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True or false – pipes must be stiff to support the weight of soil and traffic above them?
Rigid materials such as concrete and clay settle into the ground under the loads from above and around the pipe. Prevailing design practices are tailored towards rigid pipes and this conservative option ensures that the pipe is weighty enough to accommodate the loads likely to be encountered.
Flexible materials such as plastic sewer pipes make use of their lower stiffness to cleverly accommodate the loads through deflection of the pipe wall. These highly engineered materials can transfer the loads into the surrounding soil and provide pipeline engineers with the best technical and most cost-effective solution along the complete pipeline.
Of course, no two installations are the same and even over the course of one pipeline, conditions vary from metre to metre. That’s down to many contributing factors from the natural (native soil variations, gradient, weather conditions) to the manmade (workmanship, quality of soil compaction). So how well can plastic pipes adapt to these seemingly endless variations?
TEPPFA’s four-year study into the suitability of plastic pipes for sewer, storm water and other civil engineering applications investigated in the field and through advanced lab testing how good, moderate and no compaction affected ring deflection and the ability to support external loads.
Most deflection happens in the first few weeks as the installation is completed, loads applied and the soil becomes fully compacted. All loads are then being transmitted to the surrounding soil and nothing – not even heavy traffic loading – will cause additional deflection of the plastic pipe. The study showed that good workmanship ensures that the deflection of pipes remains within the tight limits set in Europe (8%) and even tighter requirements which we have set ourselves in the U.K. (5% for highway applications and 6% for adoptable sewers). This is true for all flexible pipes regardless of pipe ring stiffness and the natural conditions in which the pipeline sits.
As loading conditions change, for example as traffic loading increases, the surrounding soil simply accommodates this making plastic pipes the perfect long-term solution for civil engineering applications.