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consent records show that applications for two projects with a significantly lesser value were lodged but ultimately withdrawn over stated concern about the impact of Section 36(2) title attachment requirements.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)