Discussion: Geotechnical characterization of soil-rubber mixtures with well graded gravel

Basic SI units are shown in parentheses.

ϕ_peak

peak friction angle

ψ_max

maximum dilatancy angle

CSL

critical state line

CSS

critical state

PWP

pore water pressure

RLRG

large rubber granulated

RSM

rubber shredded material

RSML

small rubber granulated

VRC

volumetric rubber content

wgGRM

well-graded gravel-rubber mixture

This is a discussion piece on Fiamingo, A., Chiaro, G., Murali, A. & Massimino, M. R. (2025) . Geotechnical characterization of soil-rubber mixtures with well-graded gravel. Geosynthetics International, https://doi.org/10.1680/jgein.24.00177.

The authors of this discussion piece appreciate greatly the efforts and contribution of the original study, which significantly advance knowledge in the field of geotechnical engineering. However, they identify sections requiring further clarification or deeper analysis, such as methodological or interpretative aspects. Therefore, the goal of this piece is to critically examine these elements, address ambiguities, and propose alternative viewpoints. By doing so, the authors aim to complete the original findings and consequently, enhance the understanding of the treated research topic. This approach seeks to foster a more comprehensive and nuanced discussion within the field.

The original study investigates the mechanical properties of well-graded gravel-rubber mixtures (wgGRMs) under static loading conditions, providing valuable insights into their behavior. While this research contributes significantly to the understanding of the wgGRMs mechanical response, there are several points of the study that could be expanded or improved.

Firstly, the discussers would like to highlight that the study's examination of only four specific volumetric rubber contents (VRCs = 0%, 25%, 40%, and 55%) provides a useful database but overlooks potential non-linear trends or critical thresholds in the intermediate and higher VRC ranges. Additionally, the triaxial tests conducted at low confining pressures of 30, 60, and 100 kPa, while relevant for some engineering applications, may not fully capture the behavior of the wgGRMs under higher pressure states, typically found in the case of deep foundations or embankments. The use of well-graded gravel sourced solely from the New Zealand area further limits the generalization of the findings, as variations in grain size, shape, mineral and geochemical composition across different regions of the globe could significantly affect the performances of tested material. Moreover, the lack of detailed characterization of the shredded rubber, including its particle size distribution, shape, and chemical properties, introduces uncertainty about its interaction with gravel. Addressing these limitations through broader testing parameters and consequently, gaining more comprehensive material characterization would strengthen the study's applicability and robustness. The outcomes of this research are solely limited to static loading conditions, which may not adequately reflect the dynamic requirements of many geotechnical applications, such as liquefaction mitigation and dynamic isolation systems that typically involve cyclic or transient loadings. Although, the authors recognized these limitations by referring to related researches on small-strain stiffness and dynamic properties, these critical aspects were not addressed in the current study. To fully evaluate the suitability of the wgGRMs for geotechnical engineering applications, a more comprehensive analysis under dynamic loading conditions appears more than necessary, as it would provide deeper insights into their performances and behavior in such scenarios. Referring to the published studies of Anastasiadis et al. (2012), Tasalloti et al. (2020), Dai et al. (2023), Li et al. (2024), it was found that the addition of rubber material to coarse soils enhanced significantly the liquefaction resistance of tested soils. Under cyclic unidirectional loading, the liquefaction phenomenon in the RSM10% mixture was observed only at high loading amplitudes, and the sudden loss of strength during the shearing process was found exclusively a characteristic of the pure coarse-grained soils. Under cyclic bidirectional loads with shear strain amplitude of 1.0%, the liquefaction occurred in the RSM10%, however increasing the rubber content effectively prevented liquefaction in the tested mixtures. Additionally, higher levels of vertical stress had a low impact on the excess pore water pressure (PWP) generation.

Secondly, the study emphasizes the excellent energy absorption properties of the wgGRMs based on the strain energy density approach, but it overlooks their long-term sustainability under cyclic loading conditions, such as fatigue or wear that could potentially degrade these properties over time. Furthermore, while the introduction of a “critical state” surface (CSS) is innovative, its validity requires deeper exploration. Although parallel critical state lines (CSLs) for different volumetric rubber contents (VRCs) indicate consistent behavior, the uniqueness of the CSS for various initial conditions remains unpredictable. To strengthen this concept, additional experiments should be conducted, incorporating a broader range of confining pressures, VRCs, and loading rates, thereby providing a more comprehensive understanding of the tested mixtures behavior.

On the other hand, the peak friction, critical excess friction, and maximum dilatancy angles are among the most important parameters for analyzing the stress-dilatancy behavior of soils. The critical excess friction angle represents the difference between the peak and critical-state friction angles, playing a key role in determining the shear strength magnitude of soils by indicating the additional resistance developed at the peak state. Additionally, the maximum dilatancy angle (ψ_max) quantifies the maximum volumetric expansion of soils during the shearing process, which is essential to clearly understand their dilative character and its influence on the shear strength behavior. Indeed, these parameters have been widely recognized in previous studies (Azaiez et al. 2021; Cherif Taiba et al. 2019; Deng et al. 2021; Doumi et al. 2021; Mahmoudi et al. 2021; Nimtaj and Beyrag 2022; Taibi et al. 2023 2024; Xiao et al. 2019) as fundamental base to comprehending soil mechanics and behavior under various loading conditions. Tasalloti et al. (2020) investigated the strength and deformation characteristics of two types of gravel-granulated tyre rubber (G-GTR) mixtures under four levels of normal stress (6.5, 30, 60, and 100 kPa). The mixtures were prepared using rounded uniform gravel combined with small and large rubber particles at varying volumetric rubber contents (VRC) of 0%, 10%, 25%, 40%, and 100%. Their findings revealed that pure gravel exhibited the highest peak friction angle (ϕ_peak = 56° − 62°), while pure rubber showed the lowest values (ϕ_peak = 28° − 50°). For both RLRG and RSML mixtures, ϕ_peak decreased progressively with increasing VRC. Notably, the reduction in ϕ_peak was more pronounced for mixtures with VRC below 20% (gravel-controlled behavior), whereas mixtures with VRC greater than 40% (rubber-controlled behavior) demonstrated relatively consistent ϕ_peak values. In addition, a comprehensive evaluation of these parameters would provide deeper insights into the mechanical behavior of the wgGRMs, particularly in understanding their shear strength and volumetric response under various loading conditions. Future studies should consider incorporating stress-dilatancy analyses to fully characterize the material's performances and consequently, enhance its applicability in many geotechnical applications.

Finally, investigating the addition of the wgGRMs with diverse types of well-graded gravel and shredded rubber, or incorporating other waste materials like recycled plastics or glass, could promote their eventual applicability in the different fields of construction. Additionally, Banasiak et al. (2019) reported that long-term studies assessing the durability and degradation of the wgGRMs under varying environmental conditions, including resistance to weathering and chemical exposure were essential to ensure their reliability and effectiveness in the different geotechnical applications over time.

In conclusion, this discussion paper critically assesses the initial study on the well-graded gravel-rubber mixtures (wgGRMs), hence, recognizing its contribution while identifying some relevant key limitations. Although, this research offers valuable insights into the static mechanical behavior of the wgGRMs, it lacks reliable exploration on dynamic loading conditions, long-term durability, and material variability. By addressing these gaps in future studies, researchers can greatly deepen the understanding and practical application of the wgGRMs in the field of geotechnical engineering, ultimately advancing sustainable and cost-effective infrastructure solutions.

All authors made significant contributions on this discussion. The final manuscript was approved by all authors.

A response to this discussion can be found at Fiamingo, A., Chiaro, G., Murali, A. & Massimino, M. R. (2025) . Response to discussion: Geotechnical characterization of soil-rubber mixtures with well-graded gravel. Geosynthetics International, https://doi.org/10.1680/jgein.25.00039.