Performance review of roller-compacted concrete spillways and overtopping protection reveals factors contributing to success.

Roller-compacted concrete (RCC) has been used as a spillway or overtopping protection for more than 130 earthen dams. In addition to providing protection from erosive forces of flowing water, several of these projects are located in areas exposed to numerous freeze-thaw cycles. Operation frequency of RCC spillway and overtopping protection projects ranges from serving as principal spillways to frequencies less than the 500-year flood event. However, most RCC overtopping protection structures are emergency spillways designed to operate at a frequency not exceeding the 100-year storm.

Primary reasons for the popularity of RCC with designers and owners are simplicity, speed of construction, strength and durability, and economic advantages compared with alternative methods. Because RCC emergency spillway and overtopping protection projects are designed to operate infrequently during major flood events, limited information is available on the performance of these types of structures. However, the few that have operated performed satisfactorily with no evidence of excessive wear or structural distress.

Several U.S. Army Corps of Engineers (Corps) research projects have confirmed the excellent abrasion resistance and durability of RCC. Comparative tests on soil-cement, RCC, and conventional concrete showed RCC to have a greater abrasion resistance than conventional concrete of higher strength, primarily because of a greater percentage of aggregate in the mixture and less paste. In underwater abrasion tests using ASTM C 1138, the Portland Cement Association (PCA) determined that abrasion resistance was a function of both the aggregate hardness and the strength of the paste.

Despite the research findings, there is still the need to evaluate the reliability and performance of RCC under field conditions when subjected to debris-laden flows and hydraulic forces. The following examples describe a few projects that have experienced multiple flows during their service life.

Ocoee Dam #2, Tennessee
In 1980, Ocoee Dam #2 became the first known use of RCC for overtopping protection. The dam, located in Ocoee, Tenn., was constructed in 1912-1913 and operated almost continuously until 1976. The dam is a 30-foot-high, 450-foot-long, rock-filled timber crib structure. Water from the reservoir is transported via a 4.6-mile-long wooden flume and two steel penstocks to a power plant. Deterioration of the dam and the wooden flume forced the owner, Tennessee Valley Authority (TVA), to cease power generation operations temporarily at Ocoee #2. The downstream face of the rockfilled timber dam was severely damaged. Several rehabilitation alternates were considered and the owner elected to restore the flume using materials similar to the original design and to buttress the dam with RCC.

Ocoee Dam No. 2 was the first known use of RCC for overtopping protection. The RCC surface appears rough and uneven, but continues to perform well after 27 years of service.

Dam rehabilitation was completed in 1980 with placement of approximately 4,550 cubic yards (yd³) of RCC. The RCC mix used 3/4-inch maximum size aggregate (MSA); specified compressive strength was 3,750 pounds per square inch (psi) at 28 days.

Since completion, the dam has been subjected to more than 80 days per year of regularly planned overtoppings to accommodate the popular white-water rafting business downstream. The river site was also the 1996 Olympics site for canoe and kayak competitions, which required overtopping the dam 160 times that year. In addition, on Feb. 16, 1990, a major flood overtopped the dam by approximately 12 feet.

Where energy dissipation is minimal near the top of the dam, the RCC experienced very minor erosion. Further downstream where energy dissipation is greater, the water has eroded the uncompacted lift edges and in some areas several inches of the compacted RCC. The RCC surface appears rough and uneven, but the dam continues to perform well.

Brownwood Country Club Dam, Texas
In 1984, RCC was used as overtopping protection for an earthen dam in central Texas. This 19-foot-high earth embankment, constructed in 1938, is owned by Brownwood Country Club in Brownwood, Texas. In 1972, the dam was classified as a high hazard dam and subsequently was determined to be lacking adequate spillway capacity. The dam spillway capacity was rated at 2,600 cubic feet per second (cfs), far below the required probable maximum flood of 11,600 cfs.

To provide adequate spillway capability, the spillway length was increased to 300 feet and RCC armor, designed by Freese & Nichols, was placed on the downstream face of the spillway. The new spillway was designed to withstand a maximum overflow depth of 5.5 feet.

The RCC mixture contained 310 pounds per cubic yard (lb/yd³) Type IP blended cement consisting of 247 lb/yd³ portland cement and 63 lb/yd³ fly ash. Aggregate for the RCC was dolomitic crushed limestone with 1-1/2 inch MSA. The project required 1,400 yd³ of RCC placed in just two days.

The dam has been overtopped at least once every one to two years. Overtopping events are estimated to be as deep as 2 feet. Similar to Ocoee #2 dam, erosion of the uncompacted RCC at the lift edges has taken place. However, the compacted RCC a few inches away from the original uncompacted edge remains durable and the spillway continues to function properly.

Kerrville Dam, Texas
The Kerrville Dam, located in Kerrville, Texas, and owned by the Upper Guadalupe River Authority, was completed in 1980. The original dam is a water supply and recreation, clay-fill embankment with an 8-inch-thick reinforced concrete facing. The dam had two spillways with one crest elevation lower than the other. The dam suffered some damage during a storm event in 1981 that sent 4-foot-deep water over the dam. Additional and more severe damage took place when the dam was overtopped by 4.5 feet during another storm in 1982. The damage consisted of concrete cracking and displacement, loss of filter materials, and undermining of the concrete facing in spillway sections. Subsequent repairs included replacement of slab sections and grouting.

On New Years Eve 1984, about 40 percent of the service (lower crest) spillway concrete facing was lost and the clay core was eroded to bedrock when heavy rains overtopped the structure by 10 feet. The emergency (higher crest) spillway was also damaged and loss of filter materials took place at the spillways and near the abutments.

After considering several alternatives, the design team of Espey, Huston & Associates, Inc. (design engineer and project manager) and Rone Engineers (geotechnical consultant) decided to construct an RCC structure immediately below the existing structure.

The project required approximately 23,000 yd3 of RCC to complete. The mixture contained 200 lb/yd³ portland cement for the majority of the dam and 400 lb/yd³ for the upper seven lifts. Bedding mortar was also used to improve bonding strength at the lift joints of the upper several lifts. The RCC aggregate was pit-run river deposit with 3-1/2-inch MSA. Compressive strength test results of RCC cores were on the order of 1,600 psi at 90 days for RCC containing 200 lb/yd³ cement and 3,000 psi at 90 days for RCC containing 400 lb/yd³ cement.

Normal water flow at this dam is continuously passed over the RCC spillway area; however, on Oct. 19, 1985, 30 days after completion of RCC placement and before final completion of the project, as much as 11 inches of rain fell upstream of the dam. At peak flow, the water overtopped the main portion of the dam by 14.5 feet and the lower spillway section by 15.5 feet. This event was estimated to be a 50-year flood with a maximum flow of 125,000 cfs. The flow from the storm lasted five days over the entire dam and three weeks over the spillway.

The dam was overtopped again on July 17, 1987. This time the flow peaked at 162,000 cfs and the maximum depth of the flow at the dam was estimated at 16.2 feet, which is close to the 100-year storm. Additional significant events reported by the owner include one in 1988 and another in 1990, causing the water to overtop the dam by 10 feet and 8 feet, respectively.

Observations after these flood events and subsequent inspections, including an inspection conducted in 2007, revealed that the RCC performed and continues to perform remarkably. No significant damage was observed. RCC damage was limited to surface erosion exposing large aggregates and, at isolated locations, minor spalling took place during July 1987 event.

Lower Lake Royer Dam, Maryland
Lower Lake Royer Dam is a water supply dam owned by the U.S. Army and located at Fort Ritchie, Md. The purpose of using RCC at this facility was to upgrade the dam and increase spillway capacity. The Corps Baltimore District designed dam modifications that included replacing the existing concrete spillway with a new RCC gravity section and constructing an RCC overtopping protection over the left abutment.

Both the gravity section and the armored left abutment formed the service spillway for the dam. The spillway was designed with a sloped collection channel directing the flow toward a discharge culvert located downstream from the RCC gravity section. The dam upgrades also included a new reinforced concrete water intake and conduit. The downstream slope of the RCC spillway was 1.5H:1V.

The RCC mix proportions per cubic yard consisted of 200 pounds (lb) cement; 100 lb fly ash; 205 lb water; 3,530 lb aggregates; 18 ounces (oz) water reducing admixture; and 30 oz air entrainment admixture. The MSA was 1-1/2 inches. It took 16 days to place a total of 10,000 yd3 of RCC in June 1995.

The RCC steps were unformed and hand equipment was used to compact the sloped edge of each lift. This method sometimes produces zones of lower-density RCC compared with the density of formed RCC compacted with vibratory rollers. After 12 years of service, this service spillway has shown only minor erosion a few inches deep, mainly at the exposed lift edges. Considering the frequency of operation and the harsh freeze-thaw environment at this facility, the spillway has been performing as expected with no excessive wear.

Lake Tholocco Dam, Alabama
Constructed in the 1930s, Lake Tholocco Dam is an earth embankment owned by the U.S. Army and located on Claybank Creek in Fort Rucker, Ala. The lake encompasses 680 acres and has been used for training military personnel and for recreational activities for fort personnel, as well as surrounding civilians.

The dam is an earth embankment, 2,400 feet long with a maximum height of 45 feet. The service spillway is a 50-foot-long, reinforced concrete structure with a fixed ogee crest. A 1979 Phase I Inspection Report under the National Dam Safety Program showed that the dam did not meet current standards because of insufficient spillway capacity. Since it was constructed, the dam’s earthen emergency spillway was regularly overtopped, causing severe erosion.

Major storms in the 1990s breached the emergency spillway twice. The first breach was the result of a 1990 storm that dumped 14.5 inches of rain in five hours. The Corps Mobile District recommended raising the dam and increasing spillway capacity. However, because of a lack of funds, these recommendations were not implemented. Instead, repairs were made to restore the earthen emergency spillway to pre-1990 storm condition. Four years later, July 1-4, 1994, Tropical Storm Alberto failed the earthen spillway a second time, leaving the reservoir dry for six years.

The Corps Mobile District explored several upgrade alternatives and determined that the most cost-effective solution was to install an RCC auxiliary spillway with a collection channel in the embankment adjacent to the reinforced concrete service spillway. The design called for a 1,550-foot-long, 36-foot-high spillway constructed with 12-inch-thick RCC steps. The crest elevation was set to discharge water from rainfall events once every one to two years and the design maximum overflow height was 6.5 feet. The RCC lifts varied in width from 8 feet to 12 feet. The slope of the spillway chute was 6H:1V; a slope of 3H:1V was selected for the downstream side of the collection channel. The design also included large riprap placed immediately downstream of the collection channel to prevent damage to the backside of the collection channel should the estimated tailwater levels not be realized during storm events.

Approximately 26,000 yd³ of RCC were placed in the spring of 2000. The RCC mixture contained 275 lb/yd³ portland cement and 50 lb/yd³ fly ash. The MSA for the RCC was 1-1/2 inches.

Despite being overtopped at least twice since they were placed in 2000, the RCC steps at Tholocco Lake Dam, shown in this May 2007 photo, remain in excellent condition.

Onsite Corps personnel report that the spillway has been overtopped at least twice. The first was during Hurricane Ivan on Sept. 16, 2004. At peak flow, overflow height was determined to be 3 feet. The second storm took place on March 27-28, 2005. Maximum overflow depth during this storm was 1.5 feet. The RCC steps remain in excellent condition.

Red Rock Detention Basin Inlet Spillway, Nevada
Red Rock Detention Basin (RRDB) in the southwestern portion of Las Vegas Valley, Nev., is one of five detention basins on the Corps’ Tropicana and Flamingo Washes Project. The detention basins are part of a master plan for providing flood protection and erosion control in Las Vegas Valley. The Corps completed construction on RRDB in 2001. The facility included an RCC inflow spillway (or drop structure); a holding reservoir; and principal, auxiliary, and emergency outflow spillways.

The inflow spillway handles floodwater carrying heavy sediment loads without eroding or lowering the streambed, which would undermine the stability of the bridge upstream. The RCC structure consists of an approach apron, stair-stepped chute, stilling basin, and training walls. The spillway is 42 feet high and the slope of the chute is 3H:1V. The steps are 2 feet high compacted in two lifts; individual lifts are 10 feet wide. The RCC mixture contained 364 lb/yd3 cement and 74 lb/yd³ fly ash.

The RCC spillway operates during every rain event generating a flow in the wash, normally a few times a year. A 2004 storm carried heavy sediment loads, as evidenced by a large sediment pile in the basin of the RCC structure. In 2005, a stronger storm deposited boulders weighing as much as 200 lb on the RCC steps and distributed the sediment pile throughout the detention basin.

Observations in 2005 and 2006 showed that the effect of flow (carrying abrasive sediments) on the RCC is limited to surface erosion or polishing. Loss of surface materials ranged from none at the tread interior corner to 2 inches or less at the tread (step) nosing. Spalling was also observed at isolated areas; however, almost all of the spalling was confined to where localized segregation had occurred.

Conclusions and recommendations
Although RCC is still a relatively new method of construction and performance data are limited, structures that have been overtopped show strong evidence that the material is performing satisfactorily when subjected to the elements, hydrostatic pressures, and flows containing very abrasive sediments.

In addition to proper structural design, the following primary factors contributing to successful performance of these structures are related to the RCC mix design and construction methods:

• proper mix proportioning, including use of a well-graded aggregate so that the volume of coarse aggregate in the mixture is maximized without segregation while still providing an adequate amount of paste;
• use of the hardest aggregate available;
• sufficient cementitious content in the RCC mixture and proper compaction to achieve adequate strength and high density, especially where the RCC is subjected to repeated freeze-thaw cycles or frequent overtopping;
• forming and high-density compaction of the steps to limit erosion at the exposed lift edges; and
• proper bonding of RCC lifts, especially at the upper few lifts and where energy dissipation occurs in the vicinity of the stilling basin.

Fares Y. Abdo, P.E., is program manager, water resources, and Wayne S. Adaska, P.E., is director of public works for the Portland Cement Association. They can be contacted at fabdo@cement.org and wadaska@cement.org, respectively.