Monitoring of Steel Lined Pressure Shafts Considering Water-Hammer Wave Signals and Fluid-Structure Interaction

In the past, the safety margin for dynamic water pressure loads in steel-lined pressure tunnels and shafts was considered as acceptable by using conventional design safety factors. Due to high peak energy demands, existing plants are operating nowadays under rough conditions to regulate the discharge and power with relatively fast and repeated opening and closing of turbines and pumps. The economic and social costs due to production losses, when these water conveying structures are emptied for investigations and repairs, are considerable. Furthermore, the failure of pressure tunnels and shafts may produce catastrophic landslides and debris flows. An extensive literature review showed that the existing design methods have been based on the idea of keeping the allowable stress in steel liner below yielding threshold. These methods use also some rules for construction details and tolerances which minimize the risk of formation of high local concentrated stresses. Since the beginning of use of very high-strength steel liners in new hydro plants, the actual design methods and safety assessment have become inappropriate. This type of steel has a high risk of brittle failure and fatigue. Therefore, an enhancement of the existing theoretical design model for steel-lined pressure tunnels and shafts is necessary. Generally applicable approaches for estimating the quasi-static, which means without FluidˆStructure Interaction (FSI) and frequency-dependent water-hammer, wave speed in steel-lined pressure tunnels have been analyzed. The external constraints and assumptions of these approaches are discussed in detail and the reformulated formulas are then compared to commonly used expressions. For thin steel liners and weak rock mass modulus, Jaeger's and Parmakian's relationships overestimate the water-hammer velocity by approximately 3 – 4.5 %, while in Halliwell's formula this overestimation reaches 7.5 %. The quasi-static wave speed is significantly influenced by the state of the backfill concrete and the near-field rock zone (cracked or uncracked). In the case when these two layers are cracked, the quasi-static wave speed is overestimated in between 1% and 8% compared to uncracked concrete and near-field rock layers. Depending on the stiffness of steel liner and penstock, the FSI leads to significant difference in wave speeds values. As a first step, a fluid-structure interaction model is proposed as a basis for the development of new design criteria which consider fracture mechanics to access the response of high-strength steel liners. The effect of the backfill concrete and the surrounding rock mass has been mechanically modeled by a spring, a dashpot, and a lumped additional mass. The quadratic dispersion equation which results from FSI model, has been solved in the frequency domain through a numerical example. In this example and compared to the quasi-static case, the FSI approach results up to 13% higher wave speed values in the high-frequency range (