications

Consultation was undertaken with the surrounding residents at the drilling sites to ensure that they were not impacted by, in some instances, 24-hour drilling. The drilling was completed with no noise complaints from the surrounding residents.

Safety and quality

Various high-risk activities have been successfully undertaken on the scheme including connection of the HDD transfer main to the existing Blue Mountains Sewerage Tunnel at North Katoomba – approximately 70 m below ground level – which required rigorous planning and implementation.

Cascade Dam HDD

Initially a 2.4 km single shot bore was considered well outside acceptable risk parameters and a design was developed for two bores drilled end to end at the toe of the Upper Cascades Dam. However, with the involvement of AJ Lucas, the possibility of drilling a 2.4 km long 660 mm bore became a real alternative. Extensive geotechnical investigation, analysis, hydraulic design and stakeholder consultation was undertaken and eventually this became the preferred option and the longest HDD bore of its type in Australia.

The duty and contingency bores commenced in Medlow Bath and followed a route through the Cascade drinking water catchment, as seen in Figures 1 and 2. Due to geotechnical requirements and potential frac-outs of drilling fluids to the surface, the HDD alignment passed at a depth of approximately 40 m below the toe of the Upper Cascade Dam. The alignment continued through to intersect the Blue Mountains Sewerage Tunnel at a depth of approximately 70 m below ground, with a 1m wide target window within the tunnel. The vertical profile is illustrated in Figure 3.

An American Auger D1100 was used and maximum drilling rates of approximately 200 m/day were achieved. The project took just over five months to complete, with AJ Lucas working 24 hours a day in sometimes extreme weather conditions to complete the 2.4 km long bores.

Some of the hurdles that were successfully addressed included:

* Carrying drill cuttings 2.4 km in a 660 mm bore; * Achieving weight to bit for the HDD bore over 2 km; * Ensuring drill fluid did not escape the bore – frac-out; * Negating survey tool wire line failure over 1,800 m in length; * Installing product pipes for both large HDD bores including floating the 450mm HDPE pipe with an installation load of 12.5 tonne for 180 tonne DN450 pipe, and floating the 225 mm HDPE pipe with an installation load of 1.8 tonne to install 45 tonne of DN225 pipe; * Undertaking 24 hours a day, 7 days a week works with no community impact; * Undertaking drilling under the main Blue Mountains’ water supply without any impacts or incidents.

Underpinning experience with sound engineering, based on reliable data and the active involvement of all the alliance partners, was fundamental in achieving this successful outcome.

All parties – including external stakeholders – were involved throughout the whole process from initial planning approvals, development of the target outturn cost and implementation. Traditional contract delivery methods may not have produced the same success.

The drilling work encountered multiple iron stone bands dipping in the same direction as that of the bore and the steerer constantly had to adjust to prevent the driller over-steering through the hard bands into soft bands. This took its toll on the drill string and maintaining hard banding on the pipe was a constant critical path task.

A key expectation of all parties was to ensure that no frac-out of bentonite occurred, considering that the drilling works were being undertaken across a National Park and under the main water supply for the Blue Mountains. Frac plots were undertaken proving to be a useful tool and the annular pressure tracked well against the model. The added benefit of real time logging of the annular pressure was that the driller could see that swabbing the bore created more pressure than the drilling, and the swabbing speed could be adjusted to prevent over-pressurising the bore.

Mount Boyce HDD

Initially the Mount Boyce HDD was intended to follow the rail corridor. However, there were potential risks arising from the congested service corridor, environmental constraints and utilising conventional excavation techniques alongside the existing rail line in very unstable ground. Instead a 1.4 km 500 mm HDD bore at 2½ per cent grade under Mount Boyce at Mount Victoria was constructed.

The drilling rig was situated in one area and the power pack and mud handling system in another to reduce the area to be cleared. The bore was forward reamed to a diameter of 500 mm. Due to the proximity of gas and water mains to the exit point conventional reaming would have been riskier and additional controls would have been required.

HDD for pressure sewerage reticulation system

HDD was the preferred construction method for the reticulation system as it reduced restoration costs and work was able to continue during wet weather – unlike trenching which is impacted by such conditions. Trenching was also inappropriate for the road, rail and creek crossings that were required. In addition, HDD reduced residents’ inconvenience with respect to access to their properties, assisted with noise management and eliminated vibration impacts on heritage structures.

HDD technology was used to install approximately 70 per cent of the main reticulation lines and service lines to the properties. Production rates of 100 – 150m/day were achieved for the reticulation lines – refer to Figure 5 – and three services lines a day of approximately 30 – 40 m were achieved, including the set-up and demobilisation at each property for the property works.

Micro bores – transfer main

The transfer main also required four micro laser-guided bores. Micro bores were adopted as the main crossed the Great Western Railway Line and Great Western Highway. Micro bore techniques limited the impact on heritage structures and settlement requirements were critical under the rail line. Approvals required 12 months of investigations and negotiations with the relevant authorities and construction took only 4 months.

A laser-guided micro boring machine with a boring diameter of 560 mm was used. It achieved lengths of up to 90 m in hard rock. On completion of the bores, a DN560 PE100 SDR11 PN16 pipe was installed and fully grouted.

Pipe amplifications – reticulation

A major pipe amplification was located at the Blackheath Golf Course where approximately 300 m of existing sewer was amplified from 225 mm to 300 mm. This amplification was undertaken by pipe bursting. This method allowed the existing network to operate during the works, minimised noise to the adjoining properties and minimised restoration at the Blackheath Golf Course fairway.

HDD – sewerage pumping station rising mains and gravity lines

Conventional trenching of the sewerage pumping station rising mains and gravity lines was not possible as the sewers were located near a hanging swamp protected by Commonwealth Legislation. They also passed under a creek that flows into the Blue Mountains National Park and a bush-walking track that was required to remain open during the work. Traversed slopes greater than 30 degrees in inclination had to be negotiated to complete the work.

Four 100 to 120 m long bores with diameters of 350 mm and 470 mm were installed. Drilling was conducted in an uphill direction, capturing the drilling fluids at the pumping station excavation site. This ensured that the return of fluids was achieved, and the potential of frac-out to the environment was minimised. On completion of the back reams, the HDPE pipes were welded at the top of the drive and pulled down towards to the pumping station using the HDD rig. After installation, the pipes were tested and fully grouted.

Trenchless benefits

In considering Trenchless Technology, a cost versus benefit analysis is necessary and this was undertaken for all options described above. It is also imperative that early investigation work such as geotechnical, detailed design, community consultation and stakeholder management are undertaken so that the benefits of utilising Trenchless Technology over conventional trenching can be realised.

Over the last 16 months, various types of Trenchless Technology have been adopted to successfully overcome various technical, safety, cost, community and stakeholder requirements as well as environmental challenges.

In the future, as the industry and clients understand its potential, Trenchless Technology will be utilised more often to successfully meet various project challenges.

This article is a summary of a paper written by project managers Wayne Robinson, Tino Ferrero and Danial Sweeting. The paper, titled Sydney Water’s Priority Sewerage Program, Upper Blue Mountains Sewerage Scheme: Trenchless Technology utilised to overcome to scheme challenges, was presented at the Trenchless Australasia 2008 Conference and Exhibition in Sydney.