Seismic performance of buried electrical cables: evidence-based repair rates and fragility functions

Bulletin of Earthquake Engineering, Jan 2017

The fragility of buried electrical cables is often neglected in earthquakes but significant damage to cables was observed during the 2010–2011 Canterbury earthquake sequence in New Zealand. This study estimates Poisson repair rates, similar to those in existence for pipelines, using damage data retrieved from part of the electric power distribution network in the city of Christchurch. The functions have been developed separately for four seismic hazard zones: no liquefaction, all liquefaction effects, liquefaction-induced settlement only, and liquefaction-induced lateral spread. In each zone six different intensity measures (IMs) are tested, including peak ground velocity as a measure of ground shaking and five metrics of permanent ground deformation: vertical differential, horizontal, maximum, vector mean and geometric mean. The analysis confirms that the vulnerability of buried cables is influenced more by liquefaction than by ground shaking, and that lateral spread causes more damage than settlement alone. In areas where lateral spreading is observed, the geometric mean permanent ground deformation is identified as the best performing IM across all zones when considering both variance explained and uncertainty. In areas where only settlement is observed, there is only a moderate correlation between repair rate and vertical differential permanent ground deformation but the estimated model error is relatively small and so the model may be acceptable. In general, repair rates in the zone where no liquefaction occurred are very low and it is possible that repairs present in this area result from misclassification of hazard observations, either in the raw data or due to the approximations of the geospatial analysis. Along with hazard intensity, insulation material is identified as a critical factor influencing cable fragility, with paper-insulated lead covered armoured cables experiencing considerably higher repair rates than cross-linked polyethylene cables. The analysis shows no trend between cable age and repair rates and the differences in repair rates between conducting materials is shown not to be significant. In addition to repair rate functions, an example of a fragility curve suite for cables is presented, which may be more useful for analysis of network connectivity where cable functionality is of more interest than the number of repairs. These functions are one of the first to be produced for the prediction of damage to buried cables.

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Seismic performance of buried electrical cables: evidence-based repair rates and fragility functions

Seismic performance of buried electrical cables: evidence-based repair rates and fragility functions I. Kongar 0 1 S. Giovinazzi 0 1 T. Rossetto 0 1 0 Department of Civil and Natural Resources Engineering, University of Canterbury , Christchurch , New Zealand 1 Earthquake and People Interaction Centre (EPICentre), Department of Civil, Environmental and Geomatic Engineering, University College London , London , UK The fragility of buried electrical cables is often neglected in earthquakes but significant damage to cables was observed during the 2010-2011 Canterbury earthquake sequence in New Zealand. This study estimates Poisson repair rates, similar to those in existence for pipelines, using damage data retrieved from part of the electric power distribution network in the city of Christchurch. The functions have been developed separately for four seismic hazard zones: no liquefaction, all liquefaction effects, liquefactioninduced settlement only, and liquefaction-induced lateral spread. In each zone six different intensity measures (IMs) are tested, including peak ground velocity as a measure of ground shaking and five metrics of permanent ground deformation: vertical differential, horizontal, maximum, vector mean and geometric mean. The analysis confirms that the vulnerability of buried cables is influenced more by liquefaction than by ground shaking, and that lateral spread causes more damage than settlement alone. In areas where lateral spreading is observed, the geometric mean permanent ground deformation is identified as the best performing IM across all zones when considering both variance explained and uncertainty. In areas where only settlement is observed, there is only a moderate correlation between repair rate and vertical differential permanent ground deformation but the estimated model error is relatively small and so the model may be acceptable. In general, repair rates in the zone where no liquefaction occurred are very low and it is possible that repairs present in this area result from misclassification of hazard observations, either in the raw data or due 1 Introduction to the approximations of the geospatial analysis. Along with hazard intensity, insulation material is identified as a critical factor influencing cable fragility, with paper-insulated lead covered armoured cables experiencing considerably higher repair rates than crosslinked polyethylene cables. The analysis shows no trend between cable age and repair rates and the differences in repair rates between conducting materials is shown not to be significant. In addition to repair rate functions, an example of a fragility curve suite for cables is presented, which may be more useful for analysis of network connectivity where cable functionality is of more interest than the number of repairs. These functions are one of the first to be produced for the prediction of damage to buried cables. When considering the potential or observed impacts of earthquakes, the predominant focus within the engineering community is towards building damage, because of its potential for casualties. Less consideration is instead given to the impacts of the earthquake on critical infrastructure systems. Although not as important as building damage for immediate life safety, the impacts on infrastructure can be significant during the emergency phase, causing delays to repair work and impeding emergency services operations. In the later recovery phase, sustained disruption to infrastructure services can slow down reconstruction and have implications for business continuity and the health and wellbeing of local residents. An effective disaster management strategy is therefore characterised by detailed assessment of the seismic safety of infrastructure networks, the assessment of the most important infrastructure component and subsequent prioritisation of mitigation works to enhance the infrastructure network resilience to potential hazards. As discussed by Nuti et al. (2010), network safety assessment requires the analysis of a large part of the network to ensure that the interactions between components, and where applicable across networks, are considered. The general procedure is broadly similar for different types of infrastructure networks and involves the modelling of seismic actions; assessment of the structural fragility of network components; determination of the damage state of network components; construction and solution of network flow equations; and evaluation of the ability of the network to meet its customer demand. One of the key elements of such an analysis are the component fragility functions. Fragility functions estimate the likelihood of damage given a specified level of intensity measure (IM), and are the most common tools adopted for characterizing the robustness of infrastructure elements with respect to earthquake hazards (NIBS 2003; Cavalieri et al. 2014a). Whilst numerous fragility functions exist for predicting damage to (...truncated)


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I. Kongar, S. Giovinazzi, T. Rossetto. Seismic performance of buried electrical cables: evidence-based repair rates and fragility functions, Bulletin of Earthquake Engineering, 2017, pp. 3151-3181, Volume 15, Issue 7, DOI: 10.1007/s10518-016-0077-3