Search

The construction of large-scale underground caverns poses numerous challenges due to their massive scale and high complexity. Among them, stability has always been a major concern during the construction process of large underground caverns within high-stress hard rock masses. It is not only influenced by the geological background of the project but also by the engineering activities during construction. This study takes the construction of the Shuangjiangkou (SJK) hydropower station as a case study and combines field investigations to summarize engineering issues and solutions under high geostress in three aspects. Firstly, it focuses on estimating and modifying the in-situ stress field of the underground caverns in the deep-cut valley area using limited geostress testing data and the observed failure phenomena in the pilot tunnel. Secondly, it discusses the typical failure phenomena of caverns under high geostress, including global stress-induced failures and local vein-controlled failures, as well as the corresponding failure mechanisms and treatment measures. Finally, it addresses the special failure phenomena occurring in the protective layer of the rock-anchored beam and optimizes the excavation scheme based on the observed failure patterns. The research results and solutions provided in this study, based on the case of the SJK hydropower station, are expected to serve as a reference for subsequent construction of large underground caverns within high-stress hard rock masses. [ABSTRACT FROM AUTHOR]

Abstract: The failure phenomena of a viscoelastic thin film of bitumen and polymer modified bitumen under direct tension tests is investigated in detail, experimentally and theoretically. Using video recording and image processing techniques, the time evolution of failure and film disintegration and separation phenomena were studied. The following failure stages were identified and recorded: Initial (=stage 1), necking (=stage 2), filamentation with void nucleation (=stage 3) and separation (=stage 4). The technique allowed the identification of an additional failure stage for the pure bitumen than the four identified for binders, namely light transparent thinning before void nucleation. The determination of stages to final separation allowed behavioral distinction between the straight run bitumen and the polymer modified bitumen. As the direct tensile tests proceed, in addition to shear forces demonstrative of viscous effects, gravity and capillary forces act on the binder film. In order to understand the relative effect of these forces the order of magnitude of relevant dimensionless numbers, representing ratios of these forces is considered and discussed. The analysis of the above numbers confirms that the flow in bitumen is dominated by viscous effects although capillary forces do play a role at later stages of the experiment with negligible inertia and gravitational effects. The detailed analysis of micromechanical behavior of straight run bitumen and polymer modified bitumen provided further indication corroborating the superior field performance of polymer modified bitumen in porous asphalt. [Copyright &y& Elsevier]