The present paper provides an example of using two advanced failure criteria, namely "nonlocal Mohr Coulomb with softening" and "multilaminate", for modelling underground structures. The presented numerical studies have been carried out to examine cavern diggings of the Nant de Drance hydropower project which has been excavated in an Alpine foliated mica-schist with an overburden exceeding 600 m. The complexity of the rock mass texture which exhibited clear anisotropy and discontinuity urged adopting a constitutive model capable of taking the rock material features into account accurately. In this paper, after describing the fundamentals of each failure rule and the involved mechanical parameters, those are used to simulate triaxial laboratory tests of mica-schist samples. The obtained numerical predictions are compared to the experimental data. At last, uses are made to model the cavern excavation by both the criteria and the obtained results in terms of rock deformations are compared with the in-situ measurements.
While conventional Mohr-Coulomb is yet the most favorable amongst engineers, making use of more advanced modelling technics can effectively result in more sustainable, less energy-consumable thereby more environment-friendly developments. Many different material constitutive models have been developed so far to model failure behavior of rock mass. They range from finite elements plasticity models to microstructural simulations based on the discrete element method. A comprehensive review of the existing literature was presented by Kazerani (2011) and Zhang & Zhao (2014). Although the discrete element models have been quite successful in treating rock particle details in micro-scale, they all involve a fundamental limitation, that is, lack of feasibility for practical large-scale purposes where the created particle models will involve millions of elements thereby be impossible to be dealt using current computer facilities. In addition incorporating structural details (e.g., tunnel lining and rock bolt) into a particle-based rock model is hardly possible in practice.
This paper presents the application of two previously developed constitutive laws, namely "nonlocal Mohr-Coulomb with softening" and "multilaminate", for cavern excavations implemented in the Nant de Drance hydropower project in the Swiss Alps. This 1.8 billion-dollar investment is a pumped-storage scheme that will provide 900 MW pump-turbine capacity and will annually generate 2500 GWh by using the head difference between two existing reservoirs (Garin 2014). This required excavation of over 15 km of galleries and shafts as well as caverns the largest of which (called KMA) will house six Francis pump-turbines and is amongst the largest of its kind around the globe. 194 m long, 52 m high and 32 m wide, KMA is excavated in a foliated mica-schist which exhibits clear anisotropy and involves joint sets. The numerical modelling were implemented by ZSoil.PC (2011), a finite element analysis package (Zace 2011). After presenting the details of the constitutive laws and a brief of their background mathematics, the paper describes modellings of laboratory tests done on the rock cores obtained from five deep boreholes drilled around the cavern. Those include uniaxial and triaxial compression, indirect tension and direct shear tests. The numerical predictions are then discussed and compared to the laboratory data quantitatively and qualitatively. In conclusion, the results of simulating the cavern excavation by both the constitutive laws are presented and compered with the in-situ measurements.