Abstract: In order to explain the failure and breakage mechanisms of structural geological materials ,the plane strain compression tests of the samples of prism-like structural bodies assembled in three patterns are conducted under unloading conditions. In the process of reducing the lateral stress, it is found experimentally that the structural bodies will be broken after some relative movement and the main failure modes of structural bodies are splitting and shear failure; while the samples assembling of prism-like structural bodies are broken gradually, the strain hardening or strain softening can be observed and breakage bands are macroscopically formed in the end; the volume strain of the samples is always contractive but the lateral contractive strain turns to be the lateral extensive gradually.

Key words: unloading conditions, prism-like structural bodies, plane strain compression tests, breaking process, breakage bands