Abstract: Completely decomposed migmatitic granite（CDMG）is a kind of rock with special genesis, but there is little research on its mechanical properties and microstructure. In order to explore the relationship between macroscopic strength characteristics and microstructure of Lincang CDMG, triaxial tests and scanning electron microscopy tests were carried out on samples with different moisture contents, and the microstructure parameters that characterized the size, morphology and orientation of particles and pores were extracted and calculated. By investigating these microstructure parameters, the micro mechanism controlling the macroscopic strength characteristics of CDMG is revealed. The results show that with the increase of moisture content, the stress-strain curve of the sample exhibits a dissimilar hardening effect, the shear strength deteriorates significantly, the internal friction angle decreases linearly, and the cohesion fluctuates up and down. Under the condition of low moisture content, the samples are mainly characterized by micro and small pores, and the particle morphology is mostly angular and sub-angular strip with certain orientation. With the increase of moisture content, the pore content shows an upward trend, forming evenly distributed honeycomb medium and large pores, which are easier to be compressed under axial load. The shape of particles develops to be round, and the overall distribution is disorderly and poorly oriented. Comprehensive analysis of the above results shows that the mechanical properties of CDMG are essentially the result of the interaction between microscopic particles and pore structure. The increase of moisture content weakens the friction between coarse particles, resulting in uneven expansion of clay particles, which leads to pore connection and coarsening and damage of filling structure at the micro level, and then shows mechanical properties reduction at the macro level. The research results provide useful reference for the cognition of mechanical properties and microstructure damage evolution of CDMG.

Key words: completely decomposed migmatitic granite, shear strength, microstructure, moisture content, particle, pore