Parsons Brinckerhoff (PB) was engaged by the authorities to provide all technical inputs including hydraulics, constructability, cost and environmental assessments related to the local sewer networks. The local authorities had ongoing input into all stages of the projects through regular meetings of integrated working groups comprising PB specialists and key planning, operations and project delivery personnel for the local authority. This collaborative approach was the key to the robustness of the option identification and selection process.

The brief

PB completed a series of sewer feasibility studies to cater to increased population growth projections within several catchments in South East Queensland. These projects include:

• Bulimba Creek Trunk Sewer Upgrade Stage-1 Padstow Road to Coora Street, South East Queensland, Brisbane City Council Water Distribution.
• Woolloongabba Sewer Augmentation Stages 2 and 3, Woolloongabba, South East Queensland, Brisbane City Council Water Distribution.
• Woogaroo Creek Trunk Sewer Upgrade, Ipswich (Goodna), South East Queensland, Ipswich Water
.

A set of local and high level augmentation options were developed and optimised as part of the studies. Preferred options were selected on the basis of multi-criteria analysis to ensure that the projects delivered the best overall outcomes in terms of costs, environmental, social and community impacts, and improved flexibility in operability. PB’s comprehensive understanding of tunnelling and recent Trenchless Technology, coupled with effective hydraulic modelling techniques, allowed a number of innovative strategic solutions to be identified, offering significant cost savings and improved environmental and community outcomes compared to traditional augmentation strategies. The selection process

The selection process involved the detailed evaluation of options, by analysis of the qualitative and financial multi-criteria aspects. The results were only confirmed following a sensitivity analysis and careful consideration of the key differentiators between the options.

The condition and suitability of the existing sewer system was assessed followed by hydraulic MOUSE modelling to determine capacity constraints and the required gravity sewer sizes. In determining the general arrangement of the various options, it was found that the augmentations were typically sensitive to factors including pipe size, self-cleansing grades, lateral connection points, sewer operating regime, surface terrain, surface features, geology, depth to surface and environmental features such as creeks, flora and fauna.

In some highly developed areas such as Woolloongabba, the rapid ongoing development, major new transport infrastructure and complex catchments also proved to be problematic in developing practical pipeline routes. In this case, a series of strategic interceptor sewers that changed the fundamental flow pattern of the catchment were adopted. This solution addressed the key sensitivities by using 6.9 kilometres of microtunnelling up to 16 metres deep in the range of 250 mm to 1,200 mm pipe diameter. The solution also involved the relining renovation and pipe bursting renovation of existing local sewer sections. Such a favourable outcome would not have been possible using traditional augmentation strategies.

Similarly, the Bulimba Creek Trunk Sewer assessment adopted 4.5 kilometres of new dry weather flow sewer main ranging in pipe size from 800 mm to 1,200 mm by microtunnelling, which in turn allowed the relining renovation of the existing main and subsequent use as a high level relief sewer. The innovative system offered substantial benefits in system flexibility and odour management.

The feasibility of upsizing from 1,200 mm to 1,500 mm for a deep tunnel section was also assessed in view of maximising pipe jacking drive lengths and eliminating tunnel access shafts. A 1,500 mm machine allows rear cutter access at the head of the machine, and therefore the ability to deal with issues at the face, which in turn offers the benefit of longer drive lengths. However, taking into account the increased costs associated with excavation, pipe material and perceived risk the oversizing option was not included in the preferred option. This risk alternative was presented as an opportunity for later design and construction stages.

Microtunnelling was generally found to be feasible where significant lengths and complexities could be saved, as well as where costing data developed from built-up methods and recent industry surveys complemented the social, community and operational constraints of the projects.

Traditional trenched excavation was found to cater well to low lying greenfield sites, which was the case on the Woogaroo Creek project. However, the use of microtunnelling at upstream sections to traverse hilly terrain and at the downstream end to avoid deep shored trenches and bridged river crossings, substantially shortened the alignment by 30 per cent. The preferred sewer augmentation solution incorporated 2 kilometres of microtunnelling of the total 4.6 kilometres alignment, and significantly reduced the overall project cost.

These innovative designs were achieved using a minimal risk approach that ensured flexibility and sustainability in the sewer systems, kept drive lengths and tunnelling risks to a minimum and ensured the adoption of safe construction methodologies that suited the prevailing conditions and community needs.

During this period PB, also undertook the detailed design of the structural upgrade works and preparation of construction tender documentation for the Cotter Suction Main Tunnel No. 3 in Canberra, construction was completed in April 2009. The tunnel required a full structural upgrade and the adopted solution used a permanent shotcrete lining. Cotter Tunnel Structural Upgrade

Cotter Tunnel No. 3 contains one of three suction pipelines that make up the water transfer network between Cotter Dam and Cotter Pump Station, located west of Canberra. A condition assessment of the pipelines and associated assets in May 2006 identified deteriorating stability conditions within the 150 metre length of Tunnel No. 3 which was constructed around 1915.

PB was commissioned by ActewAGL, acting as an agent of the asset owner ACTEW Corporation, to undertake detailed design of the structural upgrade works and preparation of construction tender documentation.

The existing lining and tunnel support varied along the tunnel profile. A short length of in situ concrete lining is present at either end of the tunnel. Between the portals the support varied from timber sets with some timber lagging, in various states of disrepair, to sections where the tunnel roof was completely unsupported. The unsupported section of tunnel was overlain by Cotter Road and the 1,350 mm diameter Bendora Gravity Main, which supplies water to Canberra.

A range of alternative options were considered and ranked in terms of safety, construction time, cost, skill requirements, technical difficulty, future operational requirements and environmental impact. The selected option was for a primary shotcrete layer followed by a permanent steel fibre reinforced shotcrete lining designed to carry long term loadings.

The tunnel was unsupported for approximately 33 metres at the southern end. Timber sets and some timber lagging were installed at varying spacing in the remaining 105 metre length portion of the tunnel. In most cases the timber sets were not providing any support to the tunnel and several sets had kicked in at the sides and had broken connection joints. A large amount of loose material was present on the floor and in slumped zones behind the timber lining.

The figures on page 58 show the approximate dimensions of the tunnel and the condition prior to commencing the stabilisation works.

From the outset the design for the structural upgrade option, using shotcrete, was based upon a conventional approach as it was considered important to have tried and tested techniques given the fairly unique nature of the structural upgrade. The uniqueness was related to the challenging aspects of safely removing the timbers in a manner that enabled immediate support to be reapplied. Given the tight working space, the solution required all shotcrete to be hand sprayed. A high quality, in situ concrete invert slab was placed to permit for ease of access and maintenance.

Safety and functionality were the main drivers for the design and were the key aspects that steered the project right from the start. Aside from the cost and environmental approvals, it is arguable that there were more complex details for methodologies and logistics and issues surrounding safe working to deal with on this upgrade project than if a new tunnel of similar length and size was being constructed. Construction was completed in April 2009.