EFFEKTER PÅ UPPSTRÖMSSTABILITETEN I FYLLNINGSDAMMAR : Ändrade porvattentryck till följd av cykliska magasinsnivåer

Sammanfattning

Hydropower is one of the few energy sources with the ability to store a resource and thus regulate electricity production. When the water level in a reservoir changes, this affects the pore water pressures in the dam. This study includes how the slope stability of a dam changes with regard to varying pore water pressure, as a result of cyclical water fluctuations. The study has been carried out by simulations with the finite element program PLAXIS 2D, where a small-scale and homogeneous fill dam is subjected to water level fluctuations followed by a constant variation of the reservoir level. The analysis includes variation in the location and appearance of the groundwater line in the dam, flow direction and development of pore water pressure and safety factor against slope failue. In the sensitivity study, the influence of tolerated error in the finite element program, choice of material model, cycle time and hydraulic conductivity is analysed. The simulations have shown that the pore water pressures generally have a decreasing trend with an increased number of cycles of water fluctuation. This as a result of a decreasing level of water saturation in the upper parts of the dam. The decreasing trend in pore water pressure contributes to an increased effective stress, which in turn gives an increased shear strength and the dam thus has an increasing trend for safety against slope failure. The sensitivity analysis shows that both cycle time and hydraulic conductivity and to some extent choice of material model are parameters that influence the development of pore water pressure. Adaptation of the tolerated error in the finite element program is crucial for evaluation of the result. A minor influence in the choice of material model is noted, above all the applicable safety factor where Hardening soil, after a number of cycles, underestimates the safety against slope failure in comparison with the UBC3D-PLM model.