Influence of water content and organic matter on the mechanical strength characteristics of municipal sewage sludge

Municipal sewage sludge typically has high moisture and organic matter contents, which exert a significant influence on mechanical strength, dewatering and resource recovery. Organic soil specimens with varying water contents were prepared by mixing raw sewage sludge (RSS) with dry soil powder (DSP), humus, and quartz sand tailings (QST). Unconfined compressive strength (UCS) tests were conducted to investigate the effect of water content on the UCS. Results show that the UCS decreases with normalized water content ( w ₀/ w _L ) following a power-law relationship, and a quantitative formula is established. Specimens with different organic contents were fabricated by blending DSP into RSS. Unconsolidated undrained triaxial tests were performed to assess strength evolution with organic content. For organic high-liquid-limit silt, the angle of internal friction changes slightly with organic content ( w _u ), maintaining a low value of 1–3°. Cohesion decreases gradually when w _u  < 17%, drops sharply when 17% <  w _u  < 21%, and stabilizes at around 6 kPa when w _u  > 21%. An organic content of 17–21% serves as a possible transition interval for sludge structure and strength degradation. Mechanism analysis reveals that the contribution of organic matter to soil strength is predominantly governed by cementation, with negligible influence on the internal friction angle. The reduction in cohesion is primarily attributed to the decrease in dry unit weight, degradation of mineral skeleton, a predominance of the soft organic matrix, and increased plastic deformation.