Tãmaki Makaurau Auckland is Aotearoa New Zealand’s largest city, with a population of 1.7 million—roughly a third of the total population. Beneath the city, magic is happening: Watercare, the city’s water utility is building a 14.7km underground wastewater tunnel from Mãngere Wastewater Treatment Plant, across the Manukau Harbour to the central city. It’s the largest and most complex wastewater infrastructure project in New Zealand history
Written by Kerryn Caulfield, Executive Director, Composites Australia Inc. along with assistance and advice from Gavin Engelsman – GM Engineering and Operations – Infrastructure
The NZ $1.2b Central Interceptor consists of one main tunnel (interceptor) and two link sewers. The project is being delivered by Ghella Abergeldie JV.
Work on the first link sewer is almost complete, with the micro-Tunnel Boring Machine (TBM) having just completed her third drive. While, Hiwa-i-te-Rangi—the large TBM is due to complete the undersea crossing of the Manukau Harbour in early December en route to the central city. Designed to replace ageing infrastructure and reduce wet-weather overflows into the central Auckland waterways as well as handling huge volumes of wastewater from Auckland’s growing population, it consists of close to 14.7 kilometres of 4.5 diameter tunnels, close to 5 kilometres of pipe-jacked sewers, 18 shafts, a major pump station, new odour control and air treatment facilities and substantial wastewater management and network infrastructure works. The construction phase of the project began in 2019. The due date for completion is now 2026 however consent applications are expected to extend the tunnel to catch combined wastewater and stormwater flows from two more suburbs.
The landmark project is being constructed to meet a 100-year durability requirement which includes a one pass HDPE lining of the main 4.5metres diameter tunnels and the extensive use of GRP/FRP in the cascade drop shafts, which would otherwise be subjected to significant corrosion due to sewer gases. Nine cascade structures varying from 3.0 metres in diameter up to 7.5 metres in diameter, and up to 72 metres in depth, will be used to dissipate hydraulic energy and link the existing sewer network with the new tunnel.
The selection of GRP/FRP for use in the cascade structures, in lieu of reinforced concrete, which was the original intention, was adopted by Watercare as a superior long-term solution to address the expected significant corrosion problems that currently occur in all sewer networks. The GRP/FRP structures act as a corrosion protection lining by eliminating any direct contact between the sewer gases and the concrete structures, whilst also reducing the site construction time and improving site safety by minimising the amount of work required to be undertaken within the shafts.
In total over 1,000 tonnes of glass reinforced composites will be utilised, with the GRP components manufactured off site and transported as completed modules, for final assembly and placement and concrete encasement by RPC/GAJV (Gella Abergeldie Joint Venture). The project required seminal engineering and manufacturing solutions for a myriad of challenges, not least of which, are that the underground structure is required to be designed to withstand a one in 2,500 year seismic event.
From a material perspective, it is a performance intensive hydraulic environment in corrosive underground confines that are not easily accessible to repair or undertake routine maintenance. The raw material inputs span the breadth of composite enabling resins and adhesives including polyester, vinyl ester, epoxy prepregs and methyl methacrylate (MMA) adhesive. A variety of different glass and carbon fabrics was used including “C” glass tissue; ECR (acid resistant) chopped strand mat and woven rovings; Bi axial, double bias, and uni-axial fabric and continuous process roving.
To meet the 100-year design life criteria, the material properties of all raw material inputs used were tested and validated over the course of a 12-month comprehensive testing program that included:
- Long term strength properties and validation of the strength/modulus characteristics
- Fatigue characteristics – 1010 cycles ( 3hz x 100 years)
- Abrasion and erosion performance (compared to reinforced concrete and alternative materials)
- External pressure collapse strength
- Ring bending stiffness
- Beam tests
- Full scale first article tests
- Seismic deformations
The test results were subsequently reviewed and incorporated in the detailed FEA analysis modelling to verify both short term and long term properties of the laminates.
The project called for a cross section of GRP manufacturing processes including conventional hand layup, vacuum infusion moulding, adhesive bonding and filament winding including in-house purpose built automated vertical winding for the DN7500 modules.
The cascade drop structures are designed for wastewater to cascade down the shaft, dropping from a series of alternating shelves. The spacing and size of each shelf is engineered to manage the velocity; the design hydraulic flow; dissipate hydraulic energy; minimise odour and mitigate health effects and corrosion from hydrogen sulfide (H2S) caused by the bacterial processes in wastewater. The liquid falling between the shelves results in both hydrostatic and fluctuating hydrodynamic loads being transferred into the composite cascade shelves and back into the concrete annulus encasing each composite shaft structure. The shafts have been designed in two operating halves, whereby water drops down one side of the shaft, allowing air to vent and rise on the other.
The design requirements led to a series of in-house fatigue and abrasion tests to verify the wear resistance and long-term strength of GRP cascade shelves, with and without additives and/or coatings. The results demonstrated that for the areas of high wear, including the cascade shelves and dividing walls, 3 mm HDPE liner bonded to flat GRP panels would meet the performance requirements. Silicon Carbide (SiC) filled GRP in the corrosion barrier met the requirements for the areas exposed to medium wear such as the cylindrical shaft, and unfilled GRP performed adequately in less critical areas such as the vertical internal walls away from the splash zone. As a material comparison, GRP out-performed all other alternatives.
Available long term fatigue performance data validating the specified 3Hz cyclic loading and 100-year durability requirements of the GRP shelves was also required. After a global search, RPC turned to experts in the US, to reference and augment data, on the fatigue performance and condition monitoring for composite wind turbine blades, which have over 30 years of test data and proven performance.
Available long term fatigue performance data validating the specified 3Hz cyclic loading and 100-year durability requirements of the GRP shelves was also required. After a global search, RPC turned to experts in the US, to reference and augment data, on the fatigue performance and condition monitoring for composite wind turbine blades, which have over 30 years of test data and proven performance.
The horizontal cascade shelves and vertical dividing walls are manufactured using a variety of advanced materials and customised engineered processes designed to last well into the next century. Leveraging technology developed in-house by RPC, high strength/stiffness bespoke sandwich structure panel, capable of supporting up to 70,000kg/shelf were manufactured and supported using a combination of glass and carbon fibre panels.
Material and process options for the cascade shelves were dictated by high hydraulic and cyclic loads, the differential pressure across the vertical flat dividing walls as well as cost and weight. A simple yet solid GRP centric flat panel design was not feasible as the through core shear stresses were too high for traditional core products. A tailored ‘sprung yet ridged’ corrugated flat panel was developed to be manufactured in two separate halves and joined together with an epoxy prepreg adhesive tape. This construction reduced the panel weight and enabled inspection testing prior to assembly and precise adhesive distribution on the mating surfaces. Peel ply was applied during the infusion process to ensure consistent high quality surface preparation prior to bonding. The test specimen to validate the fatigue strength of the GRP corrugated/ sandwich panels passed 10 000 and 250 000 cycles which was well within the limits of BS EN 13121.1.
As the cascade structures will be subject to earthquake activity and must withstand both seismic and static loading, consideration was given to both the ‘ovalling’ and raking’ response of the GRP shaft liners, due to seismic ground motion and soil-structure interaction of the concrete/GRP shafts. The GRP shafts liners that house the cascade shelves were designed to withstand a combination of load cases including groundwater and soil loadings , along with the unbalanced seismic loads that exert the ‘maximum external pressure’ on the GRP shaft liners; ‘maximum internal pressure’ for when the shaft is flooded; ‘dynamic loading’ on the cascade shelves and walls of the shaft liner due to falling water; ‘surge loading’ on either side of dividing walls; ‘geyser loading’ on the underside of shaft roof; ‘buoyant uplift’ for when the shaft is empty and seismic deformation.
As no specific design codes are available for large diameter manholes or underground structures the design adapted BS EN 13121.3-2016 GRP Tanks and Vessels – Part 3 Design and workmanship, as the standard provides a design basis for both Ultimate Limit State Design and Serviceability Design, along with fabrication, inspection, testing and verification methods, augmented by FEA (Finite Element Analysis) modelling.
To counter gravitational disturbance on the resins imposed by the process of vertical winding, a UV cured resin system – originally developed for dental technology – was used to wind the larger flanges and ribs on the modules. This relatively new technology for composites increased productivity and improved quality by removing the need for interim cure stages and reduced cure stresses of thick laminates.
At 72 m long the largest shaft is constructed with over 20 individual ‘cans’ or modules that are bolted together. The sheer size of the ‘cans’ required they be wound vertically. Specialised equipment which was procured and commissioned in the company’s Corio plant in Victoria, designed by the RPC engineering team in Seven Hills, Sydney, along with an hydraulic collapsing mandrel to reduce labour cost and mould turnaround times.
To counter gravitational disturbance on the resins imposed by the process of vertical winding, a UV cured resin system – originally developed for dental technology – was used to wind the larger flanges and ribs on the modules. This relatively new technology for composites increased productivity and improved quality by removing the need for interim cure stages and reduced cure stresses of thick laminates.
Extreme external forces also affected the lower sections of the some of the shaft liners which was further complicated by the large intersecting pipe connections. The solution of a thicker laminate exceeded available crane capacity and was less than cost effective. Again, RPC engineered and developed a multiple layered corrugated wall construction not dissimilar to the flat panel construction but with equivalent attributes of weight reduction and production efficiencies brought about by lower exotherm and reduction in curing stages.
The ‘Central Interceptor’ is an herculean engineering, manufacturing and underground construction project that has pitched design, manufacturing, material, process and logistical engineering challenges at every stage.
Shayne Cunis, Watercare executive programme director: “The Central Interceptor is one of the largest infrastructure wastewater projects taking place in the Southern Hemisphere at the moment.
“We pride ourselves on innovation, excellence and safety in everything we do. The cascade structures made by RCP exemplify all of these goals and we’re delighted to have them on board as a construction supplier.”
