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.

Types of paving

There are two types of paving on Transmission Gully – structural (deep lift) asphalt and granular pavement (chipseal).

Deep lift asphalt is being used on the steeper parts of the road, such as Pouāwhā/Wainui Saddle, and on the busiest parts of the motorway, such as the interchanges. The remaining two thirds of the road will be chipseal.

Structural (deep lift) asphalt

If the kitchen is the heart of the home – then the temporary asphalt plant at Mackays crossing is the busy commercial kitchen cooking up the ‘structural asphalt’ (also known as ‘deep lift’) needed to finish the road.

Asphalt plant machinery

The temporary asphalt plant, operated by Fulton Hogan, will be removed once paving work is completed.

To make asphalt, simply set the oven to 180 degrees, then combine your dry ingredients (a generous heap of fine and coarse aggregate), add a chef’s pour of bitumen, and finally blend together and get cooking.

For Transmission Gully, there is also a secret ingredient… a sprinkling of runway.

Our deep lift paving mixture contains Recycled Asphalt Pavement (RAP) from Wellington Airport. Sand is a valuable resource, so old pavement from the airport was carted to the Mackays asphalt plant where it was crushed into finer aggregate-substituting sand and added to the recipe.

This is just one of the ways the project is meeting its sustainability goals and paving the way towards a green road for the future.

When making asphalt, timing is everything. Once it’s ready, the hot asphalt is poured into a truck and taken directly to where it’s needed on the alignment. Ideally, we don’t want it to lose more than 10 degrees on the trip – which is why it’s so critical to have the plant close to where it’s needed.

Once the truck arrives on site, the mixture is dumped into a paver (a type of machine that specialises in laying asphalt) and an eleven-person crew meticulously gets to work. There’s a supervisor, a paver driver, roller drivers and a quality control team. They’re checking the temperature and ensuring everything conforms to the correct quality controls.

It costs about $400 per tonne to produce and lay asphalt.

If it gets too cold, or doesn’t meet quality checks, it may have to be ripped up and the whole process begins again.

Structural asphalt is a premium paving product. It can sustain high traffic volume, can be laid in more marginal weather conditions, and has a long life. It’s being used on the steeper parts of Transmission Gully, such as the approaches to Pouāwha/Wainui Saddle and on the busiest parts of the motorway, such as the interchanges.


Granular pavement (chipseal)

Chipseal is the most common type of road surface in New Zealand. Which is not surprising as it delivers great frictional characteristics (such as skid resistance) and has lower maintenance. Chipseal is suitable for the areas along the motorway in which it will be used.

A stabiliser (following a water cart) moves over the surface mixing cement with the aggregate.

Mixing cement with the aggregate is called ‘binding’ and it improves the moisture resilience of the pavement.

Rollers compact each pavement layer, subbase and basecourse, before chip sealing begins.

Watch the video below to see how we’re laying chipseal on Transmission Gully.