Abstract: Studies on the effect of humic acid on the long-term strength of solidified sludge soil, are of great importance. In this research, sludge with low organic matter content was solidified by mixtures of cement, metakaolin,and lime, etc. On this basis, humic acid in proportion of 0.5%, 1.5%, 3% and 4.5%, 6%, respectively, was added into the sludge. The development laws of bending strength, stress-strain relationship and failure strain of solidified sludge soil under standard curing condition for 240 days were obtained.Results revealed that 1) under the joint actions of humic acid and inherent humic acid slowly released from organic matters of the sludge, the initial bending strength of solidified soil increased rapidly and then decreased to different extents; 2) when humic acid content increased from 0.5% to 6%, the bending strength of solidified soil at different ages almost degraded to a certain degree, and the balance between solidification and humic acid erosion occurred in advance, from the 180th day to the 60th day; 3) the failure mode of solidified soil with high content of humic acid experienced the tendency from plastic failure to brittle failure, and then to partial plastic failure, and the bigger content of humic acid is, the more obvious the action of plastic failure is.

Key words: solidified sludge soil, humic acid, long-term strength, stress-strain relationship, degradation, failure mode