Fadi Hachem

Detection of Local Wall Stiffness Drop in Steel-Lined Pressure Tunnels and Shafts of Hydroelectric Power Plants Using Steep Pressure Wave Excitation and Wavelet Decomposition

A new monitoring approach for detecting, locating, and quantifying structurally weak reaches of steel-lined pressure tunnels and shafts is presented. These reaches arise from local deterioration of the backfill concrete and the rock mass surrounding the liner. The change of wave speed generated by the weakening of the radial-liner supports creates reflection boundaries for the incident pressure waves. The mon- itoring approach is based on the generation of transient pressurewith a steep wave front and the analysis of the reflected pressure signals using the fast Fourier transform and wavelet decomposition methods. Laboratory experiments have been carried out to validate the monitoring technique. The multilayer system (steel-concrete-rock) of the pressurized shafts and tunnels is modeled by a one-layer system of the test pipe. This latter was divided into several reaches having different wall stiffnesses. Different longitudinal placements of a steel, aluminum, and PVC pipe reach were tested to validate the identification method of the weak section. DOI: 10.1061/(ASCE)HY.1943-7900.0000478.

Effect of drop in pipe wall stiffness on water-hammer speed and attenuation

The effect of local drop of wall stiffness in pressurized waterways on the pressure wave speed and the wave attenuation during transients is investigated experimentally. A new signal-processing procedure to identify the presence of a weak reach is introduced and validated by physical experimentation based on assessing the pressure and vibration records acquired at both ends of a multi-reach steel test pipe. Water-hammer was generated by closing a downstream valve, whereas the weak reaches were simulated by replacing the steel portions with aluminium and PVC material. The wave speed and wave attenuation factor during transients are considered as global indicators of local and large changes in stiffness of the pipe wall. The method is capable of locating the stiffness weakness along the test pipe if one PVC reach is used. The error in estimating the position of such a reach relative to the real position of its mean value varies up to 23%.

Online Monitoring of Steel-Lined Pressure Shafts by Using Pressure Transient Signals under Normal Operation Conditions

The local deterioration of the mechanical properties of steel-lined pressure shafts and tunnels induces a decrease of the celerity and an increase of the energy attenuation of water hammer waves. This deterioration may arise from the weakening of the backfill concrete and the surrounding rock mass, which provide the radial support of the steel liner. A new online monitoring system implemented and tested at the Grimsel II pumped-storage shaft in Switzerland is presented. The difficulties encountered during the in situ measurements are outlined. The new monitoring approach is based on acquiring and analyzing the dynamic pressure data generated by the normal operation of pumps and turbines. The wave celerity and the exponential attenuation coefficient of water hammer waves are estimated based on pressure records. Monitoring charts for these indicators are established using the statistical quality control method. The results show a stable monitoring scheme that needs longer acquisition data series to consolidate its control limits.