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As the saying goes, what goes up comes down. The reverse also holds true for plastic sewer pipes – especially when they are being tested and studied by experts including a number of professors in this field…
In 1996, three experts in the underground infrastructure of buried pipes called Frans Alferink, Lars-Eric Janson and Jonathan Olliff embarked on what is now called the Design of Buried Plastic Pipes Systems. The aim of this four-year project was to disprove various misconceptions about the performance of such pipes. The result was all of that and a lot more. The knowledge acquired has contributed to our understanding of how plastic pipes behave under certain conditions.
But first the six misconceptions. They were both public and hypothetical:
- Deflection increases with installation depth and with traffic load.
- Pipe ring stiffness is the governing factor determining the performance.
- Pipe looses stiffness with time, the load bearing capacity reduces.
- To predict the structural performance an extensive design method is needed.
- Flexible behaviour is a disadvantage.
- Deflected pipe looses its discharge capacity and tightness.
Faced with these various challenges, a number of objectives were defined which could provide the most important answers to the mentioned misconceptions.
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- Show the relative importance of the parameters.
- Prove flexibility to be a strength instead of a disadvantage.
- Develop a design approach in balance with achievable installation quality and actual behaviour.
- Contribute to the development of the European standards with real field trials / test results.
- Provide material to communicate the project results to the marketplace.
Sponsored by TEPPFA and PlasticsEurope, the project team consulted a wide range of industry experts from across the sector. These leading professors from across Europe were given the possibility to check their calculations for pipe installation. A steering committee was also set up to oversee and support the work in whatever shape and whatever form that was needed.
Full-scale trials with different materials, stiffnesses, soils and installation conditions were carried out in Haarle and Wons in the Netherlands. Tests carried out involved traffic load simulations, various depths, internal pressures and the effects over time. This investigation was also supported by a complete series of laboratory tests.
Furthermore, design exercises were undertaken by consulting leading European experts to compare existing calculation methods with results from field measurements. Testing these calculation methods had become paramount because experts (all part of a working group to develop a unified method) could no longer proceed. The theoretical arguments for one method or the other were exhausted and a reference to the real life situation was needed.
This approach involved a five-step process:
- Consultation Field Trials and final definition of the trials
- Calculation of pipe deflections using different methods
- Establishing field tests and carrying out measurements
- Continuing measurements at different intervals
- Evaluation of results
A series of field tests were executed under different conditions:
| FIELD TRIALS: Installed Pipes |
| Material |
Stiffness
[kN/m2] |
Cover
[m] |
Installed Length
[m] |
| Silty Sand, November 1996 |
| PVC |
2 & 4 |
1.15
1.85 |
120
60 |
| PE |
5 |
1.15 |
45 |
| Steel |
4 |
1.85 |
20 |
| Silty clay, August 1997 |
| PE |
5 |
1.15
3.0 |
60
60 |
A complete set of test data was collected, including soil conditions such as Grain size distribution, Grain shape, Proctor density, Menard test, Cone penetration test, Tri-axial test (clay), Cone-pressiometer test, Impact cone test, Oedometer (device used to test soil compression)
The pipes where characterised with data about Dimensions, Stiffness and Creep ratio. Deflections were measured taking into account:
- time dependency
- under internal pressure
- under traffic load
- under ground water
- Strain under deformation
Three installation conditions were used: good, moderate and none.
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It has been seen that pipe deflection varies from metre to metre depending on many aspects such as workmanship, native soil variations, weather conditions and logistics in the field. Consequently, "the installation variability results in variations in ring deflection along the pipeline for flexible pipes and in variations in bending moments along the pipeline for rigid and semi-rigid pipes."
In terms of Time dependency of the deflection, there was no difference between PVC and steel.
With over 20 well-documented data sets, an excellent insight has been obtained into what is happening for the various installation conditions. A comparison of the various calculation methods with the measured data, showed that none of the methods were accurate over the whole range of the installation window of pipes.
An important aspect for the installers is that ring deflection of flexible pipes is controlled by the settlement of the soil. "After settlement, traffic and other loads do not affect pipe deflection."
Collecting all the data, a design graph has been made which predicts the deflection of plastics pipes (flexible) under most of the installation conditions.
In this graph, the lower line of an area represents the average deflection and the upper line the maximum expected deflection.
To arrive at the predicted final deflection, the initial deflection can be added with a factor C that represents the additional deflection during soil settlement.
It has been demonstrated that the effect of soil loads on flexible pipes results in deflection. The amount of deflection depends of the soil settlement. The flexible pipes simply follow the settlement of the soil. So the better the compaction at installation, the less ground settlement afterwards. So the main determining factor for deflection is the quality of installation.
However, in rigid pipes, the soil will bear all the soil loadings. So the worse the installation, the higher the resulting stresses in the pipes, which can lead to failure and disfunction of the sewer system.
| At Installation |
After soil settlement |
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Moreover, it was shown that there is no need for sophisticated design methods. A simple design graph gives more reliable results then the result of an extensive calculation. It is said that "Thermoplastic pipes are very forgiving". Even if something goes wrong during installation, then this will not result in failure because of the high strainability of thermoplastics"
"Deflection is safety! When pipes are relatively more rigid than the soil, the traffic and other loads have to be resisted by the pipe."
The project clearly illustrated that depth of cover is not relevant and also that traffic load has no significant effect. "Deflection and its variation depends more on the installation quality than on the pipe stiffness."
Therefore, deflection is not an issue. Recommended maximum values according to ISO TR 7073 are 8% initial and 12.5% final. And with regard to capacity, pipes deflected up to 10% thus amounting to only a 2.5% reduction in discharge capacity.
More information can be obtained through the Structural Design under Tools (Under construction)
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