Tour dates

Tours of dam projects that use roller-compacted concrete (RCC) were a major feature of last year’s International RCC Dams Seminar and Study Tour (RCC 2004), held in September 2004 in Atlanta, US. The event attracted a capacity attendance of 42 for the fourth straight time and attendees came from seven countries as diverse as Australia, Canada, Germany, Panama, Spain, and Thailand, along with the US itself. Sponsor of the course is Schnabel Engineering, in conjunction with the Association of State Dam Safety Officials (ASDSO). Three previous offerings of the structured course on the design and construction of RCC dams and RCC for dam rehabilitations were conducted in Denver on a bi-yearly basis.

Saluda dam

Atlanta was chosen as the venue for the first time, in order to include the Saluda project, South Carolina, in the study tour – a dam that has established what is considered to be a 24-hour world record for RCC placed in a dam: 14,227m3. However, organisers had to deal with the effects of rains generated by Hurricane Ivan, a storm that proved to have both positive and negative impacts on the study tour.

Because of the rain, actual RCC placement at Saluda dam had been stopped. But attendees were still able to view the status and equipment being used to build the 63m RCC gravity section that requires more than 1Mm3 of RCC. The 709m long RCC section serves as the central portion of a ‘back-up’ dam should the original semi-hydraulic fill dam liquefy and fail during a major earthquake. If this were to occur, more than 120,000 people living downstream in Columbia, the capital of South Carolina, would be at risk. The RCC section fits in between the existing dam and the power house where base width is limited as shown in figure 1, taken in late October 2004.

Although the rain deterred RCC from being placed during the tour, the knock-on effect was that attendees were able to access the completed work close up. The RCC mixing plant, which included cooling equipment as well as quality control tests, could be better seen and explained when RCC production was temporarily halted. RCC placement at Saluda is scheduled for completion in April 2005.

Also included in the RCC 2004 study tour were four lectures by personnel closely involved with the US$250M+ Saluda dam remediation project. Presentations were made by representatives of the owner, South Carolina Electric & Gas; the design engineer, Paul C Rizzo & Associates; the contractor, Barnard Construction Co and the RCC qualify control testing firm Kleinfelder.

The event of Hurricane Ivan produced one other positive for the study tour – when the attendees visited the 27m high Big Haynes dam in Georgia, the 107m wide spillway was flowing at its highest level in history. A depth of more than 300mm of flow was cascading down the stepped spillway, which had been constructed of precast concrete steps anchored back into the RCC mass (see figure 2).

During the RCC 2004 seminar, attendees visited the small Atlanta Road Dam and Hudson river no. 11 dam that, along with the RCC, offer overtopping protection for the Yellow river no. 14 (Y-14) dam. The 7m high Atlanta Road dam serves as a storm water detention dam for a commercial development in suburban Atlanta. The smaller footprint for the all-RCC gravity dam allows for maximum temporary water storage on the cramped site. The structure is also notable in that it served as the first trial of grout-enriched RCC (GERCC) for facing an RCC dam in the US. Attendees standing on the dam designed by Schnabel are shown in figure 3.

At the Y-14 overtopping protection project, formed 0.6m high RCC steps were used for both hydraulic and aesthetic reasons. The RCC overlay was not only placed on the downstream slope of the 12m high earthfill dam, but also adjacent to each abutment. In this manner, spillway capacity was increased by being able to safely take overtopping flows over the dam during a flood. The design by ---Golder Associates, which also included curving the concrete ogee crest and RCC overlay, served to produce a converging stepped spillway. The attractive completed structure is shown in figure 4.

Hudson river no. 11 dam is also an RCC dam with an unusual shape. Because the dam is located immediately upstream of a waterfall, the minimum base width afforded by a gravity structure was desired. There are three breaks in the axis for the 21m high dam due to foundation conditions. Another feature of the design is that the uncontrolled overflow spillway has no training walls – the water is allowed to spread out somewhat from the spillway notch as it drops down a 0.8H:1.0V downstream stepped slope. The width of the stilling basin was designed to capture all the overflow. The dam is shown in figure 5. It was the first RCC dam designed by the Natural Resources Conservation Service (NRCS) in the US.

Lessons learnt

Eighteen speakers from the US, Australia, Germany, and Spain presented lessons learnt from past performance and the state-of-the-art in the design and construction of RCC dams. Shear resistance and direct tensile strength at lift lines continues to be the major item to solve in the design of high RCC dams, especially in earthquake-prone areas. Brian Forbes and Francisco Ortega held a presentation illustrating methods to increase cohesion between successive layers, including bedding mortars, rapid placement including the sloping layer method developed in China, and excess paste RCC mixtures.

A class survey indicated that the attendees of the RCC 2004 seminar were especially interested in lectures on the construction of GERCC upstream faces, as presented by Forbes, and contractor value-engineering (VE) proposals by Ortega. Ken Hansen’s lecture on lessons learned from more than twenty years of experience with RCC dams in the US was also well received.

There was considerable focus and participation in the course by major RCC dam contractors such as ASI RCC, Barnard Construction and Phillips and Jordan from the US and Dragados from Spain. For the first time, two forums were held consisting of one engineer and two contractors, discussing cost effective designs of RCC dams and improving RCC specifications.

Rehabilitation

The number of earth dams being rehabilitated using RCC in the US is greater than the number of new RCC dams being designed and built, and considerable due attention to this subject was included in the course. Randy Bass, Chuck Kahler, and Hansen, all of Schnabel Engineering, covered the design, construction, and quality control of RCC overtopping protection for existing earth dams determined to have inadequate spillway capacity. In this manner, an RCC overlay placed on the downstream slope of an embankment allows for flood flows to be taken safely over an embankment dam. More than 80 applications of RCC overtopping protection have been completed in the US. These projects, designed by 50 different engineering firms or offices of government agencies, indicate the widespread acceptance of this cost-effective method of increasing the hydraulic capacity of earth dams during infrequent flood events.

The next international RCC Dams Seminar and Study Tour is planned for either late 2006 or early 2007 in Atlanta. For further information contact Ken Hansen via email: khansen@schnabel-eng.com