Experimental and DEM investigation of geogrid–soil interaction under pullout loads
Introduction
As an important reinforcing material in practice, geogrids have been widely used in dealing with various geotechnical problems, e.g. stabilizing the earth structures, increasing the bearing capacity of base courses, reducing the rut depth in flexible pavements, etc. (Yoo and Kim, 2008, Leshchinsky et al., 2010, Al-Qadi et al., 2011, Al-Qadi et al., 2012, Heerten, 2012, Yang et al., 2012, Indraratna et al., 2013, Koerner and Koerner, 2013, Qian et al., 2013, Santos et al., 2013, Allen and Bathurst, 2014, Santos et al., 2014, Abdesssemed et al., 2015). The geogrid reinforcing effects are performed via the interaction of geogrids together with the surrounding soil. Hence, the geogrid–soil interface behaviour has been regarded as a significant factor to investigate geogrid reinforcement mechanisms (Lopes and Ladeira, 1996, Palmeira, 2009).
Fig. 1 shows a typical application of geogrids in reinforcing an earth wall in practice. Failure might happen within the geogrid reinforced structure along the potential slip plane. In order to investigate the failure mechanisms of geogrid reinforced structures, many researchers have conducted various laboratory tests on the interface behaviour between geogrid and soil as well as the general compound behaviour of the composite material (Palmeira and Milligan, 1989, Ochiai et al., 1996, Moraci and Recalcati, 2006, Naeini et al., 2013, Ruiken, 2013, Arulrajah et al., 2014, Liu et al., 2014, Ferreira et al., 2015, Qian et al., 2015). Among all the experimental approaches, the pullout test has been regarded as a better way to investigate the geogrid–soil interface behaviour (Yogarajah and Yeo, 1994). Plenty of pullout tests have been carried out based on various geogrid and soil types (e.g. Bergado and Chai, 1994, Palmeira, 2004, Teixeira et al., 2007, Moraci and Cardile, 2009, Palmeira, 2009, Sieira et al., 2009, Giang et al., 2010, Abdi and Arjomand, 2011, Esfandiari and Selamat, 2012, Moraci and Cardile, 2012, Zhou et al., 2012, Mosallanezhad et al., 2016). Those studies have illustrated the influences of various factors on the geogrid responses under pullout loads, which provides helpful hints in designing geogrid reinforced earth structures. Besides the conventional pullout tests, Ezzein and Bathurst (2014) developed a novel pullout test apparatus to evaluate the geogrid–soil interaction using transparent granular soil, which allows to visualize the relative horizontal displacement between geogrid and its surrounding soil.
The discrete element method (DEM) (Cundall and Strack, 1979), which has particular advantages in capturing the kinematic behaviour of discontinuous media at a microscopic level (Zhang and Thornton, 2007, Stahl and Konietzky, 2011, Zhang et al., 2013b), has also been used to investigate the geogrid–soil interaction under pullout loads. McDowell et al. (2006) modelled the interaction between ballast and geogrid by simulating pullout tests and the optimum aperture size of geogrids subjected to pullout loads was discussed. Zhang et al. (2007) conducted numerical pullout tests on loose and dense soil samples to study the effect of compaction on the pullout response. Ferellec and McDowell (2012) investigated the influence ballast shapes on the ballast–geogrid interlocking effects. Chen et al. (2013) and Tran et al. (2013) reproduced experimental pullout tests using the DEM and the simulation results provided detailed responses of geogrid and soil. Chen et al. (2014) investigated the effects of clump shapes on both the pullout resistance and the distribution of contact forces. Stahl et al. (2014) investigated the interlocking effect based on modelling the behaviour of soil mobilization and geogrid deformation. All the above DEM investigations provide detailed observations at the geogrid–soil interface. However, due to the varying elastic–plastic properties of geogrids and soils as well as the high sensitivity of their interaction to many influencing factors, the compound stress–strain behaviour of geogrids embedded in soil is very complex. Therefore, the geogrid reinforcement mechanisms up to now have not been described conclusively and it is still an essential issue to investigate the geogrid–soil interface behaviour.
In this paper, the results of both experimental and numerical pullout tests, which were carried out with different modified geogrids embedded in granular soil, have been presented. Those geogrid samples were modified with different numbers of transverse members. Compared with 3D modelling, 2D numerical simulations have particular advantages of providing insights into key mechanisms with less computational time. Hence, the commercial software Particle Flow Code (PFC2D), which was developed by Itasca (2008) based on the DEM, has been used for the numerical investigations. The limitations of 2D numerical modelling are discussed in the later part of this paper. The aim of this study is to investigate the geogrid pullout responses with different numbers of transverse members and to provide researchers more detailed insights into the geogrid–soil interaction at a microscopic scale.
Section snippets
Testing apparatus
Fig. 2 shows the sketch of the pullout apparatus used in this study. The pullout box was made of steel with the inside dimensions of L/W/H = 435/300/200 (unit: mm). Different from the flexible surcharge loading systems recommended in the ASTM test standard (ASTM D6706-01, 2001, Huang and Bathurst, 2009), the vertical load was applied on top of the specimen through a rigid plate in this study. It should be noted that the vertical forces on both the top and the bottom plates were recorded with
DEM investigations and analyses
In order to gain further detailed insights into the geogrid–soil interaction under pullout loads, the discrete element software Particle Flow Code (PFC2D) has been used in this study. PFC2D utilizes rigid entities (particles and walls) and soft contacts in the numerical modelling. Newton's second law of motion is used to update the positions of the rigid entities due to the forces acting on the soft contacts. The contact forces are then updated based on the force–displacement law (Itasca, 2008
Limitations of the numerical modelling
The numerical simulations in this study were carried out using 2D software, which has inherent limitations in investigating the real 3D problems, e.g. a 2D model dilates with a much higher rate than a 3D model since the 2D plane assembly can only dilate in the vertical and horizontal directions but not in the cross-plane direction (Rothenburg and Bathurst, 1992). The granular soil was modelled with circular disks in this study and large frictional coefficients have been used to compensate the
Conclusion
This study investigated the pullout behaviour of geogrid embedded in granular soil with both experimental and numerical approaches. In the laboratory tests, geogrids with different tensile stiffnesses have been used and those geogrid products have been modified into specimens with different numbers of transverse members. In the discrete element modelling, the models and the input parameters of the granular soil and the geogrid have been calibrated by direct shear tests and tensile tests,
Acknowledgements
This work was financially supported by the China Scholarship Council (No. 2010630201) and the geogrids used in this study were provided by NAUE GmbH & Co. KG. The authors appreciate the above supports.
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