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.