Unlike concrete pipeline systems, plastic pipes use a different mechanism to deal with earth movements and dynamic / static loads imposed on the pipeline as the flexibility of the pipe structure enables it to redistribute the loads through to the surrounding soil. As known, plastic pipes usually deflect during the process of distributing these loads. Deformation can become progressive throughout the service life of a plastic pipe, reaching alarming levels and leading to the pipes’ failure. A range of information on deflection is provided. There are also a number of case studies exploring the proportion of deformed plastic pipes in a number of existing sewer networks. Scroll the page downwards for more detailed information on deformation.

What Do you know About Deformation? The ability to ‘deflect’ under pressure is a major characteristic of visco-elastic pipes made of thermoplastics, known generally as flexible pipes. Gerard Mulhern (2006) defines flexible pipes as pipes that can deflect more than 2% without cracking. They have little inherent strength and depend mainly upon a properly installed soil embedment to achieve the required design strength. This factsheet offers a brief description of deformation in thermoplastic pipes, and includes a number of case studies.

Case studies

The Real Scale of HDPE Plastic Pipes’ Deformation, a US Case Study A study survey carried out in 2002 investigated levels of deformation in plastic pipe networks installed since 1987 in a number of American states. The results were significantly alarming and showed what could be the real scale of the problem in these states as 69% of the pipes inspected had deflections exceeding 5%.

Evaluation of HDPE Pipe Performance in a number of Construction Projects in Kentucky and Ohio Departments of Transport at Kentucky and Ohio investigated around 1.4 km of HDPE pipeline systems laid in different locations in the two states in the late 1980s/ early 1990s. It was found that many of the pipes investigated would not pass a 5% deformation test and most of the sites inspected have pipe sections that would not pass a 10% deformation test. Both investigation reports referred to the problem associated with progression of deformation over time.

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Texas University Arlington research on Evaluation of HDPE Pipelines Structural Performance in Texas In a Study carried out by Texas University in Arlington, the structural performance of 22 HDPE pipelines laid throughout the State of Texas was investigated. 38% of the pipelines inspected had excessive deformation (over 5%), moreover it was found that the average deformation value for all pipes was 6.8%. The study clearly indicated that the structural health and integrity of the installed HDPE pipelines tested are generally below structurally acceptable levels of serviceability.

WHAT IS DEFORMATION?

Deformation/deflection is the change in pipes’ internal diameter that results when specific loads are applied to the pipes. Unlike other types of elastic polymetric materials (e.g. thermosetting plastics, elastomers, rubber) thermoplastics are known to have a large deformation capacity (Farshad, 2006): The ability to ‘deflect’ under pressure is a major characteristic of visco-elastic pipes made of thermoplastics, known generally as flexible pipes. Gerard Mulhern (2006) defines flexible pipes as pipes that can deflect more than 2% without cracking. They have little inherent strength and depend mainly upon a properly installed soil embedment to achieve the required design strength. Deflection/ deformation is part of the mechanism flexible pipes employ to redistribute the loads to the surrounding soil. As the modulus of Soil Reaction is typically higher than the magnitude of the pipe stiffness the resistance to deflection of the pipe is mostly dependent upon the strength of the soil.

Buried sewer pipes are affected by a number of loading types: the main loadings usually come from earth pressure and traffic load. Partial settlement, upheaval, pipe crossings, and ground motions are additional load cases that affect the pipe system. The equilibrium of a plastic pipeline structure is based on how the entire structure deals with these different load cases, a change or imbalance to that equilibrium may lead to additional/ unaccounted-for loads affecting the elastic component of that system (pipes) leading to buckles, excessive deformation, and, consequently, failure. Links between deformation/ cracks and failure is something the plastic industry has been trying to dismiss for some time. However, in a conference held by the plastic pipe industry itself (Pipes 2005) a service life predictability model was presented – the model was based on the ultimate impact of live loads on the service life and eventual failure scenarios for plastic pipeline systems. Lang (2005) made a clear link between progressive deformation and specific crack developments leading to failure of plastic pipeline systems.

It should be noted that this is not the only reason why deformation occurs, according to Farshad (2006) pipe systems may undergo certain deformation capacity and stiffness changes caused by aspects such ageing and temperature.

References:

• Farshad, M. (2006) Plastic Pipe Systems: Failure Investigation and Diagnosis. First Edition 2006. Elsevier Ltd. ISBN-13: 978-1-85-617496-1.
• Lang, R. (2005) the phenomenon of slow crack growth in PE pipe materials – applicability and limitations of various test methods. PIPES 2005: “Highlighting the latest developments in markets, standards and technology in the plastic pipes industry” Conference, 1 – 2 February 2005, Brussels, Belgium.
• Mulhern, G (2006) Protecting Yourself as a Gravity Pipe Municipal Engineer. Institution of Municipal Engineering of Southern Africa (IMESA) Conference 2006.

A family of polymers that includes plastics such as Polyvinyl Chloride (PVC), Polyethylene (PE), and Polypropylene (PP).