Up-to-date with RCC14 November 2000
All aspects of design construction and quality control of roller compacted concrete dams were covered in detail at the International RCC Dams Seminar
It has long been recognised that the shear resistance and direct tensile strength at lift lines is a major factor to be considered in the design of RCC dams. This is especially true for high RCC dams located in earthquake prone areas. At the International RCC Dams Seminar, held in Colorado, US, during September 2000, Brian A Forbes of GHD in Australia reported on the increasing use of bedding mortar (rather then bedding concrete) as a proven method for increasing cohesion between successive RCC lifts. He also introduced the sloping layer method of RCC placement which has been developed in China to increase cohesion by minimising the time between placing successive layers.
Coring RCC at an angle to the hori-zontal was proposed as a means of greater core recovery — the core bit only cuts through a small portion of the lift joint surface at one time. Compression testing of inclined cores can be used as an indicator of the bond and strength of a lift joint.
Course director Kenneth D Hansen of Schnabel Engineering pointed out that lessons learned from some early RCC dam designs emphasised the need for waterstopped transverse contraction joints in water-retaining RCC dams. Upper Stillwater dam in Utah was design-ed without joints and has experienced some transverse cracking requiring repair. In order to control thermal related crack-ing in these concrete structures, both pre-cooling of the RCC and transverse joints are required. Hansen noted that the cost of reducing the temperature of RCC for each 1oC was not constant. As more costly methods are needed for the last few degrees of temperature reduction, it may be cost-effective to add more transverse joints to the design. Another solution could be to place the RCC only in the cooler months of the year or at night.
Methods for producing an attractive, watertight and cost-effective upstream face to an RCC dam remain the most controversial subject for designers. Hansen noted the upstream face needs to be part of an overall seepage control system that can include the RCC itself, waterstopped joints, drains and a collection system. Also, the need for redundancy in seepage control was emphasised in case of some unanticipated problem occurring in the performance of the system. Conventional concrete placed concurrently with RCC was the most widely used upstream facing method worldwide. Newly developed systems should be compared with the cost and performance of the concrete face.
Forbes reported on the use of grout-enriched RCC (GE-RCC); developed in China for the upstream face of RCC dams. This new development is getting considerable attention worldwide due to its low cost. The cost of GE-RCC was quoted as between US$4-7/m2, plus the cost of a relatively high paste (low Vebe time) RCC mixture into which the grout is introduced by vibrators.
Glenn Tarbox of Harza Engineering spoke about the design for Olivenhain dam, a 94m high RCC dam in Southern California, which will have an exposed membrane at its upstream face. Although the cost of the exposed membrane was noted as being considerably more expensive than some other systems, cost savings could be achieved through lesser requirements for the RCC mixture and drainage behind the membrane.
With the horizontal method of construction for RCC dams, stepped spillways are easily incorporated in the design. They are attractive, hydraulically efficient and can reduce the cost of energy dissipaters such as stilling basins. Depending on hydraulic and climatic conditions at the site, the steps can be constructed of conventional concrete or exposed RCC. The construction and hydraulics of varying step heights were discussed, including the need in some cases for 2m high steps to discourage climbing and possibly injury due to a fall on the downstream stepped dam face.
Construction of RCC dams continues to gain a high degree of acceptance in the US at a time when construction of any new dam is generally opposed by environmentalists. Thirty-five dams greater than 15m have now been completed in the US.
Completed in 2000 were Trout Creek dam, a 31m high RCC structure to produce a lake for a private owner in Colorado; and Pajarito Canyon dam in New Mexico, a 35m high dam which was built quickly to control sediment runoff from land denuded by devastating forest fires near Los Alamos.
Two RCC dams are presently under contract in the US. They are the 26m high North Fork of the Hughes river dam in West Virginia and the 30m high Hunting Run dam in the state of Virginia. They will require 75,000 and 105,000m3 of RCC respectively. Contractors for both dams are Barnard Construction and ASI-RCC.
RCC for dam remediation Considerable attention was given at the seminar to the use of RCC for the rehabilitation of older existing dams. More than 70 such remediation projects, using RCC to provide overtopping protection for embankment dams or structural buttressing for concrete or masonry, have been completed in the US to date. Overtopping protection consists of armouring the downstream slope of an embankment to increase the hydraulic capacity of the structure by designing for safe overtopping of the dam during extreme flood events.
Randall P Bass, of the Portland Cement Association, discussed construction of these small volume RCC dam rehab-ilitation projects (generally less than 5000m3). He provided recommendations to engineers for reducing costs by keeping the design simple and considering the construction process in design. Bass’ recommendations included the use of pugmill mixers which use volumetric proportioning and can be erected quickly. He also noted contractors preferred cement only in the RCC mixture and therefore only one storage silo. They also prefer the use of a single pile of highway base course for aggregate to ensure small volume projects are cost-effective.
The design of RCC dams continues to be in transition. New ideas to improve performance and reduce costs come from designers and contractors worldwide. One of the purposes of RCC 2000 was to acquaint attendees with new and various options with respect to determining mixture proportions, and selecting facing systems and construction methods. A similar RCC seminar is planned for September 2002.