Influence of particle shape on the shear strength and dilation of sand-woven geotextile interfaces
Introduction
Interfaces between soil and reinforcement have a great influence on the performance of geosynthetic reinforced structures such as retaining walls, slopes, and embankments (e.g., Madhavi Latha and Murthy, 2007, Woon and Kim, 2007, Palmeira, 2009, Portelinha et al., 2014). On this subject, Krieger and Thamm (1991) showed that the mobilized friction angle between soil and geotextile is a key factor in assessment of failure in reinforced soil walls. Moreover, Finite Element Modeling and recently, Discrete Element Method simulations carried out by Karpurapu and Bathurst, 1995, Rowe and Ho, 1996, Desai and El-Hoseiny, 2005, Basudhar et al., 2008, Bhandari et al., 2008, Ferellec and McDowell, 2012, Wang et al., 2014, Wang et al., 2016 [among others] have revealed that soil-geotextile interaction plays a significant contribution in the stability, serviceability, and bearing capacity of earth structures. Goodhue et al. (2001) reported that the peak friction angle of sand–geotextile interfaces is about 65–75% of the sand peak friction angle. Jewell (1996) suggested that for a wide variety of interfaces between woven and non-woven geotextiles and soil, the interface friction angle may vary from 65 up to 100% of the soil friction angle. In the absence of detailed information, it is recommended assuming soil-geotextile interface friction angle limited to 65% of the soil friction angle (e.g., Look, 2007, Das, 2016). Therefore, accurate estimation of the soil-geotextile interface behavior by means of laboratory and numerical methods still deserves consideration.
The mechanical behavior of soil-geotextile interfaces depends on the physical soil properties (e.g., mineralogy, particle shape and size distribution, particle mean size, density, and degree of saturation), as well as the geotextile characteristics (e.g., material type, fabric, texture, tensile strength, failure strain, and water permeability) (Giroud et al., 1985, Athanasopoulos, 1993, Lee and Manjunath, 2000, Goodhue et al., 2001, Khoury et al., 2011, Anubhav and Basudhar, 2013, Esmaili et al., 2014, Hatami and Esmaili, 2015, Ferreira et al., 2015, Vangla and Latha, 2015, Vieira et al., 2015, Prasad and Ramana, 2016, Vangla and Gali, 2016). Among various experimental techniques developed for the investigation of soil-geosynthetic interaction, pullout and direct shear tests are the most common ones. However, it is suggested that soil-reinforcement interaction can be better characterized by direct shear test when sliding at the soil-geosynthetic interface is likely to occur (e.g., Palmeira, 2009, Lopes, 2012, Vieira et al., 2013, Ferreira et al., 2015). Such a condition may happen at the toe of reinforced soil slopes. Hitherto, the direct shear test has been applied by many researchers to study the mobilization of frictional shear strength at soil-geotextile interfaces (e.g., Athanasopoulos, 1993, Lee and Manjunath, 2000, Goodhue et al., 2001, Khoury et al., 2011, Anubhav and Basudhar, 2013, Vieira et al., 2013, Esmaili et al., 2014, Ferreira et al., 2015, Hatami and Esmaili, 2015, Vangla and Latha, 2015, Vieira et al., 2015, Vangla and Gali, 2016). While some researches have employed large size direct shear boxes to study the mechanical behavior of soil-geotextile interfaces (e.g. Lee and Manjunath, 2000, Goodhue et al., 2001, Ferreira et al., 2015, Vangla and Latha, 2015, Vangla and Gali, 2016), standard box direct shear test has also been applied by many researchers to the same purpose (Khoury et al., 2011, Anubhav and Basudhar, 2013, Deb and Konai, 2014; Esmaili et al., 2014, Hatami and Esmaili, 2015). Vieira et al. (2015) reported that in general, the maximum and residual shear strengths obtained from large size and traditional direct shear devices are relatively close; however, it should be noted that the peak shear stress is usually achieved in lower horizontal displacements in large scale shear tests.
Constitutive modeling of soil-structure interfaces is a relatively young matter. Clough and Duncan (1971) introduced a hyperbolic model for soil-structure interfaces. De Gennaro and Frank (2002) proposed a plasticity model for sand–steel interfaces taking into account phase transformation. Later, Liu et al. (2006) and Lashkari (2013) suggested versatile state-dependent interface models capable of simulating the mechanical behavior of interfaces in a wide domain of density and normal stress values using a single set of parameters. For sand–geotextile interfaces, Anubhav and Basudhar, 2010, Huang et al., 2014, and Anubhav and Wu (2015) proposed modified hyperbolic elastic models. Recently, Khoury et al. (2011) and Lashkari and Kadivar (2016) applied advanced constitutive models to simulate the mechanical behavior of partially saturated soil-geotextile interfaces. The influence of particle shape on the mechanical behavior of granular soils has been addressed in the literature (e.g., Cho et al., 2006, Rousé et al., 2008, Lashkari, 2014, Vahidi-Nia et al., 2015, Vangla and Latha, 2015). This paper reports result of a study on the effects of particle shape on the mechanical behavior of soil-woven geotextile interfaces. To this purpose, mobilization of shear strength and volume change response of interfaces between woven geotextile and fine angular sand and glass-beads were studied. The testing program covers a rather wide range of normal stress and initial void ratio values. For the sake of comparison, the behavior of interfaces between similar granular materials and a serrated steel block was also presented. The outcome of this study may be useful in development of novel constitutive models for sand–geotextile interfaces.
Section snippets
Granular materials
A graded sand and a blend of glass beads with physical properties given in Table 1 were, respectively, selected as angular and well rounded granular materials in this study. Scanning Electron Microscope (SEM) images for sand particles and glass beads are presented in Fig. 1. As plotted in Fig. 2, nearly identical particle size distributions were used for sand and glass beads specimens and thus, particle size is not considered as a variable here. In accordance with the Unified Soil
Testing program
A series of soil–soil direct shear tests under σn0 = 57, 107, and 207 kPa was organized in order to study the peak and residual shear strengths (say friction angles) as well as the volume change response (that is to say dilation angle) of the angular sand and glass beads specimens. In all tests, dry specimens were poured into the shear box in three consecutive layers. For preparation of medium and dense specimens, each layer was subjected to rodding; however, layers were not compacted in
Results of soil–soil direct shear tests
The behavior of angular sand in direct shear is studied in parts “a” and “b” of Fig. 6. The tests cover void ratios of e0 = 0.65 (Dr ≈ 78%), 0.75 (Dr ≈ 56%), and 0.81 (Dr ≈ 43%), as well as normal stress values σn0 = 57, 107, and 207 kPa. For each combination of e0 and σn0, at least two duplicate tests were conducted and thus, a minimum of 27 [=3 × 3 × 3] soil–soil direct shear tests were performed on angular sand specimens. Dense (e0 = 0.65) specimens manifest a peak in shear strength and the
Impact of structural material type on the interface behavior
Through parts “a” to “c” of Fig. 9, the behavior of an interface between dense (e0 = 0.65, Dr ≈ 78%) angular sand and woven geotextile is directly compared with that of an interface test between dense angular sand and roughened steel. Data of a soil–soil direct shear test on the angular dense sand are also included in the figure. Nearly identical initial conditions were applied for the tests described above. It is observed that the peak shear strength of the dense sand is greater than those
Peak friction and maximum dilation angles
For soil–soil direct shear tests on angular sand, variation of peak friction angle (i.e., ϕp) and maximum dilation angle (i.e., ψmax), with respect to normal stress, are illustrated in Fig. 10(a) and (b). Herein, peak friction angle is calculated by (e.g., Dove and Jarrett, 2002, Fioravante, 2002, Lings and Dietz, 2005):where τp is peak shear strength. It is worth mentioning that both normal and shear stress were corrected simultaneously for reduced area in interface tests. As a
Residual friction angles
Large amplitude shearing in direct shear test brings the state of specimen to such an eventual steady state at which granular media/interfaces shear continuously without any further change in stress state and void ratio. Most of the tests shown through Fig. 7, Fig. 8, Fig. 9 reached the asymptotic state of stress described above for which dilation angle becomes zero. Frost et al. (2002) and Dove and Jarrett (2002) attributed the term steady-state, and Koerner (2005) and Tabucanon et al. (1995)
Stress-dilation law
In smooth interfaces, soil particles slide easily on the contact surface and thus, neither shear strength mobilizes, nor volume change occurs at the contact zone. Nevertheless, with the increase in size of asperities, movement of particles on the contact surface gradually becomes difficult in rough soil-structure interfaces (e.g., Uesugi and Kishida, 1986; Evgin and Fakharian, 1996; Frost et al., 2002, Frost et al., 2004, Lings and Dietz, 2005, De Jong and Westgate, 2009, Martinez et al., 2015
Discussions
The influence of particle shape on the behavior of sand-woven geotextile interfaces over a wide domain of soil density and normal stress was investigated. A uniformly graded angular fine sand, and a blend of well rounded glass beads were used to simulate sands with angular and well rounded particles. For the sake of comparison, additional interface tests between the same granular materials and roughened steel were conducted. All direct shear interface tests were terminated at u = 12 mm due to
Acknowledgements
The constructive comments, both technical and editorial, by the chief editor and anonymous reviewers are gratefully acknowledged. The assistance of Mr. Benyamin Farhadi in preparation of SEM photographs is appreciated. Also, the authors would like to express their gratitude to Narvin-Gostar-Parsian Co. for providing the geotextiles, and Glassbeads Co. (PANA) for supplying the glass-beads to the authors.
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