As part of the Federal Energy Regulatory Commission-required seismic stabilization of the Linville embankment dam, Duke Energy Carolinas, LLC (Duke Energy) has replaced the existing, early 20th century vintage powerhouse at the site. The new three-unit Bridgewater Powerhouse presented unique design and construction challenges. To maintain downstream flow and manage the reservoir, the new powerhouse had to be constructed while the existing powerhouse remained in operation. The configuration required using the existing penstock to supply the new powerhouse by means of a penstock extension welded to the existing riveted steel penstock. The existing penstock remained in service for the duration of construction. The new penstock incorporated a structurally complex, large diameter bifurcation immediately downstream of the tie-in point to the existing penstock. Another construction challenge was the confined nature of the site. Bordered by the existing powerhouse, transformer yard, and tailrace to the north and east; the Linville River to the south; and a state highway to the west, construction was limited to a small area. The confined construction area resulted in the installation of several temporary and permanent soil retention structures to facilitate the excavation of approximately 48,000 cubic yards of soil and partially weathered rock. Further constraining the excavation was a high water table and porous bedrock that necessitated an extensive dewatering effort consisting of installing a 250-foot-long concrete secant pile cut-off barrier wall and 44 dewatering wells. Upon completion of excavation, approximately 18,000 cubic yards of foundation mass concrete was placed. The desired rate of mass concrete placement dictated the need for an extensive Thermal Control Plan to control and manage the heat of hydration. The final powerhouse began commercial operation in November 2011. It will meet regulatory requirements of providing a minimum continuous flow in the river downstream of the plant, while generating up to 32 MW of renewable power for Duke Energy customers. This paper will discuss the design and construction challenges encountered during this project and the creative engineering methods utilized to address them.
展开▼
