14 Sep 2020

Pumice layer delaying $140m highway was known about before work started

2:48 pm on 14 September 2020

The Transport Agency has changed tack on what has gone wrong at a $140m highway project in Tauranga.

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Photo: 123rf.com

Foundations for two flyovers on the Baypark to Bayfair project are having to be redesigned because of the instability of a buried layer of pumice.

The agency has twice said the pumice was "unknown" and was "discovered" during ground works.

But it now says it knew the pumice was there.

The impact on costs and schedule is still being worked out on a project where the completion has already been pushed back to December 2022.

In April, the agency said: "The unknown pumice layer, which was discovered as part of the ground conditions work, could have wide implications on overall construction in the Bayfair area."

RNZ questioned this as the region is well known for pumice and the agency has had past bad experiences when piles for the nearby harbour bridges threatened to sink.

In its latest statement, the agency said it did know the pumice was beneath the Bayfair highway - in a layer 12m down.

"Extensive ground investigations" by an independent third party had found it, the agency said.

This data was provided to all the design-and-construct teams that bid for the work.

Australian builder CPB won the tender. NZTA said its flyover designs were "robust" and peer reviewed and the groundworks were "best practice".

However, a section of the pumice 300m long - one sixth the length of the 1.7km project - was not performing as expected.

"Additional investigation further detailed the depth and complexity of this layer within the pumice and highlighted that it was neither cost-effective or practical to simply dig it out and remove it."

The agency had a peer review done in the US which said the project was well thought out and high quality.

The liquefaction characteristics of pumice sands were "highly uncertain", the review by Professor Russell Green of Virginia Tech University stated. "The work performed as part of this project is advancing the profession's knowledge of the subject."

Flyover abutments, like bridge piles, need a strong foundation layer to sit on.

At the nearby two harbour bridges, pumice has twice caused headaches. Pier 1 at the Stella Passage Bridge began sinking soon after building began in the mid-1980s.

"It became clear that the [pile] casing was developing significantly less shaft capacity than had been calculated," according to a 2008 study.

Engineers tried to take account of this pumice problem when they came to build a new bridge alongside, in 2008-9, but still experienced "relatively poor performance" of the new bridge's 13-25m piles.

The study concluded it can be hard to build on pumice, with the results "somewhat erratic and highly dependant upon installation method", and not a lot of guidance on how to get around it.

A 2018 report in Waikato concluded that pumice sands "are often difficult materials to build on or use in earthworks operations".

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