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Design of geogrid anchoring in systems reinforced parallel to the surface

Abstract

In surface-parallel layered systems, such as surface sealing of landfills, geogrids are used to make these systems steeper. The geogrids absorb high tensile forces, which are anchored at the crest of the embankment in so-called anchor trenches. The previous design approach of EBGEO (2010) contains a number of assumptions that must be considered overly simplistic. Furthermore, it lacks proof of the strength of the connection points between the longitudinal and transverse tension members of the grids, even though these are designed to be subjected to stress. Particularly in view of the long service life of 100 years required in landfill construction, for example, the calculation approach should cover all material components and err on the side of caution.

In view of the large number of existing and apparently stable anchoring trenches, the question arose as to whether the sometimes contradictory effects of the simplifying assumptions made in the regulations neutralise each other or whether the required level of safety is not achieved.

To answer this question, a model for the design of anchor trenches with geogrids was developed. The review of the EBGEO (2010) approach carried out with this model showed that, due to its simplifications, this approach does not provide the mathematically required safety in all anchoring situations. This article therefore shows a way in which the EBGEO (2010) design approach can be adapted in a practical manner in order to achieve a sufficiently safe design for all anchor trenches.

Introduction

Anchoring trenches for geogrids in surface-parallel layered slope systems are designed in Germany according to the specifications of EBGEO (2010). The calculation approach includes assumptions that Müller (2011), among others, considered to be overly simplistic. However, particularly in view of the long service life of 100 years required in landfill construction, for example, the calculation approach must be sufficiently conservative (Koerner 2012, p. 454). As this is not sufficiently provided for in the EBGEO (2010) calculation approach and as there are currently no alternative calculation approaches available, the construction of anchor trenches in landfill construction is currently only permitted with restrictions (BAM 2012).

In order to eliminate the uncertainties in the previous design, a mechanically based interaction model for the design of anchor trenches with geogrids, taking into account all relevant effects, was developed at RWTH Aachen University based on many years of research into the interaction behaviour of geogrids and soil (Ziegler and Timmers (2003), Ruiken (2013)), a mechanically based interaction model for the design of anchor trenches with geogrids was developed at RWTH Aachen University, taking into account all relevant effects (cf. Jacobs 2016). This article presents the model and the review of the EBGEO (2010) approach using the developed model, and as a result, shows a way to correct the EBGEO approach.

Interaction model for the resistance of anchor trenches

Based on the findings of a comprehensive study involving around 120 geogrid pull-out and shear tests, a mechanically based interaction model for the horizontal anchoring of geogrids in the ground was first developed. This one-dimensional, discrete model is shown schematically in Figure 1. The model can be used to determine the resistance of a geogrid to extraction from the surrounding soil and its displacement-dependent mobilisation, i.e. the pull-out resistance-displacement curve. To this end, the model combines various existing approaches and explicitly takes into account the two force transmission mechanisms that occur, "friction on longitudinal tension members" and "earth pressure in front of transverse tension members", which are determined experimentally with pull-out tests on modified geogrid samples. The interaction model for horizontal anchors was successfully validated with large pull-out tests carried out on the same materials in a test rig at Clausthal University of Technology.

For the model equations, the determination of the model input functions, the model calibration and details on model validation, please refer to Jacobs (2016).

Fig. 1: Interaction model (Jacobs 2016)

To anchor geogrids in surface-parallel layered systems, such as surface sealing of landfills, they are often laid in trenches, known as anchor trenches (see Figure 2). To model anchoring trenches, the interaction model for horizontal anchors was expanded to include, firstly, deflections in the geogrid and, secondly, various possible failure mechanisms in the trench. These extensions are briefly described below.

Fig. 2: a) Surface-parallel layered system of a landfill surface seal (image source: NAUE GmbH & Co. KG) and

b) Cross-section through an anchor trench (Jacobs & Ziegler 2017)

Deflection effects occur at points where the slope of a geogrid changes. In the first case, the geogrid under tension is deformed in the direction of the soil support, thereby increasing the normal stress between the geogrid and the underlying soil (deflection pressure). In the second case, depending on the weight of the overlying soil, the geogrid lifts off the soil support together with the overlying soil, so that in this case no force is transferred between the geogrid and the underlying soil (deflection lift).

Furthermore, the model of the anchor trench had to take into account that the various mechanisms listed below can lead to the failure of an anchor trench, see Figure 3 and also compare Syllwasschy & Sobolewski (2008):

1. Pulling the geogrid out of a stable soil block (with resistance forces at the top and bottom of the geogrid

geogrid underside),

1. Sliding of the geogrid together with the soil resting on it over the soil support (with resistance forces only on

the underside of the geogrid),

1. shearing off the crown of the slope and sliding the geogrid together with the soil lying on top in the trench area (with

resistance forces in the shear joint below the embankment crest and on the underside of the geogrid in the

trench area).

Fig. 3: Possible failure mechanisms: a) pulling out, b) sliding and c) failure of the slope crown (Jacobs & Ziegler 2017)

For the determined decisive mechanism, the total resistance of the anchorage is calculated via equilibrium analysis on the cut-away fracture bodies from the sum of the geogrid-soil interaction forces as well as weight and earth pressure forces.

Taking into account the relevant mechanical effects, the model provides the complete resistance-displacement curve of the geogrid anchorage trench under consideration, from which the maximum anchorage resistance can be read.

Verification of the EBGEO (2010) approach

The approach of EBGEO (2010) was reviewed in a broad-based parameter study using the model developed for geogrid anchor trenches. The variables examined in this parameter study (geometry, geogrid, soil and composite properties) and their large spans are shown in Figure 4. For the definition and notes on determining the individual parameters, please refer to Jacobs (2016).

Fig. 4: Varied parameters and their investigated value ranges (Jacobs & Ziegler 2017)

For each of the anchorage trenches resulting from the parameter variation, the decisive anchorage resistance was determined both with the model and according to EBGEO (2010). Figure 5 shows these resistances for all anchors considered plotted on top of each other. A comparison with the model shows that the EBGEO resistance is often greater than the model resistance and thus the EBGEO (2010) approach does not provide the required safety in all cases.

Fig. 5: Comparison of the modelled anchor resistances with those according to EBGEO (2010) (Jacobs & Ziegler 2017)

Design recommendations for anchor trenches with geogrids

The model presented (see the detailed description in Jacobs 2016) can be used to reliably design geogrid anchor trenches.

However, as the use of this model requires increased testing and programming effort, Jacobs & Ziegler (2017) showed a way in which the EBGEO (2010) approach can be corrected to ensure sufficiently safe design of anchor trenches. This is illustrated below:

The required size of the model factor is currently being discussed in the EBGEO working group of Working Group 5.2 of the German Geotechnical Society (DGGT). It is expected to be in the order of magnitude of 1.2.

Summary

Due to the simplifications in the EBGEO (2010) design approach for geogrid anchor trenches, an interaction model was developed for this problem at RWTH Aachen University. The review of the EBGEO (2010) approach carried out with this model showed that the design according to EBGEO (2010) does not provide sufficient safety for all possible anchor trenches.

The model developed can be used for the safe and economical design of geogrid anchor trenches. Furthermore, a way was shown how the EBGEO (2010) approach can be used for the design of anchor trenches, taking into account three specifications, including the application of a model factor. The required size of the model factor is currently being discussed in the EBGEO working group of DGGT Working Group 5.2.

Bibliography

Federal Institute for Materials Research and Testing – BAM (2012). Preliminary guideline for the approval of plastic reinforcement grids for landfill surface sealing. 2nd edition, May 2012, Berlin.

EBGEO (2010). Recommendations for the design and calculation of earth structures with geosynthetic reinforcement – EBGEO. Munich, Ernst & Sohn, 2010.
978 3 433 02950 3.

Jacobs, F. (2016). Interaction model for the design of anchor trenches with geogrids. Dissertation, RWTH Aachen University, Aachen 2016.

Jacobs, F. & Ziegler, M. (2017). Proposals for the design of anchor trenches with geogrids. Conference proceedings of the Geotechnical Engineering Section. Interdisciplinary Forum.

6–8 September 2017 in Würzburg. DGGT, pp. 612–617, ISBN 978-3-946039-03-7.

Koerner, R.M. (2012). Designing with Geosynthetics, Vol. 1, 6th edition, Xlibris Corporation, USA.

Müller, W. (2011). On the design of plastic reinforcement grids for protecting slopes against parallel sliding. Bautechnik, Vol. 88, 347-361.

Ruiken, A. (2013). Stress-strain behaviour of the composite material "geogrid-reinforced soil". Dissertation, RWTH Aachen University.

Syllwasschy, O. & Sobolewski, J. (2008). Special solution for anchoring sealing systems on slopes. 24th symposium "Safe landfills – securing landfills and contaminated sites with plastics", Würzburg.

Ziegler, M. & Timmers, V. (2003). New design concept for determining the anchoring length of geogrids. 8th Information and Lecture Conference on "Plastics in Geotechnics", March 2003, Munich, special issue of the journal Geotechnik.