Hydrochar effects on the microstructure and hydromechanical behaviour of unsaturated compacted soils

The effectiveness of using vegetation to stabilise shallow soil slopes heavily depends on the survival of vegetation, yet the amplification of extreme events induced by climate change threatens the health of plants covering slopes. Hydrochar is an environmentally friendly soil amender that can achieve the potential benefits of promoting plant growth for slope stabilisation and facilitation of waste upcycling. The mechanism underlying the hydrochar effects on the mechanical behaviour of unsaturated soils remains unclear. This study investigated the influence of grass-derived hydrochar on the water retention, compressibility, and shear strength of a compacted silty–clay sand. Soil microstructural changes due to hydrochar amendment were measured to explain the soil–hydrochar hydromechanical interaction. The increase in suction resulted in a less significant increase in yield stress and a negligible reduction in compressibility of the hydrochar-amended soil compared with the unamended case. This phenomenon was observed because hydrochar addition reduced the large pores with diameters greater than the macropore peak of 60 μm due to pore filling by hydrochar particles, resulting in a less substantial volume contraction during drying. Hydrochar introduced more significant effects on the soil’s shear strength in an unsaturated state compared to a saturated case. Despite the similarity of the unsaturated amended soil with the critical-state friction angle to the saturated case, the former exhibited a greater shear strength because the hydrochar addition improved water retention capability. As a result, the degree of saturation and, hence, Bishop’s effective stress were higher than those for the unamended case for a given suction.