IS IT ROCK YET? FOUNDATION TREATMENT FOR THE ROCKY PEN RUN DAM

摘要

The proposed Rocky Pen Run Reservoir is a 5.5 billion gallon pumped storage facility that will serve as a major water supply reservoir for the County of Stafford in Virginia. The project consists of a 135-foot high main dam and spillway, reservoir outlet works and pump station, saddle dike, river intake and pump station along the Rappahannock River, water treatment plant, and pipelines. The owner and operator of the facility is the Stafford County Department of Utilities. The project site is located in the Eastern Piedmont Physiographic Province, which is characterized by a complex system of metamorphic and igneous rocks that have been folded and faulted throughout geologic time that have been differentially weathered. These geologic processes have resulted in complex and highly variable foundation conditions that present technical challenges to the designer that correspondingly increase the risk of costly change orders, claims of differing site conditions, and delays during construction. The preliminary engineering and design for the project began in 2004 and over the last seven years has evolved as additional subsurface information has become available. Following the initial geotechnical investigations and alternatives analysis, a roller-compacted concrete (RCC) dam with an integrated gated spillway was selected. Because of the risks associated with the Piedmont geologic conditions, the dam construction was divided into several phased construction contracts in order to conduct the foundation preparation in advance of the dam construction. This approach was selected as a means of concentrating the risk associated with the uncertain and variable foundation conditions in one contract and to subsequently use the "as-built" results to tailor the final dam design to actual foundation conditions. During the initial foundation excavation contract difficulties were encountered due to a major shear zone and highly variable rock conditions that raised the question as to the suitability of the foundation for an RCC dam. Following a re-evaluation of foundation conditions, it was concluded that the foundation was adequate for an RCC. However, by this time economic conditions had changed and following a second alternatives analysis, it was decided to change the dam type to an earth dam. The design was modified for an earth dam and a new foundation treatment design was prepared which was tailored to the encountered rock conditions. The geologic mapping of the foundation exposed during the initial excavation contract was used extensively to design measures to treat the six documented shear zones. The second foundation contract consisted of relocating the diversion alignment, some additional excavation, treatment of the irregular foundation with rock trimming and backfill concrete, dental excavation and treatment of shear zones and other geologic discontinuities, with a two-row grout curtain supplemented by blanket and stitch grouting along shear zones. The paper describes the results of the multi-faceted geotechnical investigations and foundation characterization conducted for the project, the foundation treatment design, and how the design was adapted during construction to address the complex geologic conditions, as well as other issues.