Particle shape effect on interfacial properties between granular materials and geotextile Free

¹Associate Professor, Laboratory of Material Sciences & Environment, Laboratory of Architecture, Cities and Environment, Hassiba Ben Bouali, University of Chlef, Algeria, E-mail: a.cheriftaiba@univ-chlef.dz (corresponding author)

²Associate Professor, Laboratory of Material Sciences & Environment, Hassiba Ben Bouali, University of Chlef, Algeria

³Professor, Alexander von Humboldt Foundation, Berlin, Germany

1. The authors of this discussion piece acknowledge and commend the effort made by the original paper's authors in their research. Nevertheless, certain aspects of the study require further clarification and evaluation, and the primary objective of this discussion piece is to address those issues.

The discussers highlight a crucial aspect that the experiment could have addressed regarding the impact of particle shape on the shear strength characteristics of granular soils. Previous studies, including Bolton (1986), Ferreira et al. (2015), Vangla and Latha (2015), Vangla and Gali (2016), Strahler et al. (2016), Mahmoudi et al. (2021), Azaiez et al. (2021a), Cherif Taiba et al. (2022a) and Taibi et al. (2023), have established the strength–dilatancy relationship as a fundamental soil behaviour property that characterises the volume change of soil during deformation under load. However, the authors of the paper under discussion did not explicitly investigate this relationship or provide any data or analysis on it. It is possible that they considered the impact of particle shape on the shearing resistance as an indirect indicator of the strength–dilatancy relationship, but this is not clearly stated in the paper. This discussion piece aims to highlight this issue and calls for further investigation into the strength–dilatancy relationship in the context of the authors’ research.

Moreover, the authors did not utilise the well-established Bolton stress–dilatancy equation, which is commonly used in geotechnical engineering to investigate the behaviour of granular soils under different loading conditions, in analysing the stress–dilatancy relationship of both unreinforced and geotextile-reinforced sands. As a result, it is uncertain whether the stress–dilatancy behaviour of the examined soils conforms to the predicted pattern by the Bolton equation, which could help in optimising their design and use in geotechnical engineering. In addition, the authors did not investigate how the stress–dilatancy behaviour of the tested materials was influenced by their particle shape characteristics, especially the overall regularity. As a solution, the discussers recommend that the authors should evaluate the impact of the particle shape parameters on the stress–dilatancy behaviour between their granular materials and geotextile.

The authors of this experimental study did not investigate the peak, excess and maximum friction angles of both unreinforced and geotextile-reinforced soils. These parameters are essential for assessing the soil's shear strength and behaviour under various loading conditions (refer to the published papers: Fioravante 2002; Chakraborty and Salgado 2010; Nimbalkar et al. 2012; Guzman and Iskander 2014; Vieira et al. 2015; Azaiez et al. 2021b; Cherif Taiba et al. 2022b). However, the lack of data on these parameters limits the understanding of the mechanical behaviour of the sand–geotextile interface and its potential use in geotechnical engineering applications. The discussers certainly recommend that the authors evaluate these parameters to provide a more comprehensive understanding of the interface's behaviour.

Additionally, there is no discussion in the paper about the influence of particle shape on the different friction angles of the sand–geotextile interface. Previous studies have shown that particle shape can affect peak and residual friction angles and maximum dilation angle in sand–geotextile interfaces. According to Krieger and Thamm (1991), the mobilised friction angle between soil and geotextile is crucial in assessing failure in reinforced soil walls. Jewell (1996) suggested that the interface friction angle between woven and nonwoven geotextiles and soil can vary from 65 to 100% of the soil friction angle. Similarly, Goodhue et al. (2001) reported that the peak friction angle of sand–geotextile interfaces is approximately 65–75% of the sand peak friction angle. Nejad et al. (2017) found that particle shape had an impact on peak and residual friction angles and maximum dilation angle in sand–woven geotextile interfaces, even with an identical particle size distribution. They also reported that the peak friction angle and maximum dilation angle of angular sand–woven geotextile interfaces were slightly lower than the corresponding values for angular sand in soil–soil direct shear tests. Based on these findings, the discussers strongly recommend that the authors explore and clarify the correlation between particle shape and friction angles, particularly the influence of overall regularity on friction angle for sand–geotextile interfaces, to make their work more relevant.

Despite its limitations, the experimental investigation provides a valuable contribution to the field of geotechnical engineering. However, further clarification and evaluation are necessary to explore the potential of needle-punched nonwoven geotextile as a reinforcement material in sand interfacial friction behaviour. This can be achieved by integrating the influence of the overall regularity on the stress–dilation relations, especially considering the different friction angles reported in the published literature, of the tested materials under study.

⁴Professor, Department of Civil Engineering, Osmaniye Korkut Ata University, Osmaniye, Turkey, E-mail: caferkayadelen@gmail.com

⁵Assistant Professor, Department of Civil Engineering, Osmaniye Korkut Ata University, Osmaniye, Turkey, E-mail: gokhanaltay@osmaniye.edu.tr

⁶Research assistant (PhD student), Department of Civil Engineering, Osmaniye Korkut Ata University, Osmaniye, Turkey

⁷Research assistant (PhD student), Department of Civil Engineering, Osmaniye Korkut Ata University, Osmaniye, Turkey, E-mail: mitatozturk@osmaniye.edu.tr

2. The authors greatly appreciate the positive discussion and interest by Cherif Taiba, Mahmoudi and Belkhatir in our paper entitled ‘Particle shape effect on interfacial properties between granular materials and geotextile’. In the comments, they recommend investigating the strength–dilatancy relationship, utilising the Bolton stress–dilatancy equation, evaluating the impact of the particle shape parameters on the stress–dilatancy behaviour, investigating the maximum friction angle of reinforced and unreinforced soils and discussing the impact of overall regularity on dilation and friction angles. In this reply, their comments are addressed in two sections below.

Equation (1) shows the stress–dilatancy equation suggested by Bolton (1986). Equation (1) includes the parameter $ϕ′crit$⁠.

When the stress–strain curves in Figures 8–12 are examined, the stress–strain behaviour exhibits loose sand behaviour. In other words, stress increases with horizontal displacement and continues asymptotically after reaching a peak. The critical state can be observed where the dilation–strain increment and stress increment are both zero. Therefore, Bolton's (1986) stress–dilatancy equation could not be applied to the current study.

In their study, Frydman et al. (2007) proposed a relation between peak state friction angle and dilation angle as follows.

In our study, a correlation was identified between the peak state friction angle and dilation angle (Equation (3)), which is illustrated in Figure 22. Also, Figure 22 indicates the relationship between interface friction angle and dilation angle according to Equation (1), suggested by Frydman et al. (2007). However, it should be noted that this proposed relationship is related to the stress–dilation behaviour between the geotextile and the sand.

Figure 18 illustrates the Mohr failure envelopes for all samples. In this illustration, the dashed lines indicate unreinforced scenarios, while the solid lines indicate scenarios reinforced with geotextile. It can be understood that Mohr failure envelopes move upward when geotextile is used at the interface of the shear box. Based on this evidence, it can be concluded that the use of geotextile improved the shear characteristics of the shear plane. Moreover, it is clearly seen from Figure 18 that when overall regularity (O_R) decreases for all cases, Mohr failure envelopes also move upward. In other words, as angular particle content in a mixture increases, shear characteristics start to be improved. Additionally, Figure 17 shows the change in peak shear stress with varying O_R values. It can be understood from Figure 17 that peak shear stress decreases when the O_R value increases in both reinforced and unreinforced cases. It has been observed that placing geotextile at the interface as a reinforcement material leads to an increase in the peak shear stress.

Figure 20 illustrates the variation of interfacial friction angle of granular materials for different O_R values. It appears that the interfacial friction angle remains constant despite changes in O_R values for unreinforced cases. Examination of  the interfacial shear characteristics of sand and geotextile shows that an increasing trend is present as O_R values decrease. This indicates that an increase in the angular particle content of mixtures leads to an increase in the interfacial friction angle of the mixtures. The relationship between maximum dilation angle and O_R is presented in Figure 21. Maximum dilation angle decreased with increasing O_R under 29 kPa and 58 kPa normal stress levels. However, an increasing trend is detected as O_R values increase under 116 kPa normal stress level.