Rock mechanics studies of mining in the Climax granite

The Lawrence Livermore National Laboratory (LLNL) is conducting a test of retrievable storage of nuclear spent fuel assemblies in three granite caverns, at Climax/Nevada Test Site, Nevada. During excavation of the three parallel chambers, stress changes and deformations were measured in the two intervening pillars. Originally, it was inferred from these data that arching had formed over the chambers, and that the pillars had unloaded. However, these conclusions were not substantiated, and the arching hypothesis was reexamined. A two-phase project was implemented to elucidate the rock mass response during the previous mining. First, a program of field testing was conducted. The in-situ modulus (Ef was estimated, using six different approaches. A value of Ef = 26 GPa was obtained by comparing results from the various approaches. The laboratory modulus obtained on small cores was El = 70 GPa. Thus, the modulus reduction factor for the Climax granite appears to be Ef/El = 0.37. In turn, the estimate of in-situ deformability was used to back-calculate values for the normal stiffness of the granite joints. A new analysis of former stress measurements showed that horizontal stresses in the vicinity of the caverns vary greatly with azimuth, and that the vertical stress appears to be only two thirds of the calculated lithostatic stress. New stress measurements also were performed around the chambers by two different methods. From the strain ellipsoid at mid-length in the tunnels, the in-situ Poisson's ratio of the rock mass was estimated as v = 0.246. Second, finite element modeling of the mining sequence was performed, using the refined estimates of material properties and in-situ stresses. The models provided a realistic description of the geology by discretely accounting for major joints and shears. The damage due to blasting was represented, as well. The combination of the in-situ test program and of the modeling provided a coherent explanation of the stress changes observed in the pillars. It was concluded that no arching had developed. This project also highlighted the need for redundant procedures in the analysis of rock structures. Specific recommendations regarding rock mechanics field testing are offered in conclusion.

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