Abstract: To investigate mechanism of compressive deformation of red clay contaminated by heavy metal Cu²+, CuSO₄ solutions with concentrations of 0, 2.5, 5.0, and 10.0 g/L were used to prepare the contaminated soil, and consolidation tests, thermal analysis tests, and scanning electron microscopy (SEM) tests were conducted. The results show that the changes of compression coefficient and total compressive deformation of red clay and the changes of pore water (free water, weakly bound water and strongly bound water) content in the clay show the same trend of decreasing first and then increasing with the increase of Cu²+ concentration. The structural connection between soil particles in clay soil is mainly based on adsorption water film contact. Cu²+ changes the water film thickness of pore water in soil, which leads to the change of pore water content. The water film becomes thinner, the distance between soil particles shortens, the higher the structural strength of soil body, and the greater the ability of soil body to resist compression deformation. With the increase of Cu²+ concentration, the microstructure of soil body gradually evolves from loose block-like and flaky units to stacked aggregates with face-to-face contact. When the concentration of Cu²+ increases to 10.0 g/L, scale-like units start to appear, the contact mode between units is mainly point-point contact and edge-face contact, the cohesion between soil particles becomes poor, and the structural stability of soil body decreases.

Key words: red clay, compression deformation, thermal analysis tests, microstructure