Effect of biochar on mechanical properties of purple soil in Three Gorges Reservoir Area

Biochar, as a soil amendment, is commonly used to enhance soil fertility, promote soil nutrient retention, and remediate soil pollution. However, the extent of the impact of biochar with different particle sizes and dosages on the mechanical properties of purple soil remains unclear. Therefore, this study took purple soil from the Three Gorges Reservoir Area as the research object, with biochar dosages set at (2%, 4%, and 8%) and particle sizes set at (<0.25 mm, 0.25 ~ 0.5 mm, 0.5 ~ 1 mm, and 1 ~ 2 mm). It conducted direct shear tests and unconfined compressive strength tests, while setting up a blank control (CK). The study analyzed the stress-strain curves, cohesion, internal friction angle, and unconfined compressive strength of specimens under different dosages and particle sizes, to clarify the effects of biochar particle size and dosage on the mechanical properties of purple soil. The results show that: the reinforcement effect of medium-sized biochar (0.25 ~ 1 mm) is significantly stronger than that of fine-sized biochar (<0.25 mm) and coarse-sized biochar (1 ~ 2 mm); biochar dosages of 2% and 4% can significantly improve the mechanical properties of purple soil (P<0.5); when the biochar dosage reaches 8%, the enhancement effect on mechanical properties decreases, and the unconfined compressive strength decreases by 22.95% compared with the CK specimen; the specimen exhibits the best mechanical properties when the biochar particle size is 0.25 ~ 0.5 mm and the dosage is 4%, with the shear strength and compressive strength increasing by 47.96% to 58.00% and 49.35%, respectively, compared with the CK specimen. After biochar is incorporated into soil, it affects cohesion and internal friction angle on one hand by modifying the arrangement and contact characteristics of soil particles. On the other hand, biochar particles absorb strain energy when subjected to stress, which retards crack propagation. This study can provide a theoretical basis for the application of agricultural and forestry waste resources in the field of soil stabilization.