A turbine extension and modernization program at this Pumped Storage Hydrogeneration Project required complete un-watering of the subsurface water conveyance structures that connect its Lower Reservoir (LR) and Upper Reservoir (UR). Stability of the earthfill embankment components of the project, which are sensitive to water surface elevations and changes in piezometric pressures, added complexity to the engineering process.



The operational water levels vary over a head differential of 50 feet for the UR and 40 feet for the LR. The structures consist of an 1,143-foot-deep concrete vertical shaft and horizontal power tunnel, which separates into four steel-lined 12-foot-diameter penstocks through a manifold structure located halfway from UR to LR.

The un-watering was carefully sequenced to lower the UR slightly below operational level; construct a temporary cofferdam at the UR; lower the UR an additional 30 feet; construct a permanent cofferdam; de-water the vertical shaft; and, finally, pump out the power tunnel, manifold and penstocks.

Water control for the project required extended periods of high water in the LR and a relatively rapid drawdown of the UR. Since both the Lower Dam and Upper Dike are zoned earthfill embankments, GZA reviewed their stability under the conditions and loading anticipated during the water control procedures and determined that there were adequate factors of safety at both structures. GZA was also asked to review the client’s cofferdam design, used to isolate the power shaft intake at the UR from the main body of the reservoir, which was left partially full. GZA subsequently inspected the cofferdam in the field post-construction.


GZA executed the complex engineering evaluation and review calculation in support of this important modernization project, including ensuring the proposed project would not be detrimental to the stability of the facility’s most sensitive components.