Abstract：It is recognized that the intermediate principal stress plays an important role in strength and stress- dilatancy relation of soft sedimentary rock. However，few plane strain/true triaxial experiments on soft rock have been referenced in literature due to the relatively high strength of geomaterials. By using new apparatuses，triaxial and plane strain tests of sedimentary soft rock have been conducted. An innovative plane strain test system is introduced firstly，which can obtain real isotropic consolidation before shear and measurement of inner pore water pressure within/near shear band. Then，testing procedures including issues related to specimen preparation，are introduced. Finally，the strength characteristics，stress-dilatancy relation and creep behavior are analyzed. The following conclusions have been drawn. The strength of the soft rock increases under plane strain conditions，while the dilatancy decreases. The plastic potential function based on concept of tij is suitable for soft rock. Creep failure occurs only when the creep load reaches a threshold value；and the failure time is related to magnitude of creep load.
