Flaw in $10.9m wall plan

Bureaucrats have ditched plans to use plastic as a replacement for a seawall after a report reveals it would have likely left their town with a $10.9 million bulge into the sea.

Work on the aging Paekakariki seawall, north of Wellington, has been delayed after checks revealed the new wall might ‘‘flex’’ under pressure.

Work on the $10.9 million project was due to start this summer, but Kapiti Coast District Council confirmed it had been delayed at least six months after a review uncovered problems.

The work would replace a 35-year-old timber seawall, past its expected lifespan, which ruptured as recently as February and carried an average repair bill of about $35,000.

The plan had been to use plastic, under a wood veneer, to form a retaining wall as part of the replacement structure.

The seawall protects The Parade – a road between the beach and houses. About 140 metres of the existing wall to be replaced is considered at high risk of failing. About 480m is at moderate risk, and 340m at low risk, according to the council.

But a peer review commissioned by the council into the plans revealed the plastic could bulge under Paekakariki’s conditions.

Infrastructure service group manager Sean Mallon said in worst case scenarios the plastic sheets could flex from the combined pressure of overtopping waves and earth behind it.

This could create a ’’bulge effect’’ running along the wall, in potentially one continuous deflection, he said.

‘‘Certainly it wouldn’t look very good.’’

Mallon said the plastic was used in sea walls before but not in the conditions it would face on the rough Paekakariki coastline.

He said he did not know the exact cost of any new options, but there would now be another season of bad weather when the existing seawall could come under pressure – with added maintenance costs.

The council gained consent for the new wall in May 2016, to build a concrete, timber and rock wall with an expected lifespan of 35 years, at a cost of $10.9m.

The plastic was investigated because it would have saved money over the heavier materials, he said. He said it was unlikely the work would now start before mid2018.

Deputy mayor and ward councillor for Paekakariki, Janet Holborow, said any project delay ’’almost inevitably results in some costs increasing’’.

Mayor K Gurunathan said it was good that the staff had doublechecked before they started building with plastic.

Paekakariki Community Board deputy chairman Steve Eckett said the plastic wall could have been cheaper than concrete and timber.

But during the design process engineering consultants began to realise that there would be more weight behind the wall than expected.

‘‘So under certain conditions there was a possibility that the plastic could deform.’’

He said the town refused to have a rock seawall and had consulted heavily with the council for the planned replacement.