REMEDIATION OF SALUDA DAM RCC AND ROCKFILL DAMS Paul C. Rizzo1, Scott Newhouse2, Jeff Bair3 Abstract Saluda Dam, part of the FERC regulated Saluda Hydroelectric Project located near Columbia, South Carolina, is being upgraded to resist a recurrence of the 1886 Charleston Earthquake. The existing, semi-hydraulic fill embankment Dam was completed in 1930 and, based on current technology, is viewed as being susceptible to liquefaction during the design seismic event. Several remediation options were considered with the selected approach being a back-up berm, essentially a “Dry Dam”, at the downstream toe of the existing Dam. The “Dry Dam” will consist of about 5500 feet of Rockfill and about 2300 feet of RCC. This Project is the largest active (year 2002) dam construction project in the United States and the final Project will involve the placement of 1.3 million cubic yards of RCC and 3.5 million cubic yards of Rock Fill. This paper discusses the design basis for the RCC Dam and the Rockfill Dam, with particular emphasis on the extensive laboratory and pre-construction test program for the RCC. We discuss the seismic design basis, loading conditions, finite element analysis, mix design parameters, drainage and joint design, facing considerations, thermal stress analysis and joint spacing, and the mix design testing programs. The mix design program addresses the use of aggregate from an on-site borrow area and the use of landfilled waste flyash as a constituent of the RCC. Introduction Paul C. Rizzo Associates is the engineer of record for the seismic upgrade of the Saluda Dam located in Columbia, South Carolina. The selected remediation consists of constructing a combination Roller Compacted Concrete (RCC) and Rock Fill Berms along the downstream toe of the existing Dam. The Project is the largest active (year 2003) Dam construction project in the United States. An extensive RCC testing program was developed to evaluate various RCC mix designs and to study the inclusion of landfill, waste ash as a constituent of the RCC. Utilizing the on-site waste ash made it possible to significantly reduce total project cots (through a reduced cement content) and provide for an increase in the available capacity of the existing ash landfill. 1President - Paul C. Rizzo Associates, 105 Mall Blvd. Suite 270E, Monroeville, Pennsylvania, 15146 USA 2 Project Supervisor - Paul C. Rizzo Associates, 105 Mall Blvd. Suite 270E, Monroeville, Pennsylvania, 15146 USA 3 Principal - Paul C. Rizzo Associates, 105 Mall Blvd. Suite 270E, Monroeville, Pennsylvania, 15146 USA Both the Rockfill and the aggregate for the RCC will be obtained from an on-site quarry. Upon completion of the Dam remediation project, the on-site quarry will be converted to an ash landfill, which will provide 30 to 50 years worth of capacity for waste ash. The construction was bid in two packages. The first package included the installation of the dewatering system for construction. Construction of the dewatering system began in February 2002. The contract for the Dam remediation was awarded in August of 2002. Anticipated total project costs will be in excess of $200 million. Saluda Dam owned and operated by South Carolina Electric & Gas Company (SCE&G) impounds the Saluda River to form Lake Murray near Columbia, South Carolina, one of the largest manmade lakes in North America. The Dam is a semi-hydraulic fill structure completed in 1930 following typical “puddle dam” construction technology popular in the early 1900’s. Table 1A is a summary of the main parameters describing the existing Dam and the impounded Lake. Existing Dam Hydroelectric Plant Lake Murray Table 1 Summary - Main Parameters for Saluda Hydro Lake Area 78 Square miles Lake Capacity 2,096,000 acre feet Dam Length 7,800 feet Max Dam Height 211feet Hydro Capacity 206 MW Original Construction Semi-Hydraulic Fill Original Completion 1930 The primary purpose of the Dam when originally constructed was for hydroelectric generation by the Saluda Hydroelectric Plant located at the toe of the Dam. As such, the Dam is under the jurisdiction of the Federal Energy Regulatory Commission (FERC). Today, the Lake is a source of cooling water for the McMeekin Plant, drinking water for Columbia and adjacent communities, and a major recreation and residential community with statewide economic benefits. Beginning in 1989, a series of geotechnical investigations were undertaken to assess the safety of the existing Dam, particularly under seismic loading. In this part of South Carolina, seismic design bases for critical facilities are, for all practical purposes, governed by a postulated re-occurrence of the 1886 Charleston Earthquake. The Charleston Earthquake is estimated to have had a Magnitude in the range of 7.1 to 7.3 with a recurrence interval in the range of 650 to 1000 years. This event has been established as the Design Seismic Event (DSE) for assessing the integrity of Saluda Dam. A comprehensive liquefaction analysis and a post-earthquake stability analysis were conducted with the DSE yielding typical results as shown on Figure 1. This Figure shows that under certain assumptions, a major portion of the embankment will liquefy if the DSE occurs. Figure 1 - Existing Saluda Dam Factor of Safety Against Liquefaction -0.02 0.32 0.66 1.00 1.33 1.67 2.00 3.78 5.57 7.35 Should Saluda Dam fail, approximately 120,000 people would be in jeopardy, water supplies for Columbia and surrounding communities would be lost, extreme environmental impacts would be realized and countless millions of dollars would be lost in the local economy. Consequently, a major remediation project has been developed for implementation in the 2002 to 2005 time period. Remediation Concept The Design Basis for the Remediation Concept has two major objectives and a corollary expected behavior criteria. The two major objectives are: • Prevent catastrophic flooding downstream of the Dam and, • Assure Safe shutdown of the facilities, including lowering of Lake Murray in a controlled manner. From a behavior perspective, the Owner and the FERC accept that some remediation and repair may be required after the Design Earthquake, but catastrophic flooding is not acceptable. These major objectives were translated into loading cases and factors of safety by the authors with major over-riding input from the FERC. After consideration, both technical and financial, the remediation concept was developed as a “Dry Dam” immediately downstream of the existing earth embankment. The existing Dam will remain in place and function as the primary impounding barrier for Lake Murray. The Dry Dam will become a water retention structure if Saluda Dam ever fails. Hence, the Dry Dam is designed for both dry and wet conditions with a series of loading cases that reflect hydrostatic pressures and uplift pressures in the wet case and neither in the dry case. Both cases consider seismic forces acting both directions horizontally as well as vertical excitation. Additional design criteria to be implemented with the chosen remediation concept include the following: • No excavation into the original sluiced Dam. Riprap placed after original dam construction will be removed to facilitate excavation. • No excavation or construction into the Saluda River as defined by the maximum normal tailwater at El. 179.5 NAVD. • The crest of the new Berm will be El. 372 NAVD1 (i.e., current minimum crest elevation of the existing Dam). The Dry Dam concept is illustrated on Figures 2 and 3 below. Approximately 5500 feet of the total length of 7,800 feet is a Rockfill Berm (referred to as a berm rather than a dam as it does not retain water under normal conditions) and 2,600 feet of RCC Berm. (Also referred to as a berm rather than a dam for the same reason.) Figure 2 – Typical Section, Rockfill Berm Figure 3 - Typical Section, RCC Berm In reply to the obvious question of, why not use a less expensive Rockfill Berm for the entire remediation. We refer the reader to the photo with Table 1 and Figure 4 where it may be seen that the existing Saluda Hydroelectric Powerhouse is at the toe of the existing Dam, thereby precluding a Rockfill Berm because of space limitations. It is in this zone behind (upstream) of the Powerhouse where we chose an RCC Berm as the primary remediation concept. 1 North American Vertical Datum of 1988 Figure 4 - Plan View of Saluda Remediation Rock Berms RCC Berms Impact of the Remediation Concept on the McMeekin Steam Electric Plant The McMeekin Steam Electric Plant is located at the toe of the existing Dam and has several features directly associated with the Dam and the Remediation Project. The remediation of the dam has necessitated extensive modifications to the McMeekin major changes to the plant systems that are affected by the two berms as described below. Circulating Cooling Water Lines Cooling water for the Steam Electric Plant is supplied from Lake Murray via two 90 inch diameter pipes which connect to Penstocks 1 and 3 at Saluda Hydro, directly West of the Hydro Powerhouse. The existing cooling lines which draw from Penstock Nos. 1 and 3 will remain operational until two new lines are constructed connecting to Penstocks 2 and 4. The new cooling water lines will be 72-inch diameter, and will be embedded in a new mass concrete foundation for the RCC Berm between the Hydro Powerhouse and the existing Dam. The construction of the new cooling water lines will take place after the excavation of existing riprap from the area just west of the Saluda Hydro powerhouse. This riprap was placed in the early 1940’s as part of a program of modifications undertaken to increase the stability of the dam and the hydraulic capacity of the spillway.