Revealed: Flawed chipseal behind latest Transmission Gully delay
Transmission Gully highway has been plagued by flawed chipseal and water seeping through the road’s surface before it has even opened, independent inspection reports reveal.
The chipseal defect has played a significant part in the road’s opening being pushed back once again; originally expected to open in April 2020, the road is now expected to open next year.
Extensive parts of the road have evidence of “flushing” – a phenomenon caused by bitumen seeping up through a layer of chipseal on top of it, according to two reports the Herald has obtained under the Official Information Act.
Excessive flushing can mean roads are more slippery than they should otherwise be, compromising the safety of the road. It can also mean the road becomes sticky and car tyres end up pulling the bitumen further into the chipseal.
This is primarily of concern on the southbound lane of the 27 kilometre motorway.
Much of the chipseal construction on this lane was completed over the winter months in cool temperatures. To make sure the chips were able to bed into the bitumen, extra kerosene was added to the bitumen mix – this made it softer, allowing the chips to be absorbed by the bitumen during the cooler winter months when it would naturally harden.
Unfortunately, this meant as temperatures rose again in spring and summer, the layer of bitumen softened, and began to seep through the chips, creating flushing.
It is normal for some chips to come loose from a road – this is usually remediated when traffic begins to flow, as the weight of cars and trucks depresses the chips into the road.
But it’s generally recommended to avoid construction of chipseal outside of the sealing season because this premature flushing is a known issue. Transmission Gully, however, was a road already significantly behind schedule.
A November 10 report on the state of Transmission Gully from Stantec, recommended the flushing should be “addressed before the opening of TG to highway traffic”.
Some remediation has been undertaken by putting more chipseal on the road to absorb the excess bitumen.
But a later report, from November 24 found the efforts to fix the flushing “appear not to have improved the situation”.
“Indeed,” the report said, “Southbound flushing has got worse”.
Several instances of water seepage with brown staining have also been identified between two bridges during inspections. This could be due to water getting into the asphalt during rainfall and then seeping out of the road in downhill areas later, the November 24 report suggested.
However, this is not considered an urgent issue and is different from the leaks discovered at the nearby Kāpiti expressway, which had to be resealed months after opening.
This issue will still need to be resolved by winter, when the leaks could turn into ice and become another safety issue.
Earlier this week Waka Kotahi NZ Transport Agency confirmed the road would not open before Christmas because the builder has run out of time to complete safety and quality assurance tasks.
The builder is required to meet 100 of these tests before the road can safely and legally open to the public.
Waka Kotahi transport services general manager Brett Gliddon said some of these tests relate to the quality and performance of the road surface.
“The issues with flushing that have been identified in a number of locations along the motorway have meant that CPB HEB has not been able to satisfy the Independent Reviewer with regard to tests related to the quality of the road surface, which has played a part in the road not being open before Christmas.”
Gliddon referred any questions regarding remediation to the builder.
WGP chief executive Sergio Mejia also referred questions about the motorway’s paving to the road builder, CPB HEB JV, as it was a construction matter.
“What I will say on this matter is that WGP contracted the road builder to deliver a motorway that complies with all contractual requirements, including meeting the specified quality, safety and consent requirements”, he said.
“We know the road builder is working extremely hard to achieve these requirements.”
Mejia said WPG was keen to see the motorway opened to traffic as soon as possible but would not be taking any shortcuts to do so.
CPB declined to comment and HEB did not respond to requests for comment.
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