Liquid Soil - A Technical Challenge

Apr 03, 2009

In the last decades there have been many innovations in the field of plastic pipe systems, especially the bauku GmbH had a strong share with developments like the PROFILEEN - safe or the PROFILEEN - drain clean. However in the field of installation methods and installation materials for sewer pipes there has been no real movement until lately the expression "liquid soil" played an important role in more and more symposiums and expert articles. If someone has a closer look at the definition of this word, it is quite clear that this is not a standardized expression for a certain kind of product and that the products of different suppliers have also a much different product behavior. The bauku GmbH faced this challenge and made a first experience in the static design for this application and the practical use at the job site. As such a pioneer work requires a close connection between the pipe producer and the supplier of the liquid soil, the company rss was chosen for the project.

Advantages of liquid soil
The use of liquid soil is a possible solution, wherever a bad quality soil makes it impossible or hard to use the existing ground for the backfill in the pipe zone. The rss is analyzing the existing soil first and uses special additives to improve the quality, to reach a defined mechanical strength in the finished product. A lack of space and as a result a narrow trench may also be a reason for this method. Liquid soil is able to fill gaps at the pipe, where a conventional compaction machine would simply fail.
A careful and engineered mixture of the liquid soil enables the user an optimized stability of the ground. The e-modulus values in the ATV calculation for the soil can be used in a uniform way for the complete pipe zone - There are just a few possibilities for mistakes during the installation and the conventional compaction procedure is not necessary any more.
Furthermore the rss - liquid soil offers the advantage, that the use of cement for the hardening process is more or less eliminated, the soil stays flexible even after the hardening and the structure of the soil is quite close to good conventional soil groups. This behavior makes it easy to work later in the pipe zone, for example for a house connection which has to be added, years after the job site has been finished. In contrary, liquid soils with a basis from cement can only be moved with heavy equipment later. The flexibility of the soil is a big advantage for flexible pipe systems like the PROFILEEN - pipe, as pipe and soil supplement each other in an optimal way. Rigid embedding materials will not allow the plastic pipe to take structural overloads by a defined and controlled deformation and the pipe may be damaged, when the rigid embedding material around the pipe starts to crack. The rssliquid soil offers enough possibilities for such a controlled deformation, in spite of the relatively high e-modul values.
PROFILEEN - pipe systems and rss - liquid soil (or equivalent products) would be an ideal combination, if some principal differences are taken into consideration.
Special system requirements
The use of liquid soil asks for a carful planning of the job site and the working steps on the job site. The contractor has to prepare himself intensive in the handling of this embedding material. That is why the company rss is not only offering the product, but is recommending a complete training package for consultant engineers as well as for contractors, before the liquid soil is used for the first time. The support covers even the training during the installation phase on the job site. The preparation of the liquid soil and the unloading with mixing trucks must meet the speed of the installation of the pipes exactly. As the embedding material has a higher specific weight than the pipes, the pipe is under a buoyancy load during the backfill of the trench. Without a special installation method the pipes would just swim on the surface of the liquid soil. The pipes have to be fixed against the trench support (steel support) over the complete length of the pipe crown, to avoid such a swimming. However independent from the embedding material, flexible pipes allow a small deformation. The ATV A 127 mentions a short term deformation of up to 3 % and a long term deformation of up to 6 %.
Even if PROFILEEN - pipe systems very often just show a much lower deformation in praxis, it is just a simple fact that this deformation now is not happening at the pipe crown, but at the pipe bottom.
The consultant engineer has to realize that in projects with just a very small slope in the pipeline, the use of liquid soil may lead to a local counter slope, just by creating a little deformation at the pipe bottom. To avoid this risk, a flexible pipe has to be fixed not only at the pipe crown, but also at the pipe bottom (at least pipes in a diameter range where an inside work is possible). This can be simply done with a support in the inside of the pipe, this support will be linked to the fixing point in the pipe crown and as a result will fix the pipe bottom as well.
Of course these "system requirements" can be erased easily by a careful preparation of the installation procedure at the job site.
Realization in praxis
One example for a successful planning of the job site work with liquid soil is a project that we started in year 2008 together with the contraction company Sonntag in the City of Bergheim. The installation of the pipes DN 1400 was done in working steps, which have been defined by Sonntag based on their training with the rss people.
The pipes have not been positioned on the trench bottom, but on small hills made from liquid soil. The exact leveling of the pipes could be done easily by changing the form of the soil hills. The pipes were supported on the complete pipe crown, but from the inside they were just fixed locally. A dangerous buoyancy force and a resulting bending of the unsupported pipe bottom was prevented by a step by step installation of the liquid soil. Following this special method, the pipes installed last in the trench were just filled up to the pipe axis, the pipe length already filled in the last step were filled up to the pipe crown and the pipes from phase I were filled up to the ground level.
Static specialties
The ATV A 127 is based on the embedding conditions like described in the DIN EN 1610 - liquid soil is not part of this standard. Of course the norms and standards are only able to show the well-known state of the art and they are up to 10 years behind the technical development sometimes. The supplier of this new embedding material has the responsibility to describe his product sufficiently, especially the static parameters have to be defined. The liquid soil of rss can be calculated for the long term behavior easily with ATV, as it can be compared with a good compacted normal soil, just the e-modulus for the soil has to be confirmed by the supplier. This is different with the installation procedure, often the main load for the pipe is happening here, resulting from the buoyancy load of the liquid soil. In addition bending moments in axial direction of the pipe are created, if the fixing points in the pipe crown and the pipe bottom are just done locally. The ATV only knows a 2 dimensional check, for liquid soil a 3 dimensional calculation is necessary. Of major importance is as well the time for the hardening of the soil. The e-modulus of HDPE under constant load will go down from approx. 1.000 N/mm2 to 380 N/mm2 in 24 hours. If PP is used it will sink from 1.900 N/mm2 to 650 N/mm2. The supplier of the liquid soil has to define the time frame exactly, as well as the static values in form of an expert opinion. A detailed installation guide has to be part of the package. Without this preparation a static calculation of the pipes according to ATV is not possible.
Liquid soil is not the solution for all problems, but it can be of great help for difficult installation conditions. This soil can reduce the installation costs and can improve the quality of the embedment. It is extremely important to choose the right type of liquid soil, as there are big differences in the hardening process and the stability values. All necessary static details and the installation recommendation have to be supplied to the pipe producer in due time, as the pipes have definitely to be designed for this special soil type. As the ATV calculation just has insufficient descriptions regarding the liquid soil, an expert opinion regarding the static values should be part of the package of the soil supplier. If these preconditions are respected, the combination of the flexible liquid soil and the flexible PROFILEEN - pipe system build a high quality soil-pipesystem.
Project details
project: collecting main Oberaußem, City of Bergheim
project value: 700.000,- €
product: DN 1400: 150m, DN 1200: 950 m
consultant: Peil, 52351 City of Düren
contractor: Sonntag, 56281 City of Dörth
time frame: 02.2008 - 11.2008

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