Every engineer should be keenly aware of the following relationship:
RISK = Probability of Failure *Consequences of Failure
Engineers involved in the design and construction of buried infrastructure should be especially aware of this relationship. Buried infrastructure includes storm sewers culverts, sanitary sewers and watermains. All of these pipes are expected to perform as both a conduit and a structure. Much attention is always given to the conduit aspect of the design; however, these conduits are also expected to perform as structures throughout their service life. A service life of 75 years is a common expectation for the buried infrastructure that we are building today. A pipeline must continue to function as a conduit and as a structure for all of those 75 years. The consequences of failure include fatalities and injuries, damage to vehicles and property, traffic congestion, detours and delays, environmental impacts and reconstruction costs.
Buried infrastructure by its very nature is out of sight and, because it is out of sight, it is often also out of mind. It can be difficult to realize that there is a problem until it is too late and a catastrophic failure has occurred.
The modes of failure for pipe include:
Reinforced concrete pipe is generally accepted as having a service life of at least 100 years in culvert and drainage applications. Unlike flexible corrugated steel pipe or plastic pipe, rigid reinforced concrete pipe has inherent structural strength and is much less dependent upon the surrounding soil support, known as bedding. Hence, it is very unlikely to fail suddenly due to wash-out or loss of bedding materials.
The Canadian Concrete Pipe and Precast Association (CCPPA) recognizes that there is a range of applications for various pipe products such as concrete, plastic, steel and fibreglass. CCPPA asserts that not all pipe products should be used in every application. The CCPPA advises engineers to be cautious and diligent when dealing with pipe salesmen, marketing rhetoric and glossy marketing brochures introducing new but unproven pipe products. A salesman’s job is to sell pipe and meet his sales target for the month. An engineer’s job is to design and construct infrastructure that will last over 75 years.
The following matrix can be used as a guide to assess the level of risk for a specific project:
Once the engineer has evaluated the level of risk, the engineer can then respond to that risk in a number of ways. There are four generally accepted responses to RISK:
AVOIDANCE: Specifications are written to disallow certain products for certain applications. In some provinces in Canada, only concrete pipe is allowed under highways with the highest traffic counts and under other key transportation arteries.
TRANSFERENCE: The risk is transferred to a party who is more capable of handling the risk such as a contractor or a supplier. Examples of transference are asking the contractor to submit design calculations for the pipe that are sealed by a professional engineer, and supplying a trench detail, also sealed by the professional engineer.
MITIGATION: This lessens the impact of a specific risk for a particular pipe product. Many municipalities in Canada limit the size of plastic pipes to 600mm maximum diameter.
ACCEPTANCE: The recognition of a specific risk by the engineer and a decision NOT to take action to deal with that risk. The engineer needs to ensure that a comprehensive design is completed and construction of the project is done strictly in accordance with the specifications.
Engineers involved in the design or construction of buried infrastructure should always be sensitive to the risks associated with the project and should conduct a risk assessment as appropriate. They should not be swayed by sales rhetoric and sales pitches, glossy marketing brochures and apparent lowest initial cost.
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