Tender, detailed design and construction phase services as the design joint venture (DJV). Beca and Tonkin + Taylor each carried primary responsibility for areas of the services within the DJV and contributed to the delivery of others.
Specifically, Beca was primarily responsible for: team leadership and design management; construction phase services; structures, geotechnical engineering for structures, roading and landscape design; planning; contaminated land management and Greenroads sustainability framework.
Tonkin+ Taylor was primarily responsible for: geotechnical engineering, water engineering, ecology, and construction phase support.
The Pūhoi to Warkworth project (P2Wk), due to open to the public in 2021, will extend Auckland’s Northern Motorway (SH1) 18.5km from the Johnstone’s Hill Tunnels to just north of Warkworth.
It is the first stage of the Ara Tūhono – Pūhoi to Wellsford Project and will help drive regional economic growth and development to support access and connectivity for local communities.
By delivering a safer, more efficient and resilient connection between Pūhoi and Warkworth, the new motorway will reduce journey times (especially during peak holiday periods and weekends), improve freight efficiency, and hopefully, save lives.
Under a Public Private Partnership (PPP) contract, the Northern Express Group (NX2) will finance, design, construct, manage and maintain the Pūhoi to Warkworth motorway for 25 years following the expected five-year construction period.
After the 25-year concession, NX2 will handover management and maintenance of the highway to the NZ Transport Agency.
Beca and Tonkin + Taylor, in joint venture (Design Joint Venture) are providing tender design, detailed design and construction phase services for the construction joint venture (Fletcher/ACCIONA).
A range of factors influenced our design approach for the P2Wk motorway including:
- Flexible consent conditions
- The NZ Transport Agency’s desired Key Outcomes for the project
- The PPP contract model – e.g. the optimisation of capital spend versus operations and maintenance costs over a 30-year contract, through close interaction between the design and construct, and operations and maintenance teams
- Safety in Design
Flexible consent conditions
The Resource Management Act (RMA) conditions applied to P2Wk were unique – lacking the common “Condition 1” requirement for the design and construction approach to be in general accordance with what was submitted with application documents.
In its place, the Board of Inquiry (BoI) used a set of outcomes, designation conditions and an Urban and Landscape Design Framework (ULDF) to outline the intent, and an outcomes-focused approach for the project.
We leveraged the wide designation provided by the flexible consent conditions of the PPP to optimise the partnerships’ alignment, and used the ULDF to guide our design choices.
These flexible consenting conditions supported an innovative approach to the project, but also brought with it a heavy responsibility - the NZ Transport Agency was in effect entrusting their reputation to us.
Throughout our decision-making and the Urban and Landscape Design Sector Plan (ULDSP) process, we worked closely with the project’s Iwi collective Hōkai Nuku, stakeholders and communities.
Hōkai Nuku is the authorised voice of four Iwi and Hapū mana whenua – Ngāti Manuhiri, Ngāti Mauku/Ngāti Kauae of Te Uri o Hau, Ngāti Rango of Kaipara and Ngāti Whātua.
In line with the flexible consent conditions, the NZ Transport Agency set four Key Outcomes for the P2Wk project, which drove all our design choices:
- Protecting and enhancing the NZ Transport Agency’s reputation
- Delivering a high-quality asset
- High and sustained safety outcomes
- Delivering predictable journeys for customers
To ensure we delivered value to the NZ Transport Agency, our tender initially found the lowest-cost design then sought enhancements and refinements, drawn from a register of over 100 value-add opportunities.
Our final design solution represented an optimised combination of design choices to achieve the Key Outcomes.
The PPP model influenced both our design choices and our approach to design management.
We took a whole-of-life approach to every design choice, ensuring we made the right investment choices for the Concession Period and the asset’s residual life, and provided the NZ Transport Agency with a high quality, durable asset at handback. For example, although not ‘preferred’ by the ULDF, we opted to seal the median and behind barriers to minimise maintenance activity and cost, and protect the integrity of the pavement whilst improving safety.
The highly contractual nature of a PPP contract required a very structured approach to our design management. This was particularly evident in the way we had to manage third-party reviewers with no statutory response timeframes - in a highly programme-driven environment.
We took a statistical approach to our delivery risk and set a 43-week design programme, of which only approximately 50 percent was allocated to design effort.
The remaining time was required to obtain internal NX2 reviews, peer reviewers and other approvals/reviews from key stakeholders including Auckland Council, Auckland Transport, Hōkai Nuku and the NZ Transport Agency.
Of our 236 design packages delivered to date over a 4-stage process, 97 percent were delivered on time. One hundred percent of our construction packages were delivered on time.
Geometrics and road alignment
With a significant earthworks volume, we recognised early on the importance of optimising the alignment within the designation.
We opted for a curvilinear alignment which follows the natural topography of the land, thereby minimising the footprint, improving safety for drivers by encouraging consistent speeds, and providing a better customer experience. By promoting consistent driving speeds, the curvilinear alignment also has a positive impact on fuel consumption and vehicle emissions.
Our alignment choice also enabled us to eliminate a number of bridges from the Indicative Design (the design presented as part of the BoI process), thereby minimising upfront and whole-of-life costs, improving safety and delivering a better experience for drivers.
Our alignment philosophy provided a design that minimised fills, fitting much more closely to the natural ground, lessened embankment heights and shortened bridges. This provided considerable environmental benefits both visually and for the footprint effects on flora and fauna.
For example, in one case where the alignment was required to traverse a stand of mature kauri trees, our approach of careful and detailed mapping of individual native tree locations and sizes, allowed us to minimise tree removal.
To verify our choice of alignment, we tested it against a Trimble Quantm (a specialist and powerful software package that provides an optimised alignment based on an array of inputs) review during the tender phase. The results showed that our experienced geometrics designers had already got it right.
Although noise minimising Open Graded Porous Asphalt (OGPA) pavement was only required by consent condition over two lengths of the alignment, our consortium opted for an OGPA pavement across the entire length of the alignment.
We felt this delivered value by meeting all of the Key Outcomes – improving safety by reducing water spray, durability (delivery of a high-quality asset), protecting and enhancing the NZ Transport Agency’s reputation through noise reduction and ride quality, and minimising the requirement for maintenance (resulting in less impact on drivers through lane closures).
High quality OGPA pavement also has a positive impact on fuel consumption and emissions.
The connection of the P2Wk motorway to the existing SH1 just north of Warkworth (the Northern Connection) was a key concern for the Client – with impacts on all four Key Outcomes (safety, journey time reliability, quality and NZ Transport Agency’s reputation).
Our design solution was an at-grade roundabout, providing a safe transition between the motorway and local road environment, and easing congestion with the use of slip lanes.
Our design also delivered future-proofing by providing a staged upgrade option to meet the increasing requirements of the road if required in the future.
Our D&C structures team, including bridge architectural designer Monk Mackenzie and constructors, balanced a number of considerations when designing the structures.
These included whole-of-life costs, safety, delivering a positive driver experience and meeting the requirements of the ULDF.
The ULDF called for an understated, elegant and minimal, and in some cases recessive aesthetic, so all bridges feature slender designs for the spans and deck soffit lines that are pure and uniform in construction depth.
Edge barriers incorporate precast down-stands to provide a simple and elegant elevation with no visible services. Visual aspects of the bridges from all stakeholder’s perspectives were considered in our designs. This included river and local road users, and residents.
The Pūhoi and Ōkahu Viaducts (Te Arawhiti ki Pūhoi and Ōkahu) and Moirs Hill Bridge (Te Tapuwae o Kahumatamoemoe) are all steel girder structures to provide consistency and balance elegance and cost.
We opted to construct steel girder bridges rather than steel ladder bridges (the most common typology for medium span bridges in New Zealand) based on Acciona’s experience constructing this bridge type on other projects internationally. Steel girder bridges are built extensively across Europe and are simpler, safer and lower-cost to construct and maintain.
Geotechnical challenges and solutions
The motorway notably passes through a variety of landscapes that strongly reflect the underlying geology, ranging from flat alluvial infilled valley systems to the unforgiving, forest covered, steep slopes developed on the 20 million-year-old Pākiri Formation sedimentary rocks in the central section of the project.
At the outset of the design process it was recognised that the footprint of the motorway should be kept to a minimum width to minimise its effect on the environment, especially through the steep central section. To maintain suitable grades on the carriageway, we adopted a bold approach with deep cuts in the high central section; these cuts were designed with steep, tall slopes partially covered with wire mesh drapes leaving the lower slopes in bare rock to clearly expose the geological structure.
However, this approach demanded a full understanding of the underlying geology and geological structure which, in turn, required detailed geotechnical investigations to gather the necessary information. The detailed geotechnical investigations were undertaken by NX2 following the fixing of the motorway alignment. This was achieved by drilling fully cored both vertical and angled drill holes using drill rigs lifted into place by helicopters to minimise disturbance to the natural environment. The information was pieced together to produce a geological structural map that became the central focus for the final route selection using, wherever possible, favourable geological conditions identified by the investigations.
Immediately north of the central section, the motorway designation corridor crosses a stand of kauri and a local stream in an area known as the “Eco-Viaduct”. This presented a dual challenge of preserving the kauri and designing the bridge abutment on a relic landslide. While the original concept envisaged a tall viaduct structure, we developed an alternative lower structure that would not shade the kauri and also required less geotechnical input for the abutment in the relic landslide area.
Another significant challenge arose from fill packets where the motorway passed over the pervasively sheared rock mass of the Northland Allochthon. In order to maintain the required carriageway grade, a number of options were considered for keying the fill foundations into these sheared rocks resulting in the selection of shear keys using, where possible, the local high grade rock.
The issue of utilising site-won materials for early construction is often compromised by poor quality materials that require stripping and storage to enable access to better quality materials. The Pūhoi to Warkworth motorway was no exception, however, the design team sourced quarry sites within the designation to enable early production of site-won high quality materials along with the location of fill sites for the unsuitable materials stripped to waste.
The landscape surrounding the P2Wk motorway is home to a wide variety of at-risk and threatened native flora and fauna.
We were involved heavily through design optioneering to make sure these sensitive species were considered in the overarching design approach, and we worked closely with construction teams to implement management measures, once the project commenced in earnest.
An assessment of ecological effects, undertaken by BioResearchers in the BoI phase, identified ecological values along the project footprint which informed the resource consent conditions. Thus, the resource consent, along with Department of Conservation (DoC) translocation permits for threatened fauna, defined what was to be avoided, salvaged or protected within the project area. Similarly, a range of construction-phase management regimes were developed for implementation by the construction team.
In order to find out what species were living in the project area, the ecology team had identified and measured native flora and fauna, investigated stream ecology and mapping habitats where native species could be living. The team worked closely with Auckland Council, Department of Conservation, Ministry for Primary Industries (MPI) and Hōkai Nuku to make sure that appropriate survey and salvage methods were in place.
Further work during the construction phase of the project included fish salvage, night-spotting for geckos, and harvesting of native plant species for propagation in a nursery.
Given that the project area included the Pūhoi estuary and the Mahurangi Harbour, a regular marine monitoring programme was adopted to check the sensitive marine environment for impacts from the construction. This involved monitoring of animals, plants and the marine substrate.
The construction of instream structures and the filling in of streams (e.g. as a result of spoil sites) reduces stream quality and overall habitat availability for native freshwater fauna. We assessed all streams prior to construction and calculated an appropriate amount of offset stream restoration. As a result, the mitigation includes restoration of 20 metre-wide riparian margins on either side of stream channels with plants that shade and protect the stream and allow freshwater fauna to thrive.
The project features 8.7km of stream restoration planting – 6.3km as mitigation for 31 culverts, and 2.4km for stream diversions.
The ecology team have searched and collected a variety of species in the project area to ensure ‘at risk’ or ‘threatened’ species are appropriately treated and/or relocated as appropriate. This includes a native Danhatchia orchid which had been identified within the designation as part of the Assessment of Ecological Effects within the BoI phase. The area was cordoned off to avoid damage or removal of any orchid plants during the construction phase.
NZ’s native green mistletoe, a woody epiphytic semi-parasite which has been declining since the early 1900s has been found to grow on tōtara trees within the project footprint. These plants were identified and flagged by the ecologists at the start of the project. Given their location close to a haul road, they are monitored for dust to prevent negative impact from excessive dust settling on leaves.
The project ecologists have also identified and collected seed from a colony of threatened short-hair plume grass. The seeds from the graceful, feathery grass are being propagated in the nursery where they will grow into plant stock for planting out during the landscaping phase of the project.
Native canopy trees (kauri, tānekaha, pūriri, tōtara, kahikatea, rimu, rewarewa and taraire) within the project area were measured to determine their diameter at breast height (DBH) to calculate mitigation requirements. The team measured more than 3,300 trees, of which approximately half (1,561) were felled by May 2018. To mitigate for the loss of these trees within 20 years, at least 50 hectares will be restored with these native canopy species. The proposed mitigation planting will be undertaken in two stages. In Stage 1 pioneer species (e.g. kānuka, mānuka) will be planted to allow the establishment of a shelter canopy for the second stage planting of light intolerant species such as rimu. Overall, the mitigation planting, will connect habitat across the wider landscape.
Wetland loss will be offset at a 1:1 ratio, equating to approximately five hectares of wetlands which forms an integral part of the landscape plans, along with terrestrial mitigation of canopy species and locations for short haired plume grass.
To avoid negative impact on birds, vegetation clearance took place outside of the breeding season between September and December. Where this was not feasible, shrubs and trees were checked for active bird nests and, if positively identified, were felled at a later stage. In wetland areas where the call of fernbirds was identified during the wetland bird survey early on in the project, a carefully monitored vegetation clearance approach was adopted to prevent harm to the birds.
Other activities included freshwater ecology along streams and terrestrial work with geckos, skinks, snails and bats.
Prior to the construction of the temporary causeway, ecologists and representatives from Hōkai Nuku collected more than 34,000 mud snails from the Ōkahu Inlet and relocated them. These snails are native to New Zealand and live in great numbers on mudflats. Additionally 50 Rhytid snails were salvaged and released in a predator-proof relocation site.
The construction of culverts and other stream structures required the dewatering of streams, which are home to native fish species. To ensure their safety during construction, more than 10,000 fish, kōura and native freshwater mussels were relocated beforehand.
The ecologists used the electric fishing method, whereby a current temporarily stuns the fish which float to the surface so they can be scooped into nets and relocated upstream or downstream of the impact area. Relocated fish included giant and banded kōkopu (adult whitebait), giant bullies, and long-fin eels, some of which measured over 1.5m.
However, the fish relocation near the Pūhoi River could not utilise conventional methods due to the challenging low-lying and wide profile of the gully with thick wetland vegetation to boot. The solution was a 12-ton digger fitted with large paddles which kept the digger ‘afloat’ on top to remove the surface vegetation while an ecologist assisted with the fish identification and relocation.
We adopted extensive fish passage monitoring (both pre-construction and post-construction) to safeguard the natural cycle for native freshwater fauna. This means that local fish species can continue to travel unimpeded to find habitat, food and mates; for some species this means traveling to the ocean and back to complete their lifecycle. To facilitate successful fish passage, culverts were designed with features to enhance fish passage, for example including baffles to reduce water velocities.
After 538 hours searching for geckos, the team of ecologists salvaged 36 forest geckos and one pacific gecko which were held at the Massey University Reptile Facility (MURF) and were released at the relocation site in December 2017. Despite the extensive deployment of onduline artificial cover objects across 21 possible habitat areas, only one copper skink was captured as a result of manual search.
Specialist care was required for a colony of very rare indigenous long-tailed bats (Chalinolobus tuberculatus) within the project area. These thumb-sized bats with a wingspan of barely 150mm have the highest threat ranking of “nationally critical”. To provide an indication of bat activity and roosting habitat (within tree cavities), ecologists conducted a large-scale acoustic survey and ongoing monitoring during the design and early construction phase. Follow-up surveys included the visual inspection of hundreds of trees along the 15km corridor that had to be felled for the new highway and associated access roads.
Long-tailed Bats have large “home ranges” and do not respond to manual relocation and fail to survive in captivity. As a result, prior to felling any “high risk” trees, extensive acoustic surveys were conducted to ensure bats were not at risk of being harmed. All “high risk” trees needed to be felled before the end of the survey season (1 October – 31 April) when weather conditions were favourable for bat emergence. However, a small number of bats hadn’t evacuated their roost in a small stand of trees located in a steep gully; they kept returning every morning after their night outings and seemed unlikely to vacate their existing habitat. This would have precluded the scheduled felling and would have hampered project progress. Thus, following consultation with DoC and other bat experts, the team trialled an innovative solution involving large flood lights that lit up the habitat area throughout the night and eventually caused the bats to not return to their roost in the morning, and the stand of trees could be felled without the risk of harming bats.
Tree felling protocols were successfully used for the main felling season 2016/17 and for subsequent felling seasons of remnant pockets of trees without any bats being harmed in the process.
The project features an effective and innovative stormwater design to manage motorway runoff while also catering for the large number of watercourses that cross the motorway alignment. Thus the design avoids, remedies or mitigates adverse environment effects efficiently and practically.
Along its 18.5km length, the project includes over 50 culvert installations (both concrete and HDPE) with a total culvert length of over 6.3km. At up to 2.5m diameter and 220m in length, and beneath embankments that are up to 45m high, culverts include a number of multi-barrel installations with up to 11 barrels.
The importance of facilitating fish passage through the culverts was a key design driver and was addressed with the use of fish baffles, spat ropes and submerged culverts. Within this approach, we have focused on balancing the energy dissipation while accommodating fish passage.
The issue of debris blockage was also addressed by assessing the culvert catchments to determine whether they might be susceptible to blockage. Where we identified a risk of debris blockage, we included debris screens, debris fins and relief risers to mitigate the blockage risk.
Where necessary we’ve designed a number of stream diversions with low flow channels, rock riffles, ponds, boulders and rootwads to provide habitat.
Additionally, flood modelling for the Mahurangi River demonstrates that the project impact on the existing flooded areas is compliant with the Resource Consent requirements.
A combination of rock lined swales, rock cut swales and piped reticulation conveys the motorway runoff to one of 13 wetlands for water quality treatment. These wetlands are typically 30m wide and 80 to 100m long and provide extended detention that helps channel protection for frequent storm events. They feature banded bathymetry with small topsoil bunds that will retain water during times when water levels are naturally low. What’s more, the wetland outlets can be shut in the event of an oil spill in order to contain the spill in the wetland and prevent it from entering the wider environment.
The motorway drainage features a number of spill points where flows that are greater than those needing to be treated, can spill back into the environment. This effectively mimics the natural hydrology with the added benefit of reducing the size of the stormwater infrastructure.
A key objective of the stormwater design was to assess and minimise the risk of aquaplaning which can occur when a layer of water builds between the wheels of a vehicle and the road surface. This can result in a loss of traction and control of a vehicle with dramatic consequences. To address this, we have used slot drains to intercept long flow paths and to reduce the risk of aquaplaning while complying with NZ Transport Agency standards. Comprehensive aquaplaning assessment for the entire motorway alignment has informed the placement of slot drains and fed into the geometric design.
The project’s stormwater, landscaping and ecology solutions are designed to be complementary and sympathetic. This is particularly notable at the Te Arawhiti pua Ngahere (Kauri Eco Viaduct) where the proposed motorway crosses a watercourse and a significant stand of kauri trees. At this location, an under-bridge watering system will irrigate the slopes beneath the bridge to ensure a continuous landscaping approach. The watering system utilises technology that is typically used for onsite wastewater treatment and mimics natural rainfall, while in-ground irrigation lines allow plants to flourish. This approach will provide a cohesive and consistent landscape with connectivity between significant areas.
Construction Phase Support (CPS)
As a key aspect of this project, CPS fulfils a dual role by providing certification as well as design clarification and optimisation.
For the certification requirements, the CPS team undertakes construction monitoring to CM4 level which involves frequent reviews of site conditions. The CPS team effectively represents the designers by observing key elements of the construction process to confirm that it meets the design. This gives the Design or CPS Lead confidence to sign producer statements or certificates.
What’s more, the CPS role also helps to clarify and optimise the design throughout. Primarily this is achieved by augmenting existing data with site observations; it helps to confirm that the ground conditions are consistent with design assumptions. In this manner, any deviation in ground conditions (i.e. not evident in earlier investigations) can be accurately identified and appropriately addressed during construction.
In the context of the project area’s highly variable ground conditions, which vary from very steep rock cuts (up to 85 degrees) to flat boggy land, and an overall ambitious earthworks programme, this observational input is crucial for proactively managing risks on site that can cause potential instability or help utilise resources. As such, it makes a tremendous project contribution by helping refine the design.