The effect of using agricultural waste ash on improving the mechanical properties of forest roadbed soil

Document Type : Scientific article

Authors

1 MSc. Student, Department of Forestry, Faculty of Forest Science, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, I.R. Iran

2 Associate Professor, Department of Forestry, Faculty of Forest Sciences, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, I.R. Iran

3 Assistant Professor, Department of Forestry, Faculty of Forest Sciences, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan, I.R. Iran

4 Research Expert (Ph.D. of forest management), Forest Research Division, Research Institute of Forests and Rangelands, AREEO, Tehran, I.R. Iran

Abstract

Background and Objective: Forest road subgrade soils in certain areas often face challenges such as high plasticity, low load-bearing capacity and shear strength, compaction difficulties, and settlement risks. Therefore, soil stabilization and strength improvement through additives have always been of interest to researchers and road engineers. At the same time, the agricultural industry struggles with the increasing volume of waste and by-products from production processes. Many of these residues, due to their mineral composition, possess favorable mechanical properties, making them potential substitutes for traditional soil stabilizers such as lime and cement. Moreover, the structural characteristics of organic materials can contribute to environmental preservation and the biological improvement of unstable and expansive soils.
Material and Methods: In this study, agricultural by-products of walnut and peanut shells were used to reinforce the subgrade soils of forest roads in the Shast Kalateh forest management plan. The soil samples, collected from unstable landslide-prone zones containing expansive clay soils, were obtained from five landslide sites along forest roads. From each site, at least three samples weighing 5 kg each were randomly collected, yielding a total of 15 samples (75 kg). The hard shells of walnuts and peanuts were ground into powder using a professional grinder and then burned in a furnace at 580 °C for two hours to produce ash. Clay soil samples with high plasticity (CH) were mixed with walnut and peanut shell ash in proportions of 0, 5, 10, 15, and 20% (ash size 0.04–0.4 mm). The curing period was set at 28 days. Compaction characteristics, Atterberg limits, load-bearing capacity (CBR), and unconfined compressive strength were then evaluated using Proctor, Casagrande, plastic thread, and CBR tests. Statistical analyses were conducted as a factorial experiment with three factors (waste type at two levels, waste content at five levels, and 10 treatments in total) using SPSS software.
Results: Findings revealed that all treatments increased the CBR values by 2 to 2.5 times; however, none were able to elevate the soil’s load-bearing capacity from “poor” to a higher category. The lowest CBR was recorded in soils treated with 3% peanut shell ash, while the highest plasticity index (PI) was observed in the control and soils treated with 10–15% walnut shell ash. Conversely, the lowest PI was recorded in soils treated with 3% peanut shell ash. Overall, soils treated with peanut shell ash exhibited significantly lower PI values than those treated with walnut shell ash. Due to the high plasticity of the native soil, treatments showed limited effectiveness in improving load-bearing capacity. Thus, their independent use for CH soil mechanical improvement is not recommended, but their potential in combination with additives like lime and cement could be considered.
Conclusion: Higher percentages of walnut shell ash (10–15%) did not effectively reduce soil plasticity, likely due to its stronger affinity for water. In contrast, peanut shell ash at 3% by weight showed promising results in reducing soil plasticity, suggesting its suitability for stabilizing landslide-prone slopes without load-bearing requirements. Overall, while the tested treatments demonstrated limited capacity to enhance load-bearing properties of CH soils, their combined use with lime or cement could be a promising approach for further evaluation.

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References

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Forest Research and Development
Volume 11, Issue 2
September 2025
Pages 249-265

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