Liquefaction Report For Blenheim

A geo-technical report on land to the east and south-east of Blenheim that was earmarked for possible future residential expansion has shown that it would be susceptible to liquefaction in the event of a significant earthquake.

Further testing is to be done on land to the north of the town where similar doubts have been raised about the stability of the soils.

The Marlborough District Council is aware that developers and land owners considering subdivision need clear signals about the best direction for expansion.

The northern and eastern town perimeter sites were identified in Marlborough’s South Urban Growth Strategy, a town planning project designed to find suitable land which could be re-zoned to meet the needs of long-term population growth.

However, the geo-technical report prepared for the District Council suggests the high cost of treating the land and of constructing appropriate foundations, coupled with the costs and risks to water and sewerage services and roads, makes these areas unsuitable for rezoning:

• land to the east of Riversdale off the end of Dillons Point Road, bounded by the Opawa River out to Rowberry’s Road
• land at St Andrews, between Alabama Road and State Highway One
• land between Alabama Road and Tavera Street to the east of Redwood Street.

 

The future of a site to the north side of Old Renwick Road – 35 hectares on the edge of Springlands – is not so clear. Initial testing suggests the ground conditions vary across this area and further investigation has been recommended before re-zoning decisions are made.

Land-owners are receiving letters advising them of the contents of the report.

Council staff have begun the process of examining other potential areas which may be suitable for housing expansion and welcome inquiries from landowners on the perimeter of the current urban area.

The Urban Growth Strategy was prepared before the Christchurch earthquakes which graphically illustrated the impact of liquefaction on low-lying residential land.

“Since the Christchurch earthquakes the perception of risk has changed; everyone is far more conscious of liquefaction as we have been able to see the results of it across relatively recent housing developments,” said Councillor David Dew, chair of the subcommittee reviewing the Council’s Resource Management Plans.

Since then too, new guidelines recommending geo-technical investigation have been issued by the Department of Building and Housing.

“Council has taken a risk-averse approach because we’ve seen what has happened in Christchurch.

Before any further decisions were made, we wanted to know where the risks were,” he said.

The geo-technical report says all four areas on the edges of Blenheim would be vulnerable to ‘lateral spreading of land adjacent to waterways’ and it was unlikely that treatment of the ground would be economic. Standard residential foundations would be inadequate in housing subdivisions at any of the locations, including north of Old Renwick Road.

Councillor Dew said while the Council would be unlikely to re-zone these areas in the face of this kind of evidence, landowners still wishing to develop would be entitled to seek a private plan change. In such a case, however, they would face the extra costs of dealing with liquefaction issues in relation to services and building practice, he said.

“The test results will help inform landowners to judge the point at which it might be economic to consider developing their land, given the design and construction methods that would be required on this kind of soil.”

The Council’s Resource Management Plan review subcommittee will be asked to consider the implications of the liquefaction investigations.

Population projections suggest another 2500 houses will be needed in Blenheim over the next 20 years.

DAVID DEW
CHAIRMAN RESOURCE MANAGEMENT REVIEW SUB-COMMITTEE