Abstract: The unsaturated shear strength of fly ash is necessary for precise stability analysis of ash dams in unsaturated cases. However, less studies can be taken for references in this research field currently. A series of triaxial tests of unsaturated fly ash were carried out by controlling compaction, matric suction, and net confining pressures, and the effects of compaction and matric suction on the stress-strain curve and strength parameters of fly ash were discussed. The results show that the stress-strain curve of fly ash has no obvious peak and shows strain hardening characteristic when the matric suction is low. When the matric suction is gradually increased, the stress-strain curve has an obvious peak and presents the strain softening characteristics. If the compaction degree is larger, the strain softening characteristic is more obvious and the total cohesion and the effective internal friction angle are bigger. The total cohesion of the unsaturated fly ash increases with the increase in the matric suction, and the increase rate is gradually slower and tends to be stable. The effective internal friction angle of the fly ash sample with different matric suctions has little change, and is nearly equal to the effective internal friction angle of the saturated sample. As the matric suction increases, the contribution of the internal friction angle to the shear strength of the fly ash sample becomes smaller and smaller. The shear strength characteristics of unsaturated fly ash obtained in this paper have theoretical significance and engineering practical value for the design and stability analysis of ash dams.

Key words: fly ash, shear strength, compactness, matric suction