Interface shear strength of geosynthetics: Evaluation and analysis of inclined plane tests

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Abstract

The inclined plane test (IPT) is commonly performed to measure the interface shear strength between different materials as those used in cover systems of landfills. The test, when interpreted according to European test Standards provides the static interface friction angle, usually assumed for 50 mm displacement and denoted as ϕ50stat. However, if interpreted considering the several phases of the sliding process, the test is capable of yielding more realistic information about the interface shear strength such as differentiating interfaces which exhibit the same value of ϕ50stat but different behavior for displacement less than 50 mm. In this paper, the IPT is used to evaluate the interface shear strength of some materials usually present in cover liner systems of landfill. The results of the tests were analyzed for both, the static and the dynamic phases of the sliding and were interpreted based on the static initial friction angle, ϕ0, and the limit friction angle, ϕlim. It is shown that depending on the sliding behavior of the interfaces, ϕ50stat, which is usually adopted as the designing parameter in stability analysis, can be larger than ϕ0 and ϕlim.

Introduction

Liners systems are used at the base and cover of waste landfills and can be constructed combining different components such as geosynthetics and soil. The liner system is designed taking into account the different functions and the efficiency of materials used. In cover liner systems, for example, these components are used (see Fig. 1) to serve one or several purposes, such as water tightness (geomembrane and compacted clay liner), drainage (geospacer), filtration (geotextile) and protection (cover soil). When these systems are built on the sloping sides of landfills they will likely to be unstable since landfill slopes are often steep and many interfaces present relatively low friction and can induce potential slip surfaces.

Several failures of sloping liners have occurred in the past (Koerner and Soong, 2000) and consequently the stability of these composite systems have been increasingly investigated. The direct shear apparatus, which is used for soil testing, is often employed to measure interface shear resistance. However, the test is not easy to be carried out under the low normal stress levels usual in some field conditions such as banks of reservoirs, lined canals and landfills cover. In these cases, the inclined plane test (IPT) is commonly performed to measure the interface shear strength (Chung et al., 2006, Ling et al., 2002, Narejo, 2003, Palmeira et al., 2002, Wasti and Ozduzgun, 2001, Wu et al., 2008). The IPT can be performed according to GRI test method GRI GS-7 (1991) or ISO test method ISO 12957-2 (2005). According to this latter Standard, the IPT provides the interface friction angle that is associated to a conventional displacement of 50 mm and that has been represented by ϕ50stat (Briançon, 2002, Briançon et al., 2002, Lalarakotoson et al., 1999, Purwanto, 1996).

It has been shown (Gourc et al., 2006) that thoroughly study of the diagram of the tangential displacement along the interface (δ), as a function of the inclination (β), enables distinguishing the behavior of interfaces displaying the same standard friction angleϕ50stat.

The present study extends the previous work by Gourc and Reyes-Ramírez (2004) on geosynthetic–geosynthetic interfaces including the behavior of compacted soil and geosynthetic interfaces and more complete test interpretation, especially of the entire dynamic phase of sliding. This has allowed to interpret test results according to the initial friction angle (ϕ0), corresponding to the beginning of movement at static conditions and the limit friction angle (ϕlim), measured during the uniformly accelerated movement that takes place as the dynamic condition of sliding developed.

Section snippets

Inclined plane device

The typical parts of the inclined plane test are shown in Fig. 2. In Fig. 2a, it is illustrated the case of testing of the interface shear strength between geosynthetic, which is installed bonded to the base plane (0.80 m × 1.30 m), and soil, which fills the upper box. In Fig. 2b it is shown the special arrangement for testing the soil and soil interface. In both kinds of tests, the base plane is raised at a constant speed (dβ/dt = 3°/min), measuring the relative tangential displacement (δ) of the

Performance of geotextile–compacted soil interfaces (reference tests)

In the first series of tests, the experiments were conducted using some geosynthetics known for their low friction resistance. These results will be used as reference for the denoted “Reinforced Geomats” (geotextile + geomat). Four different materials were selected (Table 1):

  • Needlepunched nonwoven geotextile (“GT needlepunched”);

  • Heatbonded nonwoven geotextile (“GT heatbonded”);

  • Polypropylene woven geotextile (“GT woven”);

  • High density polyethylene geomembrane (“GM hdpe”).

The tests were repeated

Conclusions

This study leads to the following major conclusions:

  • It was possible to distinguish the friction behavior of different compacted soil–geosynthetics interfaces under low values of normal stress using the inclined plane test.

  • The shape of the inclination angle–displacement graphics is instructive and the difference between “sudden sliding” and “gradual sliding” is easy to correlate to the gap between ϕ0 and ϕlim. In general a “gradual sliding” is more conservative than a “sudden sliding” for a

Acknowledgements

The authors acknowledge the French-Brazilian Cofecub-Capes agreement for financial support.

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    Citation Excerpt :

    In fact, Reyes-Ramirez and Gourc (2003) showed that the standardized testing procedure of the IP test assessed the (standardized) friction angle from a static analysis for conditions that were actually dynamic. Pitanga et al. (2009) showed that the friction angle corresponding to the initiation of sliding and the friction angle corresponding to sliding at constant acceleration were generally lower than the standardized friction angle. However, they did not conclude on the friction angle value that should be considered for design.

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