Abstract
In order to study the unloading rheological properties of hard rock under complicated conditions, the proposed Mengdigou hydropower station with a granite dam foundation was taken as the research object. This paper studied the unloading rheological properties of the granite by testing means, and it obtained the unloading rheological deformation characteristics and deformation rules of the granite under complex stress conditions. The granite unloading rheological characteristics of the whole stress‐strain curve were analyzed, and the nonlinear unloading rheological model (FOD‐HKVP) was proposed based on the fractional‐order derivative. When certain conditions are satisfied, it is possible to degrade this model into the classical Nishihara model, but it should be noted that the FOD‐HKVP model is a more reasonable way to determine the long‐term strength of the granite and to consider also the rock unloading rheological damage effect. It introduced the fractional‐order viscous body of combination components with a different fractional‐order derivative to describe the nonlinear creep characteristics of the granite unloading, which provides an adequate description of steady‐state creep and accelerated creep properties. Following the analysis of test data and parameter identification for the FOD‐HKVP model, the rationality of the model was validated. Compared with the Nishihara model, the fitting accuracy of FOD‐HKVP model was more accurate.
Highlights
In order to study the unloading rheological properties of hard rock under complicated conditions, the proposed Mengdigou hydropower station with a granite dam foundation was taken as the research object. is paper studied the unloading rheological properties of the granite by testing means, and it obtained the unloading rheological deformation characteristics and deformation rules of the granite under complex stress conditions. e granite unloading rheological characteristics of the whole stress-strain curve were analyzed, and the nonlinear unloading rheological model (FOD-HKVP) was proposed based on the fractional-order derivative
For many large hydropower stations in China, the buried depth of the foundation engineering of underground caverns is larger, and most of the dam foundation rock mass is comprised of granite, marble, and other hard masses. e natural condition is that rock masses are in a three-dimensional, highstress state, and so cavern excavation has the potential to lead to rock mass unloading
Liu et al [9] considered the excavation involved in unloading rock destruction, and this was followed by an investigation of marble unloading creep properties by implementing triaxial unloading creep tests. e findings indicated that the rock unloading creep rate and creep deformation increase with an increase in axial stress, while they decrease with an increase in confining pressure
Summary
In order to study the unloading rheological characteristics of hard rock, the granite of Mengdigou hydropower station was taken as the main research object. Owing to the high strength of hard rock, the creep deformation for time dependence is stronger, and so in order to observe more obvious creep deformation, unloading rheological length was set at each level for 50–70 h. This enabled the study to satisfy the accuracy requirements of unloading rheological levels, and the unloading step was set at 5–8. On the contrary, when the stress level is higher, the rock creep strain shows a steady growth in speed over time, and it is characterised by stable creep properties. When the stress level is lower than long-term strength, hard rock displays typical viscoelastic characteristics; when the stress level is greater than the long-term strength and reaches the fracture stress level, it displays typical sticky plastic characteristics
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