NZ Geomechanics News

Proposed Slope Stability Guidance Modules

An Update


MBIE and the New Zealand Geotechnical Society have produced six Earthquake Geotechnical Engineering Guideline Modules covering a range of geotechnical topics. However, these do not specifically address slope stability. In 2019 the NZGS management committee agreed to start work on a seventh module for the assessment of slopes under seismic loading. It quickly became apparent that addressing the seismic case without first dealing with the static case would be problematic, given the varied practice currently identified in the industry.

This paper provides an update on the development of the proposed slope stability guidance outlined above. It provides some background as to our current challenges as an industry dealing with slope stability hazards, what the state of practice is within New Zealand, member concerns with current practices, and the NZGS management committee’s proposed way forward. The majority of the text below is extracted from the Committee’s Position Paper, which has been developed to act as a starting point for discussions with Stakeholders.

1 Background

Landslides and slope instability are a significant natural hazard in New Zealand, and are more common here than many countries because of our terrain and unstable geology. Residential, water and transport infrastructure in New Zealand has been developed into steeper landscapes, especially in urban areas, due to the lack of level space available. These areas are vulnerable to slope instability, either naturally inherent or created by human development and activity. These issues may be already evident under normal conditions but are expected to be exacerbated and widespread in seismic events. They will become more and more evident in the future and may be compounded by the effects of climate change (Roberts, 2020).

Schuster (1996) noted that landslides are responsible for considerably greater socioeconomic losses than is generally recognised, because they often occur as one element of multiple-hazard disasters and are reported in combination with the triggering process (e.g., earthquake, flood, hurricane, volcanic eruption, and bush fires that expose the soil). The Earthquake Commission in 2006 stated that “…more than 70% of the weather-related insurance claims are most likely related to landslips…”. 

1.1 Landslides and Slope Instability in the New Zealand Context

The Motu Economic and Public Policy Research report in 2020 indicates that between 2000 and 2017 over 15,000 weather-related claims were settled (completed), amounting to approximately $450 million dollars.  Of these, about two-thirds (about 10,000) of the pay-outs are because of land damage.”  The Motu report also states “…. we deduce that more than 70% of weather-related insurance claims are most likely related to landslips.”

The landslide hazard and potential consequences were distinctively demonstrated during the November 2014 Kaikoura earthquake.  Over 80 landslides occurred along and immediately adjacent to the coastal transport section of the transport corridor, severely affecting State Highway 1 (SH1) and KiwiRail’s Main North Line (MNL).  Lengths of around 10 km to the south of Kaikoura, and 14 km to the north, were most heavily affected. Due to the time of the Kaikoura earthquake and the lack of residential development in the area most affected, there were not any fatalities as a direct result of slope instability. The toll could have been very different if the earthquake was during a busy day. However, the socio-economic cost for the region and New Zealand in general was substantial.  State Highway 1 was closed for a year while the recovery costs for the transport corridor were on the order of $1.4 billion. If a residential population had been impacted by the earthquake generated landslides, significant life loss would most likely have occurred.

From a life loss perspective, over 600 deaths in New Zealand have occurred since the 1840s because of landslide-related causes (Rosser et al, 2017) – more than from earthquakes and tsunami combined.  As an example, the Murchison earthquake of 1929 killed 17 people – 16 because of the landslides it generated, as evidenced by photograph 1. 

Photograph 1 Landslides generated by the Murchison earthquake, 1929

Despite this, landslides have historically caused few deaths in New Zealand relative to the number of events that have occurred because there have been few settlements in mountainous terrain and the population density has been relatively low; although it is also possible that some deaths due to slope instability may have been attributed to the triggering event (for example, earthquake or rainstorm).

This is changing.  As the society evolves, and as the need of space for residential development in urban areas increase, we are pushing subdivisions and development increasingly into areas that have significant landslide or slope instability potential.  Together with the effects of climate change, our existing (and already high) risks from landslides and slope instability are likely to only increase in the future. 

For overseas experience, the Thredbo landslide disaster in Australia in 1997 (photograph 2) is a characteristic example.  This landslide killed 18 people and sparked 10 years of work by the Australian Geomechanics Society to improve site investigations and risk assessment processes for slope instability in Australia, resulting in issuing the AGS Landside Risk Management Guidelines.  New Zealand has not done likewise, even though there is no doubt that several similar situations exist here (The Priscilla Crescent landslide in Kingston, Wellington in 2014 for example).  

“It also seems to me that the geotechnical community needs to evaluate the way it conducts its investigations to ensure that investigations whether of conditions of roads or the suitability of building sites, be undertaken having regard to the potential effect of instability on human life and the risk of loss of life or injury” – Mr Derrick Hand (Coroner to the Thredbo Inquiry)

Photograph 2 The Thredbo landslide, Australia, 1997a

2 Current New Zealand State of Practice

A Local Government NZ think-piece on natural hazards in 2014 stated:

Natural hazards and associated risks are not managed under a single statute. Rather, their effective management relies on the interplay of many statutes. Most of these statutes are enabling in nature, meaning they provide powers for agencies (mostly local government) rather than prescribing detailed requirements. Under this framework, effective management of natural hazards requires the many players exercising powers and responsibilities to do so in a coherent and co-ordinated way. The legislative picture is, however, a patchwork of laws from different eras and to some extent different philosophies and subject to different legislative purposes. The policy guidance within these statutes remains very high level and hence much is left to the discretion and judgement of those at the sharp end of implementation. Further, the integration of these statutes has not necessarily been thought out in a fully considered way. This is evidenced by (for example) the many different definitions of natural hazards included across the various statutes.”

Little has changed in the eight years since this think-piece was issued.  

Since the Ministry of Works was disestablished and privatised in 1988 there is a wide-spread recognition by the geotechnical engineering industry that there has been a systematic underinvestment in geotechnical training and guidance development in New Zealand.This issue was highlighted by the Canterbury Earthquakes Royal Commission (CERC) in their 2012 report with perhaps 50 of the 189 recommendations directly or indirectly related to geotechnical issues.  Many of these CERC recommendations were later addressed under the leadership of MBIE and the support of the New Zealand Geotechnical Society (NZGS) by the development ofthe Earthquake Geotechnical Engineering Practice Series and associated training.  However, the focus of these geotechnical guidance documents was largely around earthquake design and liquefaction induced damage, although rockfall hazard mitigation and retaining wall design guidance was also prepared.

It was also apparent early in the CERC guidance project to both MBIE and NZGS in the development of the geotechnical earthquake engineering module series that there was also a need for an updated geotechnical earthquake engineering landslide guidance document that had a New Zealand focus.  The large earthquake induced landslides and subsequent heavy rainfall triggered landslides of slopes damaged by earthquakes has only enhanced this viewpoint.

Leaders in the geotechnical industry have noted the assessment of the static assessment of slopes is not consistently done well in New Zealand.  In a similar manner to Module 6 retaining wall guidance, it is considered that any guidance document on assessing the stability of slopes under seismic loads should first address good practice in assessing static stability first, then move on to cover the assessment of the stability of slopes under seismic loads.

2.1 Existing documents

A literature search was undertaken on what national and international guidance exists covering the following general themes:

  •  planning/land-use
  •  static assessment of slope stability
  •  seismic assessment of slope stability
  •  emergency response
  •  case studies of landslides
  •  a variety of action plans

One type of slope instability hazard, rockfall, is a nationwide hazard to dwellings and infrastructure.  Following the widespread rockfall that occurred during the 2010-2011 Canterbury Earthquake Sequence in the Port Hills, this hazard has been addressed by the Earthquake Geotechnical Engineering Practice Series.  This efficiently addresses the rockfall hazard from slopes and the design of protection structures, however it covers only a small aspect of potential slope instability issues that occur in New Zealand. It is noted this document, similar to the Earthquake geotechnical module series has regulatory status by being issued by MBIE as guidance under Section 175 of the Building Act.

The most recent NZ based document addressing slope instability in general is planning focussed (in particular, Saunders et al, 2013).  This document does not have regulatory status.  With some exceptions, NZ based guidance is generally nearly 20 years old and is not up to date with the latest scientific and technical knowledge with respect to co-seismic assessment of stability.  The literature search suggested that there are many good practice documents available, both NZ based and internationally, ranging from land use planning, investigation, model development, stability assessment risk assessment and mitigation that can form a good basis for an updated and modern guidance document.

The literature search raised the question about what an updated NZ based guidance document should cover that is not already covered in readily available national and international textbooks and guidance, and whether the production of a new slope stability guidance document would have a significant impact. The authors believe reference to a ‘single source of truth’ for geotechnical practitioners and consenting authorities would provide clarity around both good practice and the minimum standards required to address slope instability risks, without stifling innovation. It would improve consistency and prevent confusion caused by information spread across different sources and documents, as well as ensure that the latest science and knowledge is incorporated through regular maintenance and update of this single source of truth. Such guidance would also bring in New Zealand specific advice and knowledge to ensure suitability for our regulatory regime and geology.

3 Current industry practice 

There are many examples of excellent – even world leading – practice in New Zealand. Our flexible regulatory regime and many international engineers and geologists means we have a strong skills base and we can be innovative in our approach. However, not all practice is at this level. Many slope stability assessments, particularly at the more basic level, lack technical rigour and are sometimes significantly flawed in ways that are not immediately apparent to consenting authorities. The causes of this inconsistent performance may span from the appropriate education and training of engineers, to ongoing professional development, and the availability of adequate industry standards to ensure a consistent approach to the problem and subsequently relatively uniform results. 

It is generally recognised that recent geo-professional graduates need a substantial amount of post graduate training and mentoring in order to be effective in the private sector. Some large consultants have developed formalised systems to achieve training and development of their technical staff. 

For generalist civil/structural engineers this has caused problems when it comes to recognising the warning signs of unstable terrain.  In order to address this, some Councils have put restrictions on which geospecialists and engineers can submit consent applications in higher hazard terrain e.g., Tauranga City Council.

In addition, the CPEng (Geotechnical) and PEngGeol Body of Knowledge (BOKS) documents prepared by NZGS have been developed to outline a consensus view of senior practitioners of the skill considered appropriate to work on higher risk geotechnical projects including slope stability assessment.

In  2019, NZGS sent a survey to its members requesting their feedback on a wider scope document. This survey identified several concerns about slope stability practice in New Zealand that could be resolved with more clear guidance. A few key messages arose from the feedback to this survey, as indicated from responses received to the following three questions (10 questions were asked in total).

Q1.  Which topics in the geotechnical engineering arena do you feel cause most problems in NZ?

Out of 50 responses received, 42% of respondents concluded that slope stability caused them the most concern

Q2. Can you describe examples of practices observed in New Zealand related to slope engineering (either seismic or static) that did not give a good result? (Selected responses)
  • Yes; poor attention to geological conditions, assuming material parameters and blindly modelling.
  • Yes – Inadequate understanding of groundwater conditions (or what causes failures)
  • Yes. I’ve seen single slopes analysed by different firms/ People all giving considerably different results due to different training/ references.
  • Often the geological and groundwater interpretation is poor. The method of analysis can often be less than optimum or even wrong.

In summary, the responses received showed a generallack of consistency and in some cases a lack of understanding and/or lack of appropriate engineering geological input exists within the geotechnical community in New Zealand.

Q3. Would a guidance document on slope stability be more useful in the technically focused format of the earthquake geotechnical engineering modules, or in the broader format of the ‘Planning and engineering guidance for potentially liquefaction prone land’?

4 Way forward 

The NZGS committee believes that development of a slope stability guidance document for geo-professionals is now essential.  It is proposed to develop the guidance as a series of Modules or Units, which replicate the structure used in the Earthquake Geotechnical Engineering Guideline Practice Series.  

4.1 Proposed Module 1

4.1.1 Objectives

The purpose of the proposed first project is to produce an overarching Module (Module 1), defining the general principles under which other documents will sit, and to scope subsequent modules to provide more detailed guidance on specific topics that are identified as high priority.  Where appropriate, the additional modules will reference existing documents (for example, sections of the AGS Landslide Risk Management Guidelines, or the planned update to the Saunders and Glassey (2007) Guidelines for assessing planning policy and consent requirements for landslide prone land.

The proposed guidance is intended to set a reliable baseline for good practice for different slope instability problems, and to ensure that improvements are implemented into the guidelines after reasonable consultation with the NZGS members.

4.1.2 Stakeholder Engagement

We have had discussions with MBIE and EQC, who have indicated their qualified support for development of the proposed Landslide Guidance Module 1 although MBIE have also indicated it is not their current priority.  We will continue to lobby MBIE on this issue as having some form of regulatory support, similar to the Earthquake module series would be the ideal.  With a level of support now secured, the committee is starting and continuing discussions with stakeholders with a view to secure funding for development of the Module. Following this, we will need the expertise of the NZGS members to develop and review the Module as it evolves.

NZGS will also give thought to complementary activities such as training as associated with the guidance, promotion of the new landslide data base, pushing to increasing availability to valuable data sets such as the LINZ Section 72-74 land hazard notice dataset and EQC land claim data set.

The recent landslide prediction models developed by GNS with government funding support are also a valuable resource and relevant to this discussion.

The NZGS committee is determined to make this a success.  So please watch this space!


Roberts R.C (2020) Climate change, sustainable development and geotechnical engineering: A New Zealand framework for improvement. NZ Geomechanics News, Issue 100, December 2020

Schuster R.L (1996)  Socioeconomic Significance of Landslides.  in Landslides: Investigation and Mitigation TRB Spec Rept 247 pgs 12 – 35 revealed#:~:text=The%20impact%20on%20New%20Zealand’s,at%20%24450%2D%24500%20million.

Rosser B, Dellow S, Haubrock S, Glassey P (2017). New Zealand’s National Landslide Database. Landslides DIO 10.1007/s10346-017-0843-6—Summary-and-Recommendations

Saunders, W.S.A, Beban, J.G, Kilvington, M (2013).  Risk -based approach to land use planning.  GNS Science Msc Series 67

Tags : #Landslides#Slope failure#Slope stability

NZ Geomechanics News
Eleni Gkeli, John Scott, Richard Justice, Ross Roberts
NZ Geomechanics news>Issue 103 - June 2022

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