Research on unloading nonlinear mechanical characteristics of jointed rock masses
Jianlin Li1, Lehua Wang1, Xingxia Wang1, 2, Ruihong Wang1, Zhuang Cheng1, Li Dang1
1 Key Laboratory of Geological Hazards for Three Gorges Reservoir Area of Ministry of Education, China Three Gorges University, Yichang, 443002, China
2 College of Hydraulic and Environmental Engineering, China Three Gorges University, Yichang, 443002, China
Received 12 October 2010; received in revised form 5 November 2010; accepted 16 November 2010
Abstract: Geological environments of rock mass projects are always very complicated, and further investigations on rock mechanical characteristics are needed. There are considerable distinctions in rock mechanical characteristics under unloading and loading conditions. A series of tests are conducted to study the stress-strain relationship of rock masses under loading and unloading conditions. Also, the anisotropy, the size effect, and the rheological property of unloading rock mass are investigated. The tests presented in the paper include model test and granite rheological test, which are conducted considering geological condition, rock mass structure, in-situ stress field of the permanent shiplock of the Three Gorges Project. The main differences between loading and unloading rock masses are stress paths, yield criteria, deformation and strength parameters, etc.. Different structural plane directions affect unloading rock mass evidently. With increasing size, the tensile strength, the compressive strength, the deformation modulus, the Poisson’s ratio and the anisotropy of rock mass all decrease. For sandstone samples with parallel bedding planes, the cohesion c increases but the internal friction angle decreases under unloading condition when compared with the values under loading condition. While for samples with vertical bedding planes, the trend is adverse. The rheological property of rocks has close relationship with the tensile stresses of rock masses. When the sandstone samples are tested under high stress condition, their rheological properties are very obvious with the unloading of confining pressure, and three typical rheological stages are shown. Rheological rate changes with the variations in axial stress and confining pressure.
Key words: unloading rock mass; unloading test; anisotropy of rock mass; size effect; rheological rate