Eggshell as Soil Shear Strength Amplifier for Cohesive and Cohesionless Soils

The reuse of biodegradable waste in geotechnical engineering has gained increasing attention due to its potential as a sustainable construction material. Eggshell powder (ESP), generated in large quantities as biodegradable waste, represents a low-cost and environmentally friendly option for such applications.In this study, ESP was added to cohesive and cohesionless soils at proportions of 8.5%, 12%, and 15% in order to examine its effect on shear strength characteristics. Shear strength tests were conducted on laboratory-prepared soil specimens. The test results showed that the response of the soils to ESP addition depended strongly on soil type. In the case of the cohesionless soil, an increase in shear strength was observed with ESP inclusion, and the highest angle of internal friction, equal to 38◦, was obtained at an ESP content of 8.5%. This behaviour suggests that ESP particles contributed to improved grain interaction within the soil mass. On the other hand, the clayey soil exhibited a reduction in shear strength after the addition of ESP. The maximum reduction in cohesion was measured as 22.56 kN/m2 at 8.5% ESP content, indicating that ESP did not act as an effective stabiliser for the clay under the test conditions.To predict the optimum dosage and the corresponding strength gain, a quadratic regression model was developed, suggesting a nonlinear relationship between ESP content and shear strength.Overall, the results suggest that ESP incorporation acts predominantly as a physical modifier rather than as a chemical stabilizer. Within a limited dosage range, ESP effectively enhances the frictional behaviour of cohesionless soils; however, it is not suitable as a standalone stabilizing agent for cohesive soils. These findings emphasize the importance of carefully considering soil type when using waste-derived additives in geotechnical engineering applications.