Field Monitoring Of Splitting Failure And Three-Dimensional Geomechanical Model Test For Large-Scale Underground Cavern Group

Many large-scale underground hydropower stations have been built recently or are under construction in China. However, these underground caverns are mostly located in the high initial geostress fields which are strongly influenced by high mountains, steep slopes, and tectonic history. In some instances, the high horizontal principal stress will influence the stability of the caverns. Meanwhile, the surrounding rock masses are commonly hard and brittle rocks. Therefore, brittle failure will easily appear. Field monitoring, geomechanical model tests and numerical modeling were carried out to investigate the stability of large underground cavern groups at great depth and under high in situ stresses.The Pubugou underground powerhouse is approximately 400 m below the ground surface. During excavation, a number of vertical splitting cracks appeared in the high wall of the busbar chamber. In order to investigate the numbers and distributions of these cracks, Borehole Television (BTV) observations, C-1 electrical resistivity tester and the developed self-made borehole probe were carried out in the four boreholes at regular intervals throughout testing, the distribution rules of the vertical splitting cracks and the disintegrated zones of surrounding rock masses can be obtained. Meanwhile, it can be observed that the deformation of the high wall consists of opening displacement and continuous displacement.Large-scale geomechanical model tests were carried out under true three-dimensional stress state. Major improvements have been made in terms of experimental techniques and a number of advanced measurement methods and techniques have been developed and applied:(1) A model test system, which can apply active loading on six sides with full three-dimensional stress state, was implemented; (2) Combinational ball sliding walls were newly developed and installed on each major loading surface, which significantly reduced the friction due to model deformation; (3) During model construction, precast blocks were used and monitoring holes were reserved before the analogy material was piled up. In this way, the consistency of the mechanical properties of the model material can be better guaranteed; (4) Digital photogrammetric technique and displacement sensing bars based on fiber Bragg grating were adopted. Mini multi-point extensometers with high-precision grating rulers as sensors were developed to measure the deformation in the surrounding rock masses; (5) A unique grouting technique and prestress cables were adopted; (6) The overloading tests were accomplished for different overburden depth. The failure pattern and process in the surrounding rock masses were observed during loading; In addition, three-dimensional numerical modeling was performed and the model test results were compared with the numerical modeling results.