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Numerical modelling of reinforced concrete pressure tunnels using the overlapping lattice method

Işık, Gökberk
Pressure tunnels transmit water from the reservoir to the turbines of hydroelectric power plants. The computational studies conducted to understand the behavior and shed light on the design guides of pressure tunnels were scarcely validated with experimental data as experimental work on pressure tunnels is very limited. Recent experimental studies on reinforced concrete pressure tunnels carried out at Middle East Technical University (METU), for the first time, allow computational models to be validated with experimental data. Instead of the conventional approach of using finite elements with smeared and discrete crack models, in this study, an Overlapping Lattice Model (OLM) was used to simulate the nonlinear behavior of the reinforced concrete tunnel as well as the surrounding rock body. First, parameters used in the OLM were calibrated using the fracture energy, the tensile strength and the elasticity modulus of concrete. A normalization rule was developed to make sure that energy dissipation is independent of lattice length scale. Then, the quasi-static tunnel experiments performed at METU were simulated and internal pressure-tunnel expansion curves as well as crack patterns obtained in the numerical studies were compared with the experimental observations. OLM predictions are shown to be in agreement with the experimental data. Calibrated model was then applied to study dynamic crack initiation and propagation in pressure tunnels. In line with the experimental observations and numerical inferences, in-situ stress conditions in the surrounding rock body play an important role on the behavior of the tunnel lining. Low confining stress or lack of sufficient contact between tunnel lining and surrounding rock body are possible causes of the cracks observed in pressure tunnels. As a future research opportunity, OLM developed in the context of this research can be used to evaluate the seismic behavior of tunnels.