BURIED PIPES DESIGN
Below the results of a study into the design of buried thermoplastics pipes is reported. The study is a result of co-operative research, sponsored by the European Thermoplastics pipes and fitting association TEPPFA, and the Association of Plastics Manufacturers in Europe APME.
The project has been proceeded by experts from both the industries, as well as from external organisations. Six external leading experts in the field of pipeline design, not necessarily plastics pipes design, have been involved as consultants to the project.
The steering committee advises to bring the result to the public domain, because the information contained herein is valuable for all who are involved in the design of buried pipes systems, as well as to those who are considering improvements of existing methods or for the development of new methods.
The report summarises the experimental work carried out, and analyses the pipe soil interaction process as monitored in this study. Next to this a simple design graph is presented.
In order to make navigation in the report by the reader a little easier, figures, tables and enclosures are numbered using two digits separated by a dot. The first digit refers to the main heading in which the item is used. The second number is just a counter within that heading.
In this report information is given on the performance of buried pipes, when buried under different conditions. The need for such a study became apparent when at a European level, legislation on how to design buried pipelines became demanding. Next to this, it was realised that still a lot of misunderstanding exists about how flexible pipes, and more specific thermoplastics pipes, behave in real life.
The following five objectives have been defined:
1. Several data sets to become available for validating current and future design methods.
2. One clear design and installation advice to customers.
3. Avoid overkill in installation requirements.
4. Design in balance with feasible installation methods.
5. Increase the confidence in thermoplastics pipes for buried application at the market place.
The result on the first objective is explained in detail in this report, by the presentation of 16 well-documented data sets, and several subsets.
The result on the second one is contained in the presentation of design graphs, in which the type of construction is the main parameter.The exercise with the different, sometimes extensive, design methods has shown that such designs are only appropriate when at the same time soil properties are well determined, and at the same time, the execution of the installation work is done according to the prescriptions. The Paradox became clear that under such circumstances installations are well carried out and monitored. So limit state conditions, conditions under which design really becomes important, are not likely to occur. The balance between design and installation methods has become clear by this, and also the relevance of the type of construction has become clear.
The study has shown the excellent behaviour of thermoplastics pipes under buried conditions, which success is especially caused by the huge strainability of the material. Although very poor installation condition has been used, no failure or stability problems have occurred during the 4 years of monitoring of the buried pipes. The results have shown to line up with earlier published long-term experience of buried plastics pipes.
In this report information is given on the performance of buried pipes, when buried under different conditions. The need for such a study became apparent when at a European level, legislation on how to design buried pipelines became demanding. Although several European experts are working now for over ten years to establish such a method, no method has become available yet. One of their biggest problems is the absence of well-documented experimental data. At the end of the day, amongst many others, the Plastics pipes industry shall have to evaluate the current and future methods on their ability of predicting pipe performance in accordance with actual behaviour. So also for the industry it was felt to be of prime importance that several data sets would become available for that purpose. Another important reason for the study was the fact that the plastics pipes industry has not yet thoroughly discussed the design and installation aspects of their pipes as an industry. It is felt that an unified design and installation advice is another very important objective of this project. Currently a lot of national methods are available and some of them are very detailed whereas others have refrained from such a high level of sophistication. It shall be emphasised that the objective of the industry with this study is to help in obtaining a better understanding of pipe soil interaction in general and for thermoplastics pipes more specific.
It shall be realised that establishing buried pipes systems is not a highly academic exercise. Engineers need tools to predict the performance of the pipeline in order to be able to judge if the circumstances, including the pipe properties, soil properties and workmanship will result in the establishment of the pipeline within a safe application window. This means that the balance between the design method, the input values and the execution of the construction needs to be correct. All the above items will be focussed upon in this study.
It was decided to carry out the work on pipes with a diameter of 315 mm, as this is a rather good average pipe diameter used in many applications. All pipes are of the so-called solid wall types. Solid wall pipes allow easy instrumentation, and next to this the pipes could be studied as pressure as well as non-pressure pipes. The results of the tests on the solid wall pipes can be easily transferred to structured wall pipes as well. Essential is that the structure of the structured wall pipes stay in shape when deflecting. The product standards however utilise a so-called ring flexibility test, to check upon this aspect. Where appropriate, guidance will be given about how to utilise the information for structured wall pipes.
Sand as well as clay was chosen as embedment material, in order to be able to judge the difference between granular versus cohesive soil types.
2. Installation practice
Traditionally, pipes are buried by open trench methods. Although nowadays also, so-called NO-DIG methods are used, still the major part of the installation work is carried out using the traditional methods. Most of the installations are performed using granular materials as sidefill, in combination with some kind of compaction in order to avoid huge settlements of the surface after installation.
However, installations are performed in cohesive materials and organic materials as well. Although in general they are in many countries not recommended, good experience has been gained when using flexible pipes made out of strainable materials.
ISO as well as CEN have produced several versions of 'Recommended practices for the design of buried pipes'. Next to this, many national standards as well as manufacturers recommendations exist. These documents all recommend a certain installation procedure.
In practice, non-predictable circumstances affect the true installation of the buried pipes. For instance the compaction level that can be reached for sand depends on its water content. So already depending on weather conditions one obtains a better or worse installation than anticipated. In many cases the soil profile changes over a short distance. Using the as dug material again as embedment of the pipe, does not make an extensive design of any pipe very realistic. In order to obtain more reproducible installation conditions it is sometimes recommended to import special embedment materials, like gravels and selected stones. This however makes the installation more costly. Another negative, but in most cases not considered, effect of imported backfill is that the new soil has another density then the native soil, causing on the long term other effects, as settlement differences along the pipeline. Therefore, using the as dug material as much as possible and providing flexibility and strainability in the pipeline system is the best (robust) solution for obtaining a proper functioning pipeline system. Robust here refers to the fact that the long life performance of the pipe is hardly affected by misfits in design and or installation.
3. Features of thermoplastics
For the application of buried flexible pipes of all materials, the following pipe characteristics are of importance:
- Pipe ring stiffness
- Corrosion resistance
- Elongation at break
The pipe ring stiffness is one of the aspects that will be considered in this study and as such further discussed.
Corrosion resistant properties are in general very good for all plastics materials and the pipe will therefore not deteriorate when placed in the soil. It is this corrosion resistant that provides a firm basis for any statement on durability.
With regard to the strainability properties one needs to distinguish between elongation at break as found in a tensile test, the strain developed under creep conditions and the strain under relaxation conditions. For pressure pipes the strain controlled by the stress is the covering factor, although the buried pipe transfers part of the load (internal pressure) to the surrounding soil. For gravity pipes it is known that after installation the pipes are not loaded by a constant load, but they become under a condition of constant strain, giving rise to a stress relaxation process. For the latter case several studies have been performed in Germany and Scandinavia. Tests carried out in Sweden (1) showed that PE and PVC pipes can be subjected to significant strain without any risk of cracking. Constantly deflected pipe samples with strain of the pipe wall exceeding 10 % have been tested for more than 9 years without showing any cracks. In Germany, Hoechst has tested PE pipes, which have been expanded circumferentially by 5 %. After 40 years of testing there is still no sign of cracks in the material. It is concluded that for thermoplastics pipes like PVC and PE no strain limit exists under stress relaxation conditions; however it is proposed to limit the strain to reasonable values anyway. Table 3.1 summarises the values proposed in Reference 1.:
Table 3.1: Allowable strain in buried gravity pipes
|Pipe material||Allowable strain [%]|
For pressure pipes another test exists to determine the design values. For these pipes so-called Internal Hydrostatic Pressure testing is used to determine the failure curve, which on turn is the basis for the determination of the design values. In this test the pipe is free to expand and gives as such a worse case design condition.
The fact that the design has to be treated differently for gravity than for pressure pipes is because of the visco-elastic behaviour of the material. For linear elastic materials a direct relation between stress and strain is valid. For visco-elastic materials this is not the case. The performance under load is history and time dependent. When the load consists of a pre-scribed displacement than the holding force will decline with time. The issue described here is called "stress relaxation". In order to satisfy the traditional relationship between stress and strain, one needs to apply a lower (apparent) modulus. As stated it is an apparent modulus, since any new loading of the sample utilises the true modulus again.
The same type of process exists when the load is a constant load instead of a pre-scribed displacement. In such a case the strain will increase. Again in order to satisfy the traditional relationship between stress and strain, one needs to use the apparent modulus. Also in this case the material will respond with the true modulus to any new load. The situation of creep, as the above description is called occurs during Internal Hydrostatic testing of pipes. Essential is that the load is able to follow the creep of the material. As soon as this is hindered, the stress relaxation process becomes active.
It is for the above reasons that the design stress determined in the Internal Hydrostatic pressure test can not be applied straight away to pipes that are not fully loaded to the creep condition. In any other case then free creep, the strain limits as shown in table 3.1 shall be applied.
The Minimum Required tensile Stress values valid for determining the design stress at free creep conditions are shown in table 3.2.
Table 3.2: Minimum Required Stress values (LCL values)
|Pipe material||Long term [mPa]|
*Note : X refers to ISO 12162
4. Experimental work
4.1 Test sites
The project group deliberately carried out the tests at a field instead of in a laboratory in order to fulfil the most important requirement of this study, which is that the result shall reflect real circumstances and installations. Therefore it is needed that a regular contractor is involved to carry out the tests, and that the pipeline length is sufficiently long to allow the proceeding of a normal installation. The European experts as well as the project group emphasised that the study shall at least reflect the less ideal installation circumstances, because especially under these conditions limit state situation are to be expected if any.
Therefore it was decided to create installations at the border of the normally accepted installation window.
Two different sites were selected for that purpose. One located at the foot of a hilly area, which area was created during the second ice period some 130.000 years ago, when ice was pushed from Finland /Sweden to the south. About 1 million years ago this area presented the coastline of the European continent. The soil consists of sand with small to moderate amounts of silt. The other site is located near the seaside to current where the soil consists of clay.
The project was steered by experts from the industry. A small project group consisting of the project manager and two external experts in the field of design of buried pipes supervised the project.
Next to these consultants, European design experts, where contracted, to provide the design using the different established methods, as well as to assist in the results evaluation.
The persons working in the different groups are listed below:
|Mr. F. Alferink||Wavin M&T (Project manager)|
|L.E Janson||SWECO (Supervisor)|
|J.L. Olliff||Montgomery Watson (Project consultant)|
|Mr. I. Björklund||KWH(Chairman)|
|Mr. M. Giay||Rehau|
|Mr. H. Leitner||Solvay/APME|
|Mr. T. Meijering||Polva Pipelife|
|Mr. J. Nury||Alphacan|
|Mr. C. Gonzalez||ITEPE|
|Mr. D. Scharwächter||Uponor|
|Mr. L. Wubbolt||Omniplast|
|Mr. T. Jones||Wavin|
|Mr. A. Headford||Durapipe|
|Mr. J. Kallioinen||Uponor|
|Mr. F. Alferink||Wavin M&T(Secretary)|
European design experts
|G. Leonhardt||(D+L partners)|
|W. Netzer||University of Innsbruck|
|J. Olliff||Montgomery Watson|