Chris Armstrong – developing a risk-based approach for slips management

Chris Armstrong

Held in Auckland New Zealand, the 2023 IPWEA NZ Conference brought together public asset managers and industry specialists to showcase projects and to demonstrate the application of best practice in public infrastructure which allows our communities to thrive and prosper.

Hosted by the Institute of Public Works Engineering Australasia, the theme for the conference esd Te Whakaohooho i ngā Rangatira o Āpōpō, Inspiring our Next Generation, challenged attendees to share knowledge to ensure our future communities’ needs for safe, efficient, and effective public assets.

Among the presenter line up, Tetra Tech Coffey’s Principal Geotechnical Engineer Chris Armstrong presented his joint paper “Auckland Transport pilot project to develop and test a risk-based approach for slips management using the Transport for New South Wales Guide to Slope Risk Analysis.” This exciting pilot in partnership with Auckland Transport allowed the exlplration into application of the Guide and to explore the opportunities to develop a management programme for slip sites. Here Chris shares his insights into the pilot.

What were the factors driving the need for this pilot?

Our client Auckland Transport maintains and operates a road network of 7,500 kilometres that spans dense urban areas and rural agricultural zones. Land slips within the road corridor present a particular challenge for the management of the network because of the scale of the problem, in terms of the number of at-risk sites, the reactive nature of temporary physical interventions and the often-costly nature of permanent interventions, such as retaining walls.

These challenging issues have been exacerbated by the increasingly higher frequency of extreme weather events, which trigger the slips. This increases the direct consequences on network resilience and safety risk on the network.

When did the study commence?

In 2022, Auckland Transport with Tetra Tech Coffey began a 25 site slips investigation pilot project. The pilot project would test a new process for assessment, investigation and reporting utilising the Transport for New South Wales (TfNSW) Guide to Slope Risk Analysis Manual (the Guide) as the basis for categorising risk.

The pilot project approach was designed to support the network management outcomes. Critically it included integration of the Guide into the process to form the basis of an optimised decision-making framework based on risk outcomes. Decisions at a site and programme level can be made under a consistent framework that allows for an easy comparative assessment of local intervention or network planning scenarios.

What other elements did the pilot include?

The pilot offered an opportunity to reduce subjective decision making in slips management and present a clear longer-term outlook of network management challenges. It had a robust technical basis – including a structured approach to site assessment, geotechnical testing, geotechnical engineering design, 3rd party technical reviews, detailed QS costing and preferred solution identification.

It’s worth stressing that the use of the risk- based outputs of the Guide present an opportunity for both a project level optimisation in implementation of interventions and the ability to form a network level view of asset management scenarios for longer term planning.

While some of the geotechnical work required to assess slopes has necessary technical complexity, the outputs in terms of risk level, intervention options and costs are simply stated and easy to use for decision making. The final project output was a report termed Project Feasibility Report (PFR) (one for each site). It was designed as a document that contains all the assessment, testing, analysis and optioneering engineering information for each site. It contains useful information for an asset manager, designer, geotechnical engineer or network operations manager.

What were some of the key lessons learned?

The Guide did not present any particular difficulty or limitations to be used for risk assessment under local conditions. Ultimately the pilot concluded that The Guide is applicable to local conditions in Auckland and New Zealand.

Through this pilot our client Auckland Transport was able to use The Guide as a possible approach and a potential useful tool for asset managers to manage slips under a risk-based approach.  The approach presents a powerful tool to communicate a RCA management approach or to guide organisation policy.

We were really proud to have presented these insights at the 2023 IPWEA NZ Conference. We look forward to sharing our findings with the Geotechnical community.

If you would like more information about the pilot connect with Chris at


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