By Ernest K. Schrader
Spillways and outlet works deserve special attention when designing a roller-compacted-concrete dam. Spillways at RCC dams can be built using traditional designs, with reinforced or unreinforced concrete. The amount of discharge and head and the frequency of use all affect the spillway design chosen. The primary concern with outlet works in RCC dams is that these structures can provide obstacles to RCC placement. Proper location of these structures can minimize the effect on RCC placement while ensuring the outlet works function as intended.
Spillways for RCC dams
Traditional spillway designs used for conventional concrete dams also are appropriate for RCC dams. Gated spillways that include controls, support piers, and spillway chutes built of both reinforced and unreinforced concrete have been incorporated into RCC structures. For example, the spillways for Winchester Dam in the United States and Platanovryssi Dam in Greece were built with reinforced concrete. Dams built with unreinforced concrete spillways include Copperfield and Burton Gorge dams in Australia.
Conventional stepped spillways are common, although smooth spillways also are used. Because RCC has very good cavitation and erosion resistance, unformed spillway surfaces (having the rough textured appearance of the RCC placement) also have been used for low-head spillways or spillways that are used infrequently.1 Typically, the rough unformed RCC surface of these spillways is trimmed back to sound material, with removal of major abrupt irregularities.
For dams with low spillway discharge, the spillway and outlet works may be combined. For example, at Middle Fork Dam in the United States, the primary spillway and outlet works were combined. The two structures are contained in a double-chambered tower that was placed against the upstream face of the dam and connected to conduits in a trench at the deepest section of the dam, leading to the control structure at the toe. The conduits were built before RCC was placed, thus avoiding interference with RCC placing operations.
Spillways with conventional concrete steps have become common in RCC dams. They are relatively economical. In addition, because they can be constructed lift by lift with the RCC or as a separate activity trailing RCC placement at some higher elevation, they can save time on the overall construction schedule. The steps also can be constructed after all RCC placement is completed, as is the case for most smooth spillway facings.
The spillway steps should be some multiple of the lift height, with two or three lifts being common. This is dictated partially by construction forming and convenience but primarily by hydraulic design. Larger unit discharges require larger steps. Stepped spillways can dissipate substantial energy, thereby significantly reducing stilling basin requirements for low and moderate unit discharges. Steps that are up to several lifts high are not effective for large unit discharges. However, very large steps have been modeled and used for large unit discharges. If steps are kept at a nominal height of 600 to 900 millimeters and are effective for all except extreme floods, the steps still will pass the extreme flood flow. The steps will simply create a boundary layer of low-velocity turbulent flow, with the mass of high-velocity water flowing over this protective layer. With this choice, the dam owner must be willing to accept the duration and frequency of flow for which the steps do not effectively dissipate energy.
Stepped spillways have been constructed with essentially no reinforcement and, conversely, with substantial reinforcement all with good success. Reinforcing does not improve resistance to cavitation or erosion. Its purpose is to connect the conventional facing concrete to anchors in the RCC mass and to control the width of drying shrinkage cracks in the internally vibrated concrete.
If the facing mix has minimal shrinkage potential and includes contraction joints, reinforcing can be as simple as two longitudinal bars tied to the anchors. Experience has shown that contraction joints with no continuous reinforcing should be placed at all monolith contraction joints in the RCC mass. Primarily for aesthetic reasons, intermediate contraction joints normally are placed at about 2- to 4-meter spacings, typically with half of the reinforcing being continuous through the joint.
The design of anchorage for stepped spillways is primarily based on judgment, with wide variations in practice. Typically, two anchors are used for each step height and contraction joint spacing, but many other configurations have been used. Because it is either monolithic with the RCC or well-bonded to it, the tendency is to have less anchorage when the facing concrete is placed concurrently with, or very soon after, the RCC.
As with stepped spillways, smooth internally vibrated concrete slabs placed over RCC have a wide range of anchoring system designs based on judgment. However, smooth slab spillways always require more robust anchorage than stepped spillways.
In addition to uplift pressure from potential leaking through lift joints that is trapped by the slab, the slab also can be subjected to negative pressures from high-velocity surface flows. The slab typically is designed with waterstops to prevent velocity head from getting under the slab at construction joints. Reinforcing steel for smooth slab spillways typically is designed using a combination of judgment, experience, structural design for the slab being supported by the anchors, and reinforcement to keep shrinkage cracks small enough to prevent velocity head from penetrating the slab.
Copperfield Dam in Australia has been in service for more than 20 years with routine small discharges and occasional major floods with high velocities. It has a smooth spillway surface that was constructed by placing low-shrinkage, internally vibrated concrete lift by lift with the RCC and compacting the RCC into the fresh internally vibrated concrete.
The spillway has no contraction joints, reinforcing, or anchors. However, construction of this spillway required extreme quality control during construction, and the practice has not been readily adopted by others.
Outlet structures and conduits can provide obstacles to RCC placement. The preferred practice for placing outlet works for an RCC dam is to locate the conduits in or along the rock foundation, which minimizes delays in RCC placement.
Rompepicos Dam in Mexico features one conduit (see arrow) combined to serve as both a passageway or roadway through the dam for normal conditions and an ungated outlet during floods.
Conduits usually are constructed of concrete-encased steel pipe or conventional concrete before beginning RCC placement. Locating the intake structure upstream of the dam and the control house and energy dissipator downstream of the toe also minimizes interference with RCC placement. Outlet conduits usually are installed in trenches beneath the dam or along an abutment.
Sometimes it may be possible to route outlets through diversion tunnels. When conditions dictate that waterways must pass through the dam, the preferred approach is to locate all the penetrations in one conventionally placed concrete block before starting the RCC placement. This permits proper cooling of the conventional concrete and eliminates interface problems between the RCC and conventional concrete.
At least one RCC flood control project, Rompepicos Dam in Mexico, has combined one conduit to serve as both a passageway or roadway through the dam for normal conditions and an ungated outlet during floods.2
There are many ways to deal with overflow spillways and outlets for RCC dams. Some spillways can be as simple as allowing overtopping of an unformed RCC face, while others use various facing options, with steps being common. Typically, outlets are placed in a notch excavated into the abutment or foundation, using conventional concrete that can be placed with minimal interference to the RCC operation. The design for a spillway or outlet works should minimize interference with the rapid placement of RCC. s
Dr. Schrader may be reached at Schrader Consulting, 1474 Blue Creek Road, Walla Walla, WA 99362 USA; (1) 509-529-1210; E-mail: [email protected].
1 Schrader, Ernest K., and J. Stefanakos, “RCC Cavitation and Erosion Resistance,” Proceedings of the International Symposium on Roller Compacted Concrete Dams, Spanish National Committee on Large Dams, Madrid, Spain, 1995, pages 1,175-1,188.
2 Schrader, Ernest K., and J.A. Balli, “Rompepicos Dam at Corral des Palmas with Final Design During Construction,” Roller Compacted Concrete Dams, Swets & Zeitlinger, Netherlands, 2003, pages 859-864.
Further reading on RCC
The following HRW articles were authored by Dr. Schrader:
“Building Roller-Compacted-Concrete Dams on Unique Foundations,” Volume 14, No. 1, March 2006.
“Designing Facings and Contraction Joints for Roller-Compacted-Concrete Dams,” Volume 16, No. 3, July 2008.
“RCC Dam Design: Analyzing Stress and Stability,” Volume 16, No. 1, March 2008.
“Roller-Compacted-Concrete Dams on Difficult Foundations: Practical Examples,” Volume 14, No. 2, May 2006.
“Roller-Compacted Concrete: Understanding the Mix,” Volume 12, No. 6, December 2004.
“Some Considerations in Designing an RCC Dam,” Volume 15, No. 5, November 2007.
If you have suggestions for future departments for the magazine, such as “Ideas in Action” or “Lessons Learned,” please send them to the editor via E-mail: [email protected]
Ernie Schrader, PhD, P.E., is a consultant with more than 30 years of experience in roller-compacted concrete (RCC). He has been involved in more than 30 RCC dams that are complete and operational, several under construction, and many undergoing design and feasibility studies. The projects range from the world’s highest and largest to the smallest RCC dams. Schrader is a fellow of the American Concrete Institute.
This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.