Effect of unloading velocity on surrounding rock deformation

The excavation unloading effect of roadway / tunnel surrounding rock is directly related to the stability of sur- rounding rock,the design of support and the selection of construction technology. Fast and slow unloading experiments were carried out to study the excavation unloading effect of small surrounding rock specimens made from cement mortar by using the independently developed experimental system of excavation and unloading. Deformation laws and failure patterns of the excavation unloading process were obtained. The test results show that:① The relation curves of radial strain and tangential strain in fast and slow unloading and time show three characteristic stages:specimen compaction stage,rapid deformation stage,and slow deformation stage. ② In the same cross section,the tangential strain of the in- ner side reacts faster and more significantly to the unloading action,and greater deformation is produced. The total de- formation produced by the inner side radial is greater than that on the outer side,while the total deformation produced by the inner side axis is less than that on the outer side. ③ No matter fast or slow unloading,the tangential strain at the same point is the most important and occupies an eminent position. The radial deformation is concentrated at unloading phase,and the axial deformation is concentrated in the maintenance phase after unloading. ④ In the unloading phase, with the increase in pressure difference,the tangential and axial strain of slow unloading show an accelerated develop- ment trend,and the radial strain difference value of inner and outer is about 200. The tangential and axial strains of quick unloading show a tendency of convergence,and the radial strains of inner and outer are basically the same. Dur- ing the maintenance phase,the tangential and axial inner and outer deviations of the two unloading methods continue to increase. In the fast unloading phase,the radial strain on the inside and out lateral starts to differentiate and the differ- ence becomes larger and larger. ⑤ The specimen is compressed and becomes shorter,and the annular penetrating sur- face appears on the outside and is accompanied by splitting failure,and the inner wall shows a slice-like shear failure zone. There are two damage zones in the cross section parallel to the ring damage zone.