The tsunami refuge platform planned for Papamoa's Gordon Spratt Reserve will cost an extra $734,000 because the land underneath it would turn to putty in the event of a major earthquake.

Tauranga city councillors are today being asked to approve additional construction costs for the $1,034,000 evacuation infrastructure.

The cost of Papamoa's tsunami refuge platform is set to rise. Photo: File.

As well as extra funds for the raised earth platforms, council is being asked to pay an extra $300,000 for three new escape bridge foundations.

The reason behind the extra costs relates to ground conditions, which is suffering from greater-than-expected liquefaction and lateral spread issues.

The Gordon Spratt Reserve VES (vertical evacuation structure) was brought forward in January this year, as it was believed it could be accelerated without incurring additional costs from other projects. The price was originally set at $566,000.

However, the ‘geotechnical constraints' were identified during the design and tender phase of the project, which was completed in November.

The same ground conditions were encountered during the NZ Transport Agency's construction of the Tauranga Eastern Link. Papamoa East is a layer of sand over a former swamp, and liquefaction would likely be a major problem in a decent-sized earthquake.

Any sufficiently large pile of dirt, such as a 6-7 metre VES platform, would risk sinking into the ground unless ground water is squeezed out from underneath first.

The same ground conditions and associated increased costs are likely to be found at the VES localities planned for Wairakei East ($390,000) and Wairakei West ($764,000).

The escape platforms, which would need to accommodate thousands of people, also need to cover more area than originally expected, with original evacuation planning carried out before the introduction of Special Housing Areas.

The increased population will need to be taken into account in the final designs for VES East, West and South.

Council's Long Term Plan (LTP) budget does not take any increase in costs associated with special housing areas into account.

Building all tsunami escape infrastructures was originally budgeted at $2.9 million, with $1.1 million to be spent over the first three years of the LTP and a further $800,000 in 2019. The rest is set to be spent in 2021.

Another option put to councillors today is to buy more land – an option that will be discussed in the confidential section of the meeting.