Kia ora and welcome to the Winter 2020 update from the Transmission Gully team.

Kia ora

Without doubt this has been an extraordinarily challenging period for the Transmission Gully Project. Closing down a 27km construction site at short notice, maintaining site safety and environmental controls for five weeks, and then planning a staged site remobilisation under restricted workplace rules, is not something any of us could ever have adequately prepared for.

Add to this the complexities of an international workforce, a nationwide supply chain, and a tightly programmed schedule of works, impacted by loss of the crucial pre-winter period – and the operational issues come into even sharper focus.

I am exceptionally proud of the commitment and resilience the Transmission Gully project team has shown over these unprecedented times.

First and foremost has come the safety of our people. It is understandable that our crews from out of Wellington and further afield left the site to be with their families during the lockdown. When the project returned to work as the country moved to COVID-19 Alert Level 3 on 28 April, we did so under strict new measures designed to maintain staff safety including the required physical distancing. Inevitably this will impact site productivity – but we are a resourceful team and are working hard to regain the project momentum.

We hope you enjoy the drone footage of current active work fronts along the project that we’ve included in this newsletter, in the construction update below.

Sergio Mejia, Chief Executive, Wellington Gateway Partnership

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Construction activity underway since move to COVID-19 Level 3

With the lifting of Level 4 lockdown restrictions for construction activities on 28 April, the Transmission Gully project was able to begin a staged recommencement of construction works operating within strict new COVID-19 requirements.

Ahead of the restart, much work went into planning site operation under the tightly controlled new hygiene, physical distancing and contact tracing measures that have been established for everyone working on the project.

The subsequent move to Alert Level 2 on 14 May has meant more personnel have been able to return to site and for construction activities to step up, with the COVID-19 work site protocols remaining in place. While the work that can be done is proceeding at pace, we are operating fewer work fronts with smaller teams to maintain physical distancing, and as has been reported in the media, it is expected that some project work progamme milestones will be delayed.

With the announcement from the Government on Monday 8 June, and the quick move to Alert Level 1 overnight,  we are currently identifying when we can increase our personnel onsite, and the necessary health and safety requirements.

We continue to remind all members of the public to stay safe and not enter the project site at any time. Things may not appear to be as busy as pre COVID-19 lockdown but work is underway in multiple areas along the 27 km route and it remains a high hazard area.

If you have any questions as we enter this next phase of the project, please contact us at info@tg.co.nz or if you have an urgent matter please call the 24-hour project hotline on 0800 TGINFO.

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Construction works underway

Construction works are now back up and running five days a week in most areas. If we are working outside of our normal work hours and you’re one of our neighbours, we’ll be in touch with you directly. In all areas, there is environmental maintenance activity, fencing and other works mitigation for properties neighbouring the construction zones, as well as landscape planting continuing throughout. Our latest drone footage from north to south shows the activity that’s continuing along the new motorway route.

Paekākāriki and through Wainui Saddle towards the SH58 Interchange

Between the new motorway’s connection to SH1 at Paekākāriki and the Wainui Saddle area, we have a lot of drainage and earthworks underway . The huge crestal cuts through this zone will be stabilised, retaining walls built and final earthworks completed ready for the pavement team.

The earthworks to the north of the SH58 interchange at Pauhatanui are back in full swing and we are also prioritising the creation of a number of new private underpass accesses for some of the project’s neighbouring properties north of this area.

Bridge over Cannons Creek

To the north of the now completed Bridge 20 over Cannons Creek, there is ground stabilisation work continuing before we get ready to place subgrade in coming weeks. Around the bridge we have backfill earthworks underway thanks to the good weather in Autumn. Structural crews are also working on bridge joints and permanent traffic barrier installations.

Transmission Gully’s Bridge 20 is the project’s most significant structure. For more background on some of the innovations and challenges involved, see the Waka Kotahi NZ Transport Agency’s website.

Bridge 20 over Cannons Creek

SH1 Linden area

On the Kenepuru Link Road bridge across SH1, we are ready to install settlement slabs and are working on reinforcing the earth walls around this structure. The goal is to be able to drive our construction vehicles across the bridge in the coming weeks. Behind our traffic management barriers on the shoulders of the existing motorway through Linden, we’ve being doing testing work on the existing pavement. (Read about why we undertake ongoing testing of the quality of pavement works later in this newsletter.)

SH1 Collins Avenue overbridge and Kenepuru

We have started installing the permanent traffic barriers both north and south of the Collins Avenue overbridge and have completed around 100 metres of barrier in both directions. We are continuing to finish the noise walls for the bridge and plan to finish drainage works around Arthur Carmen Park and reinstate Collins Avenue over the next couple of months.

We are continuing the installation of drainage in this area at the Kenepuru interchange and near the future Kenepuru Drive roundabout. A small amount of earthworks has been completed by Linden School during Level 3, ready for work on the noise walls to recommence in this area in the coming month.

Planting, pavements and structures continue towards completion

There is stream diversion work underway near the Willowbank Quarry and other environmental maintenance activity, fencing and mitigation and landscape planting in construction areas throughout the alignment. The quarry is due to reopen to deliver aggregate for pavement work along the new motorway.

Finishing work has started on many of the completed structures, including ducting for services, retaining walls and barriers to separate traffic once the road is ready to open. Throughout the motorway route we have hundreds of kilometres of ducting to lay followed by the cabling that will enable Transmission Gully’s traffic information systems (ITS) to operate.

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Environmental measures during COVID-19 Level 4

While construction of the new motorway was on hold during the COVID-19 Level 4 lock down, essential site activities were carried out by our safety, traffic and environmental control teams.

There was a strong programme of environmental vigilance along the full length of the closed-down worksite. Environmental teams were on duty 7-days a week performing tasks including:

  • Inspecting and maintaining the hundreds of environmental controls along the length of the site.
  • Maintenance of sediment treatment ponds included cleaning out the trapped sediment, and checking levels of the chemical floculant that helps particles settle out more efficiently in the ponds.
  • Checking ahead of any rain that exposed earth was properly stabilised, for example by applying the green polymer spray or hay, and constructing temporary drains to intercept and slow down runoff.
  • Inspecting during and after rainstorms to manage any minor effect to the site and to repair any damage to environmental controls.

Our environmental mitigation works continued in all construction areas during Level 3 and now also in our busier Level 2 operations. During Level 3 our teams also continued completing a permanent stream diversion and fish ladder that runs around the Willowbank Quarry area as a priority before winter.

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Understanding the technology behind Transmission Gully’s road surfaces – digging deeper into pavement

With a lot of work done over Summer on pavement works along sections of the Transmission Gully route where earthworks as well as sub-grades are complete, we take a deeper look at how the pavement is constructed so that it’s ready for the final surface that will make the road driveable for motorists.

Some of this information was provided in an earlier newsletter but it’s timely to take a slightly more detailed focus on our paving procedures, to help correct some recent misleading commentary.

When we use the word ‘pavement’, we’re actually talking about many layers of material, with compacted granular layers towards the bottom and bitumen layers forming the top. There are several layers of each material compacted into the overall pavement to ensure the final road surface can handle heavy duty traffic and weathering. The pavement and subgrades are designed to distribute the weight and impact of traffic evenly through the layers, so that not just one layer is taking the punishment of that load.

The graphic (below) shows how these layers are built up to form a robust pavement.

Cross section showing different layers of pavement that makes up a road.

How the pavement is constructed

The main steps for constructing the structural pavement layers are:

  1. Quarried gravel (up to 40mm or 65mm in size) is mixed through a pugmill that adds water and cement (usually 1.5 per cent) to the gravel. This creates a uniform, workable paving material that is applied in an even layer to the top of the sub-grade (earth) layer, by a paving machine. The paver also does an initial compaction of the gravel mix, using tamping bars (just like making an espresso!)
  2. Heavy rollers then quickly do a further compaction (within two hours of the gravel being mixed) to make sure the layer is stable and that minimal water evaporates from the gravel material stabilised by the cement, before it starts to set.
  3. During this first stage of paving, testing of the surface is done to check the level of compaction is correct, and machinery is adjusted to increase or decrease the compaction effort being applied. When the right compaction is reached a grader will trim the layer to the correct final level specified by the engineering design – that means a sub-base layer with a nominal thickness of 180mm and a basecourse layer of 190mm above that.
  4. To make sure those tests are correct, independent lab testers will come to the site and check the density, compaction and strength of each finished pavement layer. A surveyor will then also check the final layer level is correct. Test results are then all checked by another independent quality reviewer.

These same steps are used during construction of both the sub-base and basecourse layers. At any point in this process if these rigorous checks detect a weaker spot that needs to be replaced, then we do that rework right away. This is to make sure we don’t need to dig through any of the overlying layers later and that traffic won’t be disrupted by any unplanned maintenance work after the motorway is opened. It’s all about building an extremely robust, high quality road.

Once all the quality checks have been done and approved for each of the pavement layers, we can go ahead and lay the road’s surface.

The materials used to build the pavement

We get most of our pavement materials from near our work sites, at the Willowbank Quarry at Judgeford, where it is produced to the Waka Kotahi size and shape specifications. The rock from Willowbank is classified as greywacke. We also purchase smaller volumes of material from other quarry sources which must meet the same requirements. If the material isn’t up to our sourcing and the Waka Kotahi construction standards, we won’t accept delivery.

Final surfacing

Once all the quality checks have been done and approved for each of the pavement layers, we can go ahead and lay the road’s surface.

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Keeping you up to date

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We hope you enjoy reading these updates, and we’d love to hear from you. If you have any questions, comments or story suggestions, you can email them to info@tg.co.nz

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