Rapid urbanization and expanding infrastructure have increased the demand for construction-ready land, often requiring building on problematic soils like those with a high clay content. These fine-grained soils can pose significant challenges, including low permeability, poor bearing capacity, and considerable volume changes with moisture fluctuations, which can threaten the stability of buildings and other structures. While traditional methods like lime or cement treatment are effective, they are also costly and can harm the environment. In a pre-print article posted on Preprints.org, “Effect of Wood-Based BiocharBiochar is a carbon-rich material created from biomass decomposition in low-oxygen conditions. It has important applications in environmental remediation, soil improvement, agriculture, carbon sequestration, energy storage, and sustainable materials, promoting efficiency and reducing waste in various contexts while addressing climate change challenges. More on the Engineering Properties of Medium Plasticity Clay,” authors Kalehiwot Nega Manahiloh, Samuel Fetene, and Emma Detwiler investigated a more sustainable alternative: wood-based biochar. Their research, which is not yet peer-reviewed, explores biochar’s potential to improve the geotechnical performance of clayey soils by testing mixtures with varying biochar content.

The study’s findings on the clay’s plasticity and compaction were particularly interesting. The research team used a medium plasticity clay from Perryville, Maryland, and amended it with biochar at volumetric contents ranging from 0% to 15%. The tests revealed that as the biochar content increased, so did the liquid limits and plasticity indices of the soil. This suggests that biochar, with its porous structure and large surface area, enhances the soil’s water retention capacity and alters how clay interacts with water. In terms of compaction, the researchers observed a decrease in the soil’s maximum dry unit weight and an increase in its optimum moisture content as more biochar was added. This is consistent with biochar’s low density and high-water absorption properties.

The research also looked at how biochar affects the clay’s strength and compressibility. Consolidation tests showed that adding more biochar increased both the compressibility and the final void ratio of the soil. For instance, the compression index (Cc) rose from approximately 14.4 for unamended soil to about 21.1 for soil with 15% biochar. This behavior is likely due to the porous nature of biochar, which introduces additional voids into the soil matrix, trapping air and water and making the soil more compressible. However, this effect is contrasted by the findings of Bian, et al., who observed that increasing biochar content decreased the compression index in high-moisture-content soil. The study’s authors suggest this could be due to the differences in the soil’s plasticity, where the more plastic clay in their study absorbs more water than the silty sand used in a similar study by Lamprinakos and Manahiloh.

To evaluate the soil’s strength, the researchers conducted direct shear and unconfined compression tests. The results from the direct shear tests were promising, showing an overall increase in the soil’s shear strength as more biochar was added. Both the cohesion coefficient and the internal friction angle improved, suggesting that biochar enhances the bonds between soil particles and increases the soil’s resistance to shear deformation under drained conditions. For example, the friction angle for the soil increased from 22.8 degrees with no biochar to 40.1 degrees with 15% biochar. Similarly, the unconfined compression tests demonstrated a general increase in both the soil’s peak strength and its ability to deform before failure. The peak strength rose from 53.4 kPa for the unamended clay to 63.4 kPa for the clay with 15% biochar. The strain at failure also increased from 13.7% to 20.1% over the same range, indicating that biochar makes the soil more ductile.

These findings collectively suggest that wood-based biochar is a sustainable and effective material for improving the engineering properties of clayey soils. By enhancing strength, ductility, and compressibility, biochar offers a low-cost, environmentally friendly alternative to traditional soil stabilizers. The researchers propose that future work should investigate the long-term performance of biochar-amended soils in real-world conditions and explore how different types of biochar might affect geotechnical properties.

Source: Manahiloh, K. N., Fetene, S., & Detwiler, E. (2025). Effect of Wood-Based Biochar on the Engineering Properties of Medium Plasticity Clay. Preprints.org.