Response to discussion: Geotechnical characterization of soil-rubber mixtures with well-graded gravel

Mixtures with volumetric rubber contents (VRCs) of 0%, 25%, 40%, and 55% were selected based on the findings from the above-mentioned studies on GRMs, where angular and rounded gravels mixed with rubber inclusions of different sizes and VRCs up to 100% were investigated, effectively capturing key trends and critical thresholds extensively analysed in the literature (Kim and Santamarina, 2008; Lee et al. 2007; Pasha et al. 2019; Tasalloti et al. 2021a, 2021b, 2021c), in terms of rigid gravel-like behaviour, transitional (dual) behaviour and soft rubber-like behaviour (Chew et al. 2022). It is accepted that the adoption of any soil-rubber mixtures in geotechnical applications must satisfy stringent design considerations such as strength, compressibility, permeability and environmental leaching characteristics. The authors previously found that GRMs with VRCs greater than 55% (soft rubber-like behaviour) do not satisfy serviceability design considerations under static load, in terms of compressibility/settlement requirements (Tasalloti et al. 2021b); moreover, from an environmental viewpoint, they are likely to exceed threshold values of leached heavy metal, for example zinc (Banasiak et al. 2021), posing a significant risk for soil and groundwater contamination. Thus, in the current study concerning wgGRMs, VRC was limited to 55%.

As for the triaxial test conditions used in Fiamingo et al. (2025a), the confining pressures up to 100 kPa were set based on the results of direct shear tests performed earlier by Tasalloti et al. (2021a, 2021b); they represent typical field stress conditions where GRMs, and thus wgGRMs, are used primarily as lightweight backfill materials, shallow underground layers for mitigation of liquefaction phenomena, and geotechnical seismic isolation for low-to-medium rise lightweight structures/infrastructure (Abate et al. 2023; Chiaro et al. 2023a; Hazarika et al. 2020; Maleska et al. 2024; Massimino et al. 2023). Testing wgGRMs under higher confining pressures was out of the scope of the study on wgGRMs; nevertheless, additional triaxial tests may be necessary in the future when dealing with specific applications where higher confining pressures are envisioned.

Since the experimental study was undertaken in New Zealand, the tested well-graded gravel was sourced from a quarry that produces gravelly soils used in actual geotechnical applications. The authors agree that such gravel may differ from other gravels sourced elsewhere around the globe. Nevertheless, the main scope of this study was not to generalise the outcomes of the study to all types of gravels, but rather to provide for the first time worldwide an understanding of the engineering performance of wgGRMs and their potential use. This study can, therefore, serve as a reference for future studies that are indeed required to characterise other wgGRMs mixtures, accounting for potential effects of the gravel mineralogy, particle size and shape and particle size distribution (PSD).

In the study on wgGRMs, mixtures were produced using the same rubber inclusions used in the previous study on GRMs by the authors, for which details of the size and shape, PSD, index properties and environmental leaching aspects have been reported extensively. Moreover, in the study on wgGRMs, only a specific size of rubber inclusions (4–8 mm, named large rubber) was used, since previous findings on GRMs indicated that the use of medium (2–4 mm) and smaller (1–2 mm) sizes is problematic in terms of strength, compressibility and environmental leaching (Banasiak et al. 2021; Chiaro et al. 2023a; Tasalloti et al. 2021b), as well as expensive from a practical viewpoint.

Additionally, the discussers indicated the study on wgGRMs lacks data on the behaviour investigation under dynamic loading conditions, which is a critical component of material characterisation. As stated in the manuscript, the authors not only recognise this aspect, but in a recent publication (Fiamingo et al. 2025b) have addressed it. Specifically, the small-strain stiffness and strain-dependent dynamic properties of the wgGRMs were carefully investigated by conducting a series of bender element tests integrated with cyclic triaxial tests. Such investigations complement very well the characterisation of the dynamic response of GRMs, previously investigated by the authors (Chiaro et al. 2022).

The discussers highlight the lack of a comprehensive evaluation of the mechanical behaviour of the wgGRMs, in terms of shear strength and volumetric responses under various loading conditions, as well as stress-dilatancy analyses. In the paper, the mechanical behaviour was deepened in the sections “Isotropically-consolidated drained compression response” and “Stress and strain paths towards the critical state”, where the deviatoric stress-deviatoric strain curves, volumetric strain-deviatoric strain relationships and dilatancy characteristics of the investigated wgGRMs were reported and compared. Additional analyses regarding the critical state surface (CSS) locus and effective friction angle, as suggested by the discussers, may indeed be beneficial and worth considering, particularly if the results are compared to those available for GRMs.

The discussers noted that it is necessary to investigate the long-term sustainability under cyclic loading conditions of wgGRMs, such as fatigue or wear, that could potentially degrade the engineering properties of the materials over time. The authors indeed agree with this statement, and would like to point out that such an investigation would be particularly important in the case wgGRMs are used in applications where the material is subjected to repeated cyclic loads, like traffic conditions, where thousands on loading cycles are applied and accumulation plastic deformation are expected. This was outside the scope of the study.

The prospect of mixing a variety of recycled materials with wgGRMs hold indeed practical significance. It is worth mentioning that, the second author has investigated the performance of GRMs mixed with crushed green glass inclusions (Chiaro et al. 2023b) and more recently HDPE (high-density polyethylene) plastic beads (to be published in due course). The results of such pioneering studies indicate that such synthetic materials can be used as geomaterials. Due to the many similarities between GRMs and wgGRMs, it is therefore expected that compounds made of wgGRMs mixed with glass and/or plastic will also have adequate strength, compressibility and minimal leaching aspects, making them suitable geomaterials. Future investigation will confirm this hypothesis.

Because the durability of soil mixed with rubber inclusions is a primary concern, testing on GRMs produced with thermally aged rubber inclusions has been the subject of an ongoing long-term study conducted by the second author. The results of such a study will be published elsewhere in due course and will serve as a foundation for future study on wgGRMs made with aged rubber inclusions.

All the authors made significant contributions to this discussion, and the final manuscript was approved by all authors.