Rigid and Flexible Pipe Design, Installation and Whole Life Cost

Understanding the key differences between pipe materials and their design philosophies is important and often overlooked. Failings here can lead to long-term impacts on infrastructure and its whole-of-life cost. This paper explains some of these issues and the cost implications of them.

Some think that Flexible (plastic) pipes and Rigid (concrete) pipes are installed the same way. Actually the design concepts are entirely different. Rigid pipelines rely mainly on inherent pipe strength. Flexible pipelines rely principally on the soil pipe interface and a slight deflection to activate the support of the bed and surround (embedment) of the pipe to give the installation its strength. Only a slight deflection is appropriate as defections over 5-7% are indicative of failure (the exact percentages are a function of pipe, material and diameter).

Some think that the design lives of the different pipe types are the same but they are not. Although the materials may last a long time, the ring strength of plastic varies with age and designers have to calculate short term and long-term strains as the plastic creeps. Regression data is used to estimate the strength and deflection at 50 years. Concrete gains strength with age. In addition to the material strength, the flexible pipeline relies on the soil interface to give it strength. Deflection changes this interface and can weaken the installation if excessive. Additionally, any disturbance of the material surrounding the pipe from excavation nearby or from seismic activity can render the design assumptions invalid.

There is a common misconception that plastic pipe has a cheaper installed cost than concrete pipe. Plastic Pipe is often thought to be easier and therefore cheaper to lay because it is lighter. Yet, limits on safe manual handling, mean that both types of pipe will likely require lifting equipment to handle them on site. Weights do differ significantly e.g. a 6 m long DN 300 SN 4 PVC pipe weighs around 70 kg whereas an approximately 2.5 m long DN 300 Class 2 concrete pipe weighs around 240 kg but both of these exceed safe manual handling limits. If installed safely and correctly to the Australian and New Zealand Standards, then concrete pipe may have a cheaper installed cost than plastic.

Whilst there is no fundamental reason to exclude either pipe type from consideration in a project the differences in design and construction requirements, which have an impact on installation and lifetime costs, need to be understood and considered during design and construction to provide the best-value installation.