Infrastructure refers to the fundamental facilities and systems serving a country, city, or area,
including the services and facilities necessary for its economy to function. Infrastructure systems typically
consist of interrelated constituent systems forming what is known as system of systems (SoS). Infrastructure
systems present numerous challenges throughout their lifecycles. This paper addresses one of these
challenges that is presented during operation, when managers need to report ‘how well’ the system is
performing and finding ways to address the consequences of unexpected events that often degrade the
intended performance. This state of system ‘wellbeing’ will be referred as system integrity (SI).
When applied to infrastructure systems this paper proposes a model suggesting that system integrity is a
combination of operational performance, safety and resilience which become the set of criteria to assess SI.
Each of these three factors is assessed by considering their specific ‘key performance indicators’ (KPI):
Operational KPIs (KO), Safety KPIs (KS) and Resilience KPIs (KR). KOs could include KPIs for quality of
service, reliability, availability, maintainability and cost; KSs could include KPIs for number and severity of
accidents; and KRs could include KPIs for level of disruption and time for recovery to acceptable levels.
In accordance with the proposed model system integrity can be defined as the “state of a system where it is
performing its intended functions safely without being degraded or impaired by changes or disruptions in its
internal or external environments”. When the system achieves the state of perfect condition its system
integrity is 100% or 1.0. Infrastructure systems may operate at lesser levels of system integrity (SI) and it is
important to assess and monitor SI to make sure the system is operating within acceptable levels and to
envisage ways to improve SI in the event of unexpected situations.
The proposed model based on the on-going operational performance, safety and resilience of the each
constituent system in the SoS is then developed into a method that applies the Analytic Hierarchy Process
(AHP) (Saaty 1994) to create a quantitative assessment derived from qualitative and quantitative information.
The method assumes that there is a set of KPIs for each of the agreed assessment criterion for operational
performance, safety and resilience which were defined, agreed and can be individually assessed. The method
uses qualitative experience-based information to weight the KPIs for each of the three criteria relatively to
each other using AHP to obtain the overall assessment for operational performance, safety and resilience for
each individual constituent system. These three criteria are also compared and weighted using the same
approach to determine their level of contribution to SI which is then calculated using the actual value
measured or estimated for each KPI. The method is then expanded to calculate the SI for SoS by applying the
concept of ‘supermatrix’ proposed by AHP to address systems with feedback loops where individual
components and assessment criteria influence each other. The SoS SI method is then applied into a
hypothetical urban transport system for illustration purposes.