Abstract: One of the effective ways to study the excavation and unloading effect of roadway (tunnel) is to carry out excavation and unloading test of small-scale rock specimen. In order to make it easier to prepare and control the strength of the small surrounding rock speimens, firstly the high-strength gypsum-based materials are selected to carry out the ratio test. The proportion of the corresponding materials with the strength range of 5-20 MPa and the influence of the similar materials composition on the strength of the specimen is obtained. Secondly, a small wall rock specimen (thick wall cylinder type: 290 mm high, 100 mm with diameter, 200 mm outer diameter) is processed. The tunnel excavation unloading model test system with independent intellectual property rights has been used to simulate the unloading and failure of the surrounding rock specimens. The excavation unloading process can simulate the excavation conditions of the actual engineering on the principle of mechanics; that is, the axial compression, external pressure and internal pressure are applied to the surrounding rock specimen to simulate the original rock stress. Excavation (that is, when unloading), unloading the internal pressure and keep the other two pressure constant. During the test, the relationship between the strain in two directions along the axial direction and the tangential direction of the surrounding rock specimen is collected. The strain-time curve and the relationship between the strain and the pressure drop are obtained. And present: the inner strain is larger than the outer one and the specimen expands inward during unloading; at the same measuring point the tangential strain is larger than the axial strain playing a leading role. When the unloading damage occurs, the failure direction of the surrounding rock specimen destroys toward the inside of the cave, that is, along the radial direction. Failure pattern presents stratified rupture.

Key words: high strength gypsum material, mixture ratio test, model test, small surrounding rock specimen, excavation unloading, surrounding rock deformation