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The slope stability of saline soil always poses a hidden danger. Excessive changes caused by pore saline in suction and volumetric can damage the stability of green infrastructure with slopes, such as landfill liners. There is a scarcity of studies that have utilized sedimentation tests to examine the influence of different pore fluid salinities on the hydromechanical behavior of clay. Furthermore, research investigating the coupling effects of biochar and salinity on clay’s hydromechanical behavior by calculating effective stress is limited. Based on the research background, water retention, volumetric shrinkage, and sedimentation tests were conducted in a temperature-controlled chamber in this study, with continuous monitoring of the change in soil volume (including height). Soil-water characteristic curves were determined under different relative humidity conditions by using Kelvin’s equation. An increase in porewater salinity led to a lower void ratio at a given gravimetric water content, while biochar addition decreased the volumetric expansion of clays by 0.4–14%. Biochar addition could decrease the initial volumetric state (i.e., void ratio) by 12–26%. When the porewater salinity increased from 0 to 10%, the air entry value improved by 26 times, and the addition of 10% of biochar was 3 times higher than that of bare soil at 10% pore fluid salinity. Under highly saline porewater (r> 5%), biochar-amended kaolin settled 2–9% less than bare kaolin. This was attributed to the high sodium ion adsorption capacity and porous structure of biochar, resulting in a lower sensitivity of clays to an increase in porewater salinity. This study suggests that adding biochar is likely to enhance the hydromechanical resistance of clay’s structure to porewater salinity.