Pipeline construction with geosynthetics
During pipeline construction, geosynthetics such as Secutex® protection nonwovens and robust Secugrid® geogrids can be used in several ways to prevent damage to the system at all stages of the project.
In the construction of pipelines, protection sand has become a substantial cost factor due to the volume of material required and associated transportation and installation costs. Secutex® protection nonwovens are a cost effective alternative to this conventional construction method. Depending on the grain size distribution and the existing confining stress, various Secutex® protection nonwoven geotextiles can be used.
For soils with a maximum grain size of 8 mm, a 4.5 mm thick Secutex® protection nonwoven would be suitable, whereas for more gravelly cover materials, a 20 mm thick protection nonwoven may be necessary. As a reference value to aid in the selection of the correct geotextile, the achieved elongation of the geomembrane (as determined in the plate load bearing test) with a minimum confining stress of 2.5 times higher than the on-site final confining stress after 100 hours can be assumed. The actual permissible elongation depends on the requirements of the pipe and can lie between 0.25 % and 1 %.
Secutex® is resistant to all naturally occurring soil chemicals as well as microbiological deterioration. It also has high thermal resistance, which can be particularly important for heating lines etc. Wrapping pipes with Secutex® also provides a barrier that prevents intimate contact between the pipe and the soil, helping to reduce electrolytic corrosion. Upon request, Secutex® protection nonwoven geotextiles can also be supplied in various pre-cut widths. The finished product is easy to handle and quick to install, allowing pipes to be wrapped prior to placement in the pipeline trench.
The stability of a pipeline against flotation in soils that are subject to alternating groundwater conditions should also be considered. Where this is the case, concrete U-channels are often placed over the pipes to provide greater resistance to the uplifting forces. To protect the pipes from the rough surface of the concrete U-channel, as well as high point loading, Secutex® polypropylene (PP) nonwoven protection geotextiles with a mass per unit area of > 1,700 g/m² are used to line the U-channels, providing a cushion effect even under high pressure. The PP raw material has a high long-term chemical resistance, even to the alkaline concrete, and it can be bonded to the concrete with suitable adhesives.
When constructing pipelines over soft or unstable soils, where differential settlement is expected, soil stabilisation measures will be required to ensure a homogeneous load distribution. Traditionally, soil stabilisation has been achieved by replacing soft soil layers with higher strength soils (often up to 1 m), or by the construction of reinforced concrete pads.
Alternative economic solutions for soil stabilisation can be achieved using Combigrid®, a bonded composite product produced with Secugrid® high strength low elongation geogrids and with the excellent separation and filtration properties of Secutex® GRK rated geotextiles. The Secugrid® portion of the Combigrid® product evenly distributes the pipeline loading over a much larger base area resulting in a lower soil bearing capacity requirement.
The physical interlocking between Secugrid® and the coarsegrained fill material absorbs horizontal forces while the Secutex® component of the composite functions as a membrane, separating the finer grained subsoil from the coarse-grained fill soil. This membrane allows the passage of groundwater without the build-up of hydraulic pressures that may damage the pipeline.
Secugrid® is a monolithic laid geogrid of extruded flat bars that is welded at the crossing points. It makes differential settlement in the soil more homogeneous through its high force absorption at low elongation. The interlocking of Secugrid® with coarse-grained base course material provides optimum horizontal force absorption. This eliminates the need for additional soil replacement, such that the necessary EV2-value of the bearing capacity can be economically achieved with little soil material.
Depending on the subsoil and the loads, various construction variations are possible. A structure with either one or two layers may be necessary. Where small or uniform settlement is expected, single or double layers of Combigrid® or Secugrid® may be placed along the base of the trench to provide planar reinforcement. If the settlement stability requirements are particularly high or in case of very soft subsoils, the U-shaped concrete forms with Secutex® protection are preferred over the planar construction method. The vertical layers of Secugrid® with their “disc-effect”, substantially augments the bearing capacity of the pipe trench and creates a kind of rigid beam of crushed stone. At the same time, a connection between the backfill soil and trench wall is created, further reducing settlement.
The long-term performance of the base course can only be ensured if the fine particles of the fine-grained subsoil do not intermingle with the coarse-grained layer. As Secutex® filter nonwoven geotextiles are manufactured from long-term resistant PP, filter stability is assured when the geotextile has a robustness of class 3.
Secutex® nonwoven geotextiles provide protection to pipeline systems against mechanical puncture and abrasion damage. Settlement that can occur in soft subsoil conditions can be controlled using either Secugrid® geogrids or Combigrid® composites. Secugrid® geogrids and Combigrid® also enables installation over soft subsoils possible, making expensive soil replacement unnecessary.