Taking a tour...of RCC dams4 May 2002
As part of the RCC 2002 international seminar to be held in Colorado, US, from 14-18 October 2002, delegates will be offered the opportunity to take an informative tour of five notable RCC dams in the US
An international RCC dams seminar held every two years is scheduled for 14-18 October 2002 in Denver, Colorado, US. Called RCC 2002, it is a structured five-day course on the design, construction and quality control for roller-compacted concrete (RCC) for new dams and the rehabilitation of existing dams. Registration is limited to 40 attendees in order to facilitate greater information exchange from 15 engineers and contractors experienced in the design and construction of RCC dams.
Following five days in the classroom and a tour of the US Bureau of Reclamation's laboratories and several social events, a three-day study tour has been planned. The schedule will take attendees from the scenic mountains of Colorado to the desert landscape of southwestern US to view five RCC dam projects.
The tour will offer delegates the opportunity to visit three RCC water supply storage dams built in three different decades, namely Stagecoach dam in Colorado, Quail Creek South dam in Utah, and Olivenhain dam in southern California. A visit to two flood control dams in Nevada will also form part of the tour. These dams differ in that Town Wash (now Jim Wilson) dam is an RCC gravity structure while Windmill dam is an embankment dam that incorporates a large RCC overlay spillway.
Located in the mountains of Colorado near the ski resort of Steamboat Springs, the 46m high Stagecoach dam was originally designed as an earth embankment in the early 1970s. However, economic constraints prevented the construction of the dam at this time. In the 1980s, Woodward-Clyde Consultants became involved in the project and discovered that the emerging RCC technology could reduce the cost of the dam by 20%.
This enabled the Upper Yampa Water Conservancy District to construct the dam across the Yampa river to provide a 42M m3 capacity reservoir for water supply and recreational use. Working two shifts, seven days a week, the contractor placed 34,000m3 of RCC in just 37 days in 1988. The dam incorporates a concrete stepped spillway, but no transverse contraction joints in the structure, except sealed notches in the conventional concrete face.
However, the seepage performance of the dam has been considerably good. After reaching a peak of 4.4litre/sec eight months after filling, current seepage through the dam is about 1litre/sec with a variation of about 0.2 to 0.3 litre/sec between the winter and summer seasons.
The project also includes a small hydroelectric plant on the left bank downstream of the RCC dam.
Quail Creek South dam, which has an RCC structural height of 42m, is a part of two dam system that impounds the Quail Creek reservoir. The reservoir provides water for the fast-growing City of St George in southern Utah. The RCC structure replaces an embankment dam that failed due to internal erosion on 1 January 1989. RCC was selected by the Washington County Water Conservancy District due to its inherent erosion resistance and higher degree of reliability, as well as its low cost.
The design of the dam by Morrison-Knudsen Engineers (now Washington Group) called for an RCC gravity section rising 24m above ground and 23m below grade to act as a mass RCC cutoff wall. The lowest 6m of the cutoff is constructed of conventional concrete in a 0.76m wide trench. Nearly the entire 610m length of the dam acts as an emergency spillway. With this design, overtopping the south dam upgrades the safety of the main dam to 100% probable maximum flood (PMF), whereas its original hydraulic design was for only 25% of PMF. Instead of the very infrequent high rainfall flood overtopping the main dam and possibly causing failure, the RCC South dam is now safety overtopped.
The contractor, ASI RCC, who also built Stagecoach dam, placed the 135,000m3 of RCC in exactly two months. The contractor worked 2-10 hour shifts, seven days a week in an area where weather seldom stops construction. Sixty-four percent of RCC was placed above grade with the remainder acting as the cutoff. The downstream slope of the dam is exposed RCC, which blends in with the colourful landscape near Zion National Park.
Town Wash and Windmill detention
It may seem unusual that flood control dams are an important issue in the arid areas of Southern Nevada where the annual average rainfall is less than 12mm. Historically, high intensity thunderstorms have produced flash floods which have caused extensive property damage and some loss of life in Clark County, which includes Las Vegas as its focal point. Funding for flood control is derived from a percentage of sales tax in this rapidly growing area.
Included in Clark County's flood control programme are two dams in the northern part of the county that use RCC. Town Wash dam (now named Jim Wilson dam) is a 15m high RCC gravity dam protecting the city of Mesquite, while Windmill dam, a 19m high earthfill dam with a large RCC over the embankment spillway, protects the nearby town of Bunkersville, Nevada Both dams were designed by Consultants CH2M -Hill. Their reservoirs are basically dry for nearly the entire year, and only detain water when the high intensity rainfall occurs. Due to the sandy nature of the desert soils, erosion of banks and thus property damage would occur if the flood flows were allowed to flow freely through the communities.
The difference between the two sites which influenced the dam type selection was the foundation condition. At the Town Wash site, weakly cemented sandstone provided sufficient bearing capacity for an RCC gravity structure. However, the downstream slope was flattened to 1.5H:1.0V to reduce bearing pressure. The design also included a cement-bentonite cutoff wall and an RCC stilling basin.
An uncontrolled stepped spillway occupies the central 183m of the 274m long dam. The 0.6m high steps are exposed RCC. The 43,000m3 of RCC was placed in just five weeks by the contractor, Progressive Construction, along with the RCC placing subcontractor Gears, in 1992.
Seven years later, Windmill dam was completed by American Asphalt & Grading. It features a 122m long RCC spillway on the downstream 4.0H:1.0V embankment slope. The side or training walls were also built of RCC. The entire dam is founded on sand and includes a low level outlet pipe to accommodate low flows.
Similar to Town Wash dam, this detention dam is designed to store the 1 in 100 year flood event. It then overtops for all flows up to the PMF event, which produces a discharge of 980m3/sec. This RCC spillway is also constructed with 0.6m high steps. The spillway overlay and stilling basin required 24,600m3 of RCC. Soil-cement was also used to plate a portion of the detention basin.
The tour will culminate with a visit to the tallest RCC dam in the US under construction. Before touring the dam site at night, the contractor, Kiewit-Pacific, will give a presentation on the entire construction process and the results to date in building the 97m high RCC dam. Designed by the Parsons-Harza team, the dam will require 1,037,000m3 of RCC and cost more than US$130M.
Features of the construction include an RCC batch plant which includes four compulsory mixers, a conveyor to the dam and trucks on the dam to deliver the RCC to the placement area for the 732m long dam.
The downstream face will have formed RCC steps on a 0.8H:1.0 slope, except near the top where it curves to form a 6.1m wide crest width. The vertical RCC upstream face will be covered with an exposed PVC geomembrane for seepage control.
The dam site is characterised by an excellent rock foundation consisting mainly of exposed granodiorite with an intrusion of tonalite. The foundation could accommodate any type of dam. Feasibility studies for the San Diego County Water Authority in the 1980s focused on either an RCC gravity dam or a concrete-faced rockfill dam. The RCC alternative was chosen due to its lower estimated cost and shorter construction schedule.
Olivenhain dam, which is located about 32km north of San Diego is a main feature of the county's emergency storage plan (ESP). The ESP, which consists of a system of interconnected dams, reservoirs, pipelines and pumping stations is designed to provide six months of emergency water storage should a major earthquake disrupt water transport to the 2.9M people in San Diego county.
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