The determination of appropriate backfill materials and methodologies is an essential decision that plays a critical role in the sustainability and safety of underground infrastructure projects. In the recent scholarly literature, there is an indication that there are many fundamental considerations that engineers must make.
Geotechnical and mechanical properties
The underlying mechanical properties of the backfill material are still an essential consideration. Gomaa & El-Nagdy (2023) emphasizes the need to focus on the uniaxial compressive strength (UCS) of the material, especially in the case of cemented backfill materials used in underground mining operations. According to their study, the composition of the material, including the amount of cement, water, and industrial by-products such as filter dust, needs to be optimized in order to achieve the required UCS. Kimmling et al. (2025) also found that the compressive strength of flowable backfill materials, especially in urban infrastructure, is crucial in determining the future excavability of the material, stating that materials with compressive strength greater than 0.3 N/mm² after 28 days can hinder future removal.
Hydro-mechanical and durability considerations
Permeability and particle size distribution are two critical parameters in the application of hydraulic backfilling. In this regard, Manohar et al. (2025) proposed the following criteria as essential for the application of the backfilling material: the material must have a minimum permeability of 100 mm/h and must contain no more than 10% silt-sized particles. The application of other materials, such as fly ash-plastic aggregates, has also been proposed as a means to address the problem of scarcity. In the application of hydraulic backfilling in nuclear waste disposal sites, the hydraulic conductivity must be lower than 10⁻¹⁰ m/s, as in the case of the application of bentonite aggregate mixtures (Dixon et al., 2023).
Environmental sustainability and material sourcing
Environmental aspects of the materials used in backfill materials have been recognized as a critical factor in material selection. Current trends in research are focusing more on the utilization of industrial waste materials, which can help in achieving a reduced carbon footprint. In a study published by Liu et al. (2024), the incorporation of supplementary cementitious materials, such as fly ash, ground granulated blast furnace slag, and silica fume, was found to result in a substantial reduction of cement usage while enhancing the stability of the grout and long-term strength of the mixture. Kumar et al. (2025) also reported the potential of controlled low-strength materials, consisting of manufactured sand, silt, and fly ash, in achieving both technical and environmental sustainability, as indicated by a carbon footprint of 246.2 kg CO2-eq as per the life cycle assessment.
Thermal and environmental interaction
In the case of backfill materials, especially in applications where the system is exposed to thermally active environments, the concepts of thermal conductivity, along with the response of the material to temperature changes, become pertinent issues in this regard. According to Patwa et al. (2023), synthetic binders such as cement, in spite of their positive effects on the mechanical properties of the system, tend to impair the thermal insulation properties of the system. In their study on biochar-biopolymer soil composites, the authors have successfully demonstrated the potential for the development of systems that exhibit enhanced thermal insulation, along with sufficient mechanical strength. (Lu & Meguid, 2026) state that in the course of mining, the deeper levels are accessed, and in this regard, the thermo-hydro-mechanical response of the cemented backfill system becomes a critical parameter in ensuring system stability.
Workability and construction considerations
The construction requirements of flowability, setting time, and placement are key factors that impact the selection of materials for use in construction. Al-Taie et al. (2023) showed that blends of recycled glass, plastic, and tire aggregates have self-compacting properties and are less sensitive to moisture variations, which are beneficial for deep trench backfilling where quality control of compaction is limited. Additionally, Manohar et al. (2025b) found that morphological properties, such as spherical fly ash particles, provide ball-bearing properties for easy pumpability and pipe wear resistance in placement.
In conclusion, it is pertinent to assert that in choosing the most suitable materials for use as backfill materials, it is essential to ensure that there is a balance between their performance, hydraulic properties, environmental sustainability, thermal properties, and constructability, with optimal solutions in most cases being achieved by making creative use of waste materials.
References
Al-Taie, A., Yaghoubi, E., Disfani, M., Fragomeni, S., & Gmehling, E. (2023). Field Performance Evaluation of Recycled Aggregate Blends Used for Backfilling Deep Excavated Trenches. International Journal of Geomechanics, 24. https://doi.org/10.1061/IJGNAI.GMENG-7588
Dixon, D., Stone, J., Barone, F., Kim, C. S., Birch, K., & Schneider, G. (2023, August 27). BACKFILLING OF SHAFTS IN A DEEP GEOLOGICAL REPOSITORY: EVALUATION OF POTENTIAL MATERIALS.
Gomaa, E., & El-Nagdy, K. A. (2023). Optimal Backfilling Materials with High Compressive Strength Based on Multiple Linear Regression. Mining, Metallurgy & Exploration, 40(6), 2183–2191. https://doi.org/10.1007/s42461-023-00850-x
Kimmling, N., Rubinato, M., Bosseler, B., Liebscher, M., Klameth, M., Salomon, M., & Ulutas, S. (2025). Evaluation of temporarily flowable self-compacting backfill materials in large-scale sewer applications in Germany. Tunnelling and Underground Space Technology, 164, 106754. https://doi.org/10.1016/j.tust.2025.106754
Kumar, V., Mangottiri, V., & Balu, S. (2025). A confirmatory approach for sustainable utilization of silt fraction from quarry sand mining as flowable fill for backfilling applications. Clean Technologies and Environmental Policy, 27, 7577–7595. https://doi.org/10.1007/s10098-025-03310-w
Liu, C., Li, Z., Bezuijen, A., Chen, L., & Cachim, P. (2024). Optimizing the shield tunnel backfilling grouts with supplementary cementitious materials by response surface methodology. Construction and Building Materials, 421, 135575. https://doi.org/10.1016/j.conbuildmat.2024.135575
Lu, G., & Meguid, M. (2026). Cemented Backfill Applications in Challenging Geothermal Environments: A Brief Review and Future Perspectives. Rock Mechanics and Rock Engineering, 59(2), 2789–2809. https://doi.org/10.1007/s00603-025-04923-9
Manohar, M., Choudhary, B. S., Skrzypkowski, K., Zagórski, K., & Zagórska, A. (2025a). A Performance Evaluation of Fly Ash–Plastic Aggregate in Hydraulic Backfilling: A Comparative Study. Materials, 18(12), 2751. https://doi.org/10.3390/ma18122751
Manohar, M., Choudhary, B. S., Skrzypkowski, K., Zagórski, K., & Zagórska, A. (2025b). A Performance Evaluation of Fly Ash–Plastic Aggregate in Hydraulic Backfilling: A Comparative Study. Materials, 18(12), 2751. https://doi.org/10.3390/ma18122751
Patwa, D., Dubey, A. A., Karangat, R., & Sekharan, S. (2023). Investigation of thermal and strength characteristics of a natural backfill composite inspired by synergistic biochar–biopolymer amendment of clay loam. Canadian Geotechnical Journal, 61. https://doi.org/10.1139/cgj-2022-0528
