As part of the FERC Potential Failure Mode Analysis (PFMA) process at a large gravity dam associated with a hydropower project, the project Owner identified potential for dissolution of gypsum from the dam foundation. The uplift control system at the dam includes a line of relief wells which collect seepage from the foundation and discharge it into an internal gallery. There was a concern that soluble gypsum was being carried by seepage water and lost through the gallery drainage system. The concern was that this loss of rock material from the dam foundation could negatively affect the stability of the dam or lead to operational problems for the turbines.
GZA was engaged by the Owner to investigate the potential problem and overall conditions in the dam foundation. GZA first undertook a review of construction era and post-construction and identified the need for additional confirmatory investigations and more extensive water chemistry sampling and analysis. GZA therefore developed and implemented a program of supplemental test borings, testing, and instrumentation within the gallery. GZA used geophysics perform in-situ logging of the holes and performed extensive flow testing in the relief wells and both in-situ and laboratory analysis of water chemistry. Using this data, GZA was able to identify the source of gypsum flux as very thin interbedded stringers within the foundation.
GZA then designed a grouting program to address the seepage and gypsum flux from the foundation. High-mobility balanced stabilized grout was specified with a provision to use chemical grouting if necessary. GZA assisted NYPA in bidding the project and then provided resident engineering services throughout the grouting project. The project was successful in reducing both seepage and gypsum flux both by almost 90 percent.