Tahunanui Slump

Housing Development on a Large, Active Landslide: The Tahunanui Slump Story, Nelson, New Zealand

Paul C Denton
Mike R Johnston
Soils & Foundations Ltd, Nelson

Above: Ground cracking on the Tahunanui Slump, Nelson, July 1929 (Jones Collection, Alexander Turnbull Library, Wellington, NZ. Ref No. 28916 1/2

Nelson has a wide range of slope instability which largely reflects the varied geology of this city. One area of significant instability involves about 26 ha of hillside overlooking popular Tahunanui Beach.

This area, the active Tahunanui Slump, is part of a complex rotational landslide. The active slump, some 700 metres wide, extends from sea level to the crest of the Port Hills and contains about 120 houses.

Significant, extensive movements and related damage are documented within the Tahunanui Slump in the 1890s, 1929 and 1962 and involve roads and houses. Ongoing resurveys of the area confirm that less damaging movements are continuing, albeit at different rates in different areas.

Residential development, which commenced in the 1920s, has been restricted since 1985 by the Nelson City Council who assumed responsibility for the Tahunanui area in 1950. The Building Act of 1991, and more specifically Section 36(2) has changed the basis by which land development can by regulated. The Resource Management Act 1991 specifies legal requirements to identify, inform and protect the public from landsliding hazards. The Tahunanui Slump poses a hazard to development including the possibility that developments on parts of it could be a danger to life. We present the results of our investigations to date.

Above: Geologic cross-section through the Tahunanui Slump showing slices of displaced Port Hills Gravel resting on in-situ Port Hills Gravel and Magazine Point Formation. Also evident is the characteristic benched profile of the slump surface.

Housing Development on the Tahunanui Slump, Nelson – An Update

Paul C Denton
Geo-Logic Ltd, Nelson
Abstract for a paper presented at the LaNZslides ConferenceAuckland, New Zealand, 3 – 5 May 2002

A short history of development, damage and the geotechnical setting of the Tahunanui Slump was presented at an earlier geotechnical conference (Denton & Johnston, 1996)(1).

This large (26 ha) active rotational slump is located in an exclusive residential hillside area of Nelson City which, in the six years since that paper was published, has seen major redevelopment simultaneous with continuing damage. As a result of damaging movements in the 1890’s, 1929, 1962 and confirmed continuing ground movements specific engineering controls had been imposed on further development by the Nelson City Council (NCC) with a prohibition of further subdivision since 1985. The Building Act 1991, and more specifically Section 36(2) changed the basis by which land development could be regulated. At the time of paper presentation (1996) no significant development had taken place since 1970.

Following an in-depth geotechnical investigation(2) for the proposed redevelopment of one property on which the original house was severely damaged by ground movement in 1962 and ultimately removed, a building consent was subsequently issued under Section 36(2) and a new house built. Since then building consents for major site redevelopment of 6 properties, including two new houses, have been issued and work completed with an estimated value of around $5 million. Building consent records show that applications for two projects with a significantly lesser value were lodged but ultimately withdrawn over stated concern about the impact of Section 36(2) title attachment requirements. Damage has continued, including the “writing off” by the Earthquake Commission (EQC) of one house for major settlement damage (in 1998), as well as other damage recorded around the perimeter of the main body of the Slump.

This paper examines, some 6 years and $5 million of property development later:

• Changes in the public perception, acknowledgement and response to the risk highlighted by Section 36(2) [replaced by Section 72 (2004)] • The nature and distribution of on-going damage
• EQC claims and associated issues of information restriction
• The variable standards applied, until recently, by the NCC for geotechnical investigation the Slump

Prepared: 12 May 2002

(1) Denton, P. C. and Johnston, M. R (1996), “Housing Development on a Large, Active landslide: The Tahunanui Slump Story, Nelson, New Zealand” presented at the Geotechnical Issues in Land Development Conference, 16-18 February, 1996, Hamilton (IPENZ Proceedings of Technical Groups Volume 22 Issue 1(G) ISSN 0111-9532)

(2) Soils & Foundations Ltd (1996), “Geotechnical Site Investigation, 33 Grenville Terrace, Tahunanui, Nelson”.
Consultant’s report issued July, 1996 (ref 93141.00)

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