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Stabilisation of unbound soil layers with geogrids

Current developments

1. Introduction

The DIN EN 13249 ff (12/2016) series of standards regulates the functions, function-related essential characteristics and applicable test methods for geosynthetics. The functions and applications are defined in DIN EN ISO 10318-1 (10/2018 edition) and the associated pictograms are standardised in DIN EN ISO 10318-2 (10/2018 edition).

Fig. 1a: Functions according to DIN EN ISO 10318-2

Fig. 1b: Application Stabilisation according to DIN EN ISO 10318-2 (10/2018)

In addition to the pictograms shown in Figure 1a, the pictogram for stabilisation (Figure 1b) was introduced in the 10/2018 edition of the standard as part of the CE marking of geosynthetic products and manufacturer information.

The stabilisation of unbound soil layers, a technical term that has been used in earthworks and road construction for decades in very different contexts, both nationally and internationally, and is therefore not clearly defined, describes an effect that, in contrast to the function of reinforcement, leads to a summary positive influence on the mechanical behaviour of the soil layer under cyclic loads through separation, drainage and/or the small-scale effect of reinforcement products, leading to a cumulative positive influence on the mechanical behaviour of the soil layer under cyclic loads and thus reducing deformations. A geosynthetic can therefore exert a stabilising effect through the functions of separation, filtration, drainage and/or reinforcement, or combinations of these functions [Vollmert].

The new edition of DIN EN ISO 10318-1 (10/2018 edition) introduces the concept of stabilisation in addition to the already known functions of geosynthetics as

"improvement of the mechanical behaviour of unbound granular material by one or more geosynthetic layers, so that deformation due to applied forces is reduced by minimising movements of the unbound granular material."

2. Short-term tensile stiffness

In principle, the direction of stress on the geogrid plays a decisive role. If a product has a pronounced main tensile direction (with increased strength or increased stiffness), it is referred to as a uniaxial product; if it has similar tensile strength-elongation behaviour in the direction of production (usually the main tensile direction, md) and transverse to it (secondary tensile direction, cmd), it is referred to as a biaxial product. Depending on the manufacturing method, the products initially differ in their product structure and dimensions.

To standardise the material description, reinforcement products are characterised according to their tensile stress-strain behaviour, among other things. According to Müller-Rochholz [MüRo], the tensile strength-elongation behaviour is influenced by the raw material on the one hand and the manufacturing process on the other. The secant slope of the tensile strength-elongation lines at a selected elongation indicates the elongation stiffness J [kN/m] according to Figure 2.

In applications under point and moving traffic loads, geogrids are subjected to stress in several directions simultaneously in the plane of the reinforcement.

If geogrids are stressed diagonally to their pronounced main tensile direction (md and cmd), shear deformations can occur in principle, e.g. in a rectangular basic structure. Due to the manufacturing process and the different stiffness of the connection points between longitudinal and transverse elements, when the products are clamped in air in a tensile test diagonally to the main or secondary tensile direction, the tensile stiffness value may be lower than in the main or secondary tensile direction, as openings can deform unhindered in a shear-like manner.

An essential characteristic is therefore the radial tensile stiffness under diagonal tensile stress or – depending on the geometry of the grid – at any angle of rotation relative to the main or secondary tensile direction.

Fig. 2: Secant tensile stiffnesses of geosynthetics [EBGEO]

3. European Assessment Document

For the marketing and trading of products in accordance with the European Construction Products Regulation (EU CPR), the harmonised standards DIN EN 13249 ff (12/2016) do not contain any essential characteristics for geosynthetics with regard to the stabilisation of unbound soil layers. Consequently, there are also no regulated test methods with which manufacturers can specify characteristic values to explain the performance of a geogrid with regard to its tensile stiffness under biaxial stress.

This can be remedied by the possibility of a European Technical Assessment (ETA), on the basis of which a CE marking for the characteristics of a function of a geosynthetic product or a geosynthetic product series can be obtained by the manufacturer (see Figure 3).

Fig. 3: Paths to CE marking

The basis for an ETA is the European Technical Assessment Document (EAD). As with harmonised standards, this document describes, among other things, the essential functional characteristics and the test methods to be used. These must relate to at least one of the basic requirements listed in the Construction Products Regulation (Table 1). For geosynthetic products, reference is usually made to mechanical strength and stability.

Table 1: Basic requirements for construction works according to the Construction Products Regulation

To describe the stabilisation function, the characteristics listed in Table 2 were developed on the basis of basic requirements Nos. 1 and 7 (see Table 1) of the Construction Products Regulation and agreed with the Technical Assessment Bodies (TABs) in accordance with Article 20 of the EU Construction Products Regulation. Accordingly, Nos. 3, 4 and 8 of Table 2 are of particular importance. No. 3 describes as an essential characteristic the secant tensile stiffness in the main and secondary tensile directions in angular increments of 15° to each. Feature No. 8 requires, for basic requirement No. 7 Sustainable use of natural resources, a life cycle assessment for the manufacture of the products in accordance with DIN EN 15804 and ISO 14025 for production phases A1 to A3 (cradle to gate). Due to the surface properties (roughness) of the grid bars, there is an influence on the frictional resistance of the grid depending on the unbound soil layers. Therefore, the influence of the surface properties of the grid bars on the shear resistance must be recorded (No. 4, Table 2). To this end, shear tests with loads of 20 kPa, 40 kPa and 60 kPa are to be carried out in the large shear box (300 mm x 300 mm) between a surface consisting exclusively of grid bars and the soil layer.

Table 2: Essential characteristics for the stabilisation function according to EAD No. 080013-00-0102 [EAD]

Fig. 4: Left: Tensile testing machine with hydraulic clamping device (max. clamping width 480 mm)
Right: Nominal width for clamping the grid samples and testing angular rotations deviating from the main tensile directions

The tensile forces are determined in accordance with or based on EN ISO 10319 on a tensile testing machine with a wide clamping device (see Fig. 4). This ensures that, in the case of diagonal stress or stress deviating from the main or secondary tensile directions of the grid, a sufficient sample width and number of nodes of the grid are recorded.

4. Results

The test results are used to determine the respective stiffnesses for the strain ranges 0 to 0.5% and 0 to 2.0%. The calculated stiffnesses can be plotted as shown in Figure 5 and the minimum stiffnesses of the grid can be determined for each strain range. In the case shown (raw material PET), the minimum stiffness for the strain range 0 to 0.5% (outer rosette) was determined to be J_0-0.5% = 784 kN/m and for the strain range 0 to 2.0% to be J_0-0.5% = 655 kN/m. Due to the square grid structure and comparable tensile strengths in the main tensile directions, the minimum tensile stiffnesses for the tested grid are at an angle of 45° to the main and secondary tensile directions.

Fig. 5: Determination of radial tensile stiffnesses using the example of the Secugrid 40/40 Q6 (PET) geogrid

Figure 6 shows the tensile stiffnesses determined in Figure 5 in comparison with other geosynthetic products. According to this, the tested woven and knitted grids achieve similar stiffnesses in the diagonal direction as the stretched multiaxial grid, whose characteristic values were taken from a data sheet. The Secugrid grid made of PP (2nd characteristic curve from the outside) is below the PET grid due to its tensile behaviour.

Fig. 6: Radial tensile stiffnesses J_0-0.5% of various geosynthetic products in comparison

5. Conclusions

The tests shown can be used to determine the radial tensile stiffness of a geogrid in its uninstalled state, i.e. in air. This provides a characteristic value that can be used to assess the stabilising effect of a geogrid in an unbound soil layer or base course. The dimensional stability, i.e. the high tensile force absorption with low deformation under simultaneous stress in several directions, can be described as a material characteristic value using the radial tensile stiffness. The integration of the test methods into a European Assessment Document and the subsequent assessment of the geosynthetic products or product series enables the manufacturer to market or sell their products with this property.

Irrespective of this, it remains to be clarified to what extent the magnitude of the radial stiffness influences the verification of the serviceability of an unbound soil layer as a material characteristic value. Further research is needed in this area.

5. Literature

[Vollmert] Vollmert, Lars: On the serviceability of geogrid-reinforced base courses under cyclic dynamic loads. Geotechnical Engineering and Mining Engineering Series. Published by: Institute for Geotechnical Engineering and Mining Engineering at Clausthal University of Technology, Issue 24, 2017

[MüRo] Müller-Rochholz, Jochen: Geosynthetics in earthworks and road construction. Werner Verlag / Wolters Kluwer, Munich/Unterschleißheim, 2nd edition, 2008

[EAD] European Assessment Document: Rectangular geogrid for the stabilisation of unbound granular layers under applied load, No. 080013-00-0102, EOTA, Draft Version, unpublished

[EBGEO] Recommendations for the design and calculation of earth structures reinforced with geosynthetics (EBGEO). Ernst & Sohn Publishers, 2nd edition, 2010