Repairing the ravages of time and tide

A return to construction and maintenance techniques similar to those used in the 19th century is ensuring the future of the historic sea walls around Otago Harbour, writes Sam Stevens.

• Sea walls' narrative kept alive

Time, tides and decades of neglect have taken their toll on some sections of the Otago Harbours sea walls.

However, under the guidance of the Dunedin City Council, work is progressing on repairing the damage and neglect.

No-one is happier than Dunedin-based independent archaeologist Dr Jill Hamel, who has researched, photographed and inspected much of the harbour's sea wall.

She was enthusiastic about the structure's historical and cultural significance. Much of it is classed as category one by the Historic Places Trust, the majority of it being built between 1920 and 1940.

"It involved local people and communities . . . It is a vernacular structure and at 40km, it's one of biggest masonry structures in New Zealand . . . made from millions of hand-placed rocks," Dr Hamel said.

But in the middle of last century, traditional construction methods gave way to more modern thinking.

After World War 2, machinery was increasingly used to dump rock to reclaim parts of the foreshore, and the nature of many walls changed as concrete, brick and demolition materials were used to make repairs.

The traditional ideas made their return at the end of last century, but by then there had been nearly 40 years of mending and meddling with concrete, with some 1960s and 1970s sections of wall even repaired with brick and other demolition material.

"By then they were in a shocking state."

Dr Hamel said concrete structures were often ineffective in the long term, as happened in Edwards Bay where about 20 concrete blocks were piled up in the 1960s, only to be undermined by sea water.

More recently, modern methods have proven more effective, such as the rocks and earth compacted with machinery on Portsmouth Dr more than 10 years ago.

But since the late 1990s, stone workers have often been seen setting stone into new and existing walls. "They are in far, far better condition than in the 1980s," Dr Hamel said.

The Dunedin City Council spends between $750,000 and $1 million on walling projects - in construction and maintenance.

The first large-scale construction of a "dry" wall, similar to the historic structures, took place in late 1999 near Vauxhall, when Portobello Rd was widened.

Dunedin stonemason Paul Cahill, now a civil contracts manager for Delta Utility Services, worked on the 1km-long project and says aesthetic considerations were balanced with the need to protect roading assets.

"The Romans were doing it thousands of years ago, so I can't claim ownership of that way of making walls, and it was part of the specification and resource consent.

"Laying walls out that way made sense, but it still involved a steep learning curve. We certainly spent a week or two feeling our way."