Numerical simulations of pore pressure on concrete buttress dams

Abstract: Uplift pressure is one of the most dominant forces on a dam structure. Difference in head water and tail water creates a hydraulic gradient resulting an upward oriented pressure called uplift pressure, which reduces the dam safety against sliding. However, there are great uncertainties determining the actual magnitude and distribution of the uplift pressure. Therefore, finite element methods were used in order to study the pore pressure in greater extent to increase the knowledge about the field measurements.    In this MSc report, the pore pressure in the rock foundation at monolith M42 in Storfinnforsen hydropower dam has been investigated. The aim of the project is to study the realistic pore pressure distributions and its magnitude underneath the dam. The aim is also to compare and find correlation between the numerical results and field measurements as well as the analytical results based on guidelines for dam safety.    Pore pressure in rock foundations can be influenced by many factors such as, the presence of drainage, grout curtain and rock fractures. Therefore, several numerical models have been developed containing different combinations based on these factors. Numerical results were then compared to both field measurements and analytical results. Furthermore, the uplift pressure from the best calibrated model and models representing critical states of the dam are used to calculate stability safety factor against sliding. The finite element program Abaqus is used to perform all the numerical pore pressure analyses. Field measurements is obtained from two pore pressure sensors installed underneath the dam and analytical results are calculated based on guidelines for dam safety. Based on the results from the case study, comparison of the numerical pore pressure with the field measurements showed that the drains have the largest impact on the pore pressure compared to the parameters. Models without drains have showed significantly larger pore pressure than the field measurements. When drains are in operation, considering empty drains underestimate the pore pressure while water filled drains slightly overestimated the pore pressure.    The influence of grout curtain varies if the effect of drains are included or not. Considering fractured rock foundation, the grout curtain showed large reduction of the pore pressure in the analyses without drains. If drains are in operation, this reduction becomes quite small. Moreover, all the numerical analyses showed that the uplift pressure in the rock fracture are lower than what is defined in the design guideline (RIDAS, 2020).