Contractor

Guidelines for resilient underground utilities

John Pfahlert, CEO, Water New Zealand and Philip McFarlane, Opus water sector leader, asset management. 

WE CAN’T PREDICT when an earthquake will occur but it is possible to manage infrastructure to ensure that as little damage as possible occurs and that underground utility networks are able to function during and after earthquakes.

In order to recover quickly from major earthquakes, underground utilities must be able to absorb, accommodate, and recover from the effects of a seismic event in a timely manner.

It’s with this in mind, following the Canterbury quakes, that Opus International Consultants and GNS Science worked together to develop new guidelines and comprehensive standards for underground utilities in a specifically New Zealand context.

The project, Underground Utilities – Seismic Assessment and Design Guidelines, was funded by the Ministry of Business, Innovation & Employment (MBIE).

The guidelines have been designed in compliance with the National Infrastructure Plan, Civil Defence Emergency Management Act (2002), Local Government Act (2002) and Health and Safety at Work Act (2015).

They are aimed at providing a user-friendly framework for asset managers, designers and local councils to integrate seismic resilience into buried infrastructure and help drive infrastructure resilience into businesses.

Specifically the guidelines enable practitioners to:

  • 
Assess the vulnerability of existing underground utilities to seismic events.
  • 
Identify and prioritise measures to improve the resilience of current networks. Resilience is defined as making a network less susceptible to damage incurred by seismic activity.
  • 
Design and install new utilities that have an acceptable level of resilience to earthquake events.

These guidelines integrate data generated and models based off the Canterbury quakes along with international research. This has produced a unique New Zealand specific model for seismic resilience design. While the model can be applied to all underground services, the report mainly focuses on wastewater, stormwater and potable water infrastructure.

Preventative actions and smart design are essential

The seismic resilience framework breaks down the complex challenge of infrastructure resilience upgrades into a simple, easy-to-follow process.

Establishing a target post-event Level of Service (LOS)

Establishing a target post-event level of service is an iterative process. Frameworks are provided for balancing the legally required level of service needed by importance level (IL) 3 and 4 infrastructure with flexible, consultation based levels for IL 1 and 2 structures.

The cost benefits of resilient infrastructure

The guidelines report points out that the cost-benefit ratio of resilient infrastructure has been estimated by the United Nations to be up to 1:10.

In Canterbury, the quakes caused extensive damage to 300 kilometres of sewer pipes and 124 kilometres of water mains. The cost to rebuild all horizontal infrastructure was estimated, in mid-2013, at just over $3.3 billion.

Some of the findings from the Canterbury quakes and incorporated into the guidelines include:

  • 
The earthquake motion and the way the ground responds has far more influence on damage than shaking and other forces resulting directly from earthquakes.
  • 
Axial forces along pipes cause the majority of the damage. Most of the damage occurs at pipe joints. Bending and other transverse loading tends to only cause damage in brittle pipes.
  • 
All utility materials sustained damage in the earthquakes but modern flexible pipe material generally suffered a lot less damage than older, more brittle pipe materials.
  • 
Larger pipelines typically sustain less damage than smaller pipelines. Service pipe connections sustain the most damage. Even modern PE service pipes sustained significant damage in the earthquakes. This was attributed to failure at mechanical couplings where inserts had not been used.
  • 
Gravity pipes located in areas where liquefaction or lateral spread occurred experienced significant differential ground deformation, causing their grade to be reduced and dips to occur. This affected all pipe materials.
  • 
The performance of the ground influences the ability of the system to remain in service. Experience in Christchurch was that if the ground liquefied then the wastewater system blocked regardless of the amount of damage sustained. This is because of sand and silt entering through gully traps and manholes even where pipelines were undamaged.
  • 
The time it takes to restore service is affected by both the amount of damage incurred and by the ground conditions. Ground conditions affect ground stability and liquefaction during aftershocks which hinders access for repair and inspection.
  • 
The quantum of damage sustained to non-critical pipes often controlled the time it took to restore service. For example, the lifting of the boil water notice on the potable water system was largely governed by the time it took to repair the multitude of small leaks that occurred on service connections rather than the condition of the larger pipelines that the services were connected to.
  • 
Alternative means of providing service, such as the provision of portable toilets can be used but they take time to install and the public can only tolerate them for so long.
  • 
Restoration of service involves several phases. It may take many years to fully restore service to the pre-earthquake condition. Priorities and needs change as restoration progresses through these phases.

The guidelines specify increasing levels of design sophistication based on the importance level of the utility. For instance, most utilities will not require any further specific design but utilities in the two most important categories will require the equivalent static design method and finite element modelling.

Assessing the system’s vulnerability

The guidelines establish parameters for earthquake design using peak ground accelerations that can be readily derived or calculated. The damage of earthquake movements to a utility can then be estimated using fragility function processes and break rate modelling provided in the guidelines.

The response of the ground to earthquake activity is also taken into account by procedural outlines. This is a lesson taken from the Canterbury earthquakes which revealed permanent ground deformations significantly influence the amount of damage to buried utilities, the extent of service lost and the required time to restore service.

Adopting procedures outlined in the guidelines can mitigate or avoid damage due to liquefaction, surface rupture and slope. More importantly the limitations of damage predictions can also be assessed using the guidelines, and appropriate redundancy can be designed into the system.

Improving resilience of existing systems

This section uses economic analysis to quantify resilience improvement measures by reducing exposure to hazards, increasing the speed and effectiveness of response, increasing the flexibility of the system to adapt and improving the robustness of utilities. The guidelines illustrate how this can be achieved in a cost-effective way entailing minimal expenditure.

Providing new utilities that are seismically resistant

The guidelines provide structure for resilient utilities by strategically avoiding areas of poor ground performance, limiting consequential damage to other utilities as well as providing sufficient redundancy, robustness and ease of repair.

The road to recovery is paved with prior planning

The most effective post-emergency systems have been carefully planned well in advance and can move away from an emergency stage quickly. The use of this guide comprehensively enables service providers to do so in a uniquely New Zealand way best suited to tailor recovery to local needs.

• For more information go to Underground Utilities – Seismic Assessment and Design Guidelines

This article first appeared in Contractor April 2017.

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