Roller compacted concrete (RCC) has been in use since 1980 in water control type applications. Because of the economics of RCC construction and its rapid rate of placement, RCC has just about replaced conventional concrete for gravity dams and at certain sites can be competitive with earthfill and rockfill dams. In addition, due to the large number of existing dams that have inadequate spillway capacities, RCC has become the choice of many engineers for providing overtopping protection.

A number of interesting projects have made use of RCC and the following US case studies illustrate how RCC can be utilised for both new construction as well as rehabilitation schemes.

Yellow River Watershed Dam No. 14

Constructed in 1967, the Yellow River Watershed Dam No. 14 (Y-14) is located approximately 48km northeast of downtown Atlanta near the city of Lawrenceville in Gwinnett County, Georgia. The dam was built as part of the Soil Conservation Service Resource Conservation and Development Program. Originally constructed to protect pasture and farmland, the dam now protects subdivisions, apartment complexes, office parks and retail businesses in a rapidly growing Atlanta suburb. The watershed area is approximately 8km2. The original height of the dam was 12.2m. The structure was constructed with a two-stage principal spillway riser and pipe outlet and a 15.2m wide earthen auxiliary spillway located east of the dam site.

When officials found that the dam was not in compliance with current Georgia rules for dam safety, they analyzed options for modernising the structure. The most cost-effective solution was construction of an RCC spillway overtopping the existing dam and abandonment of the existing earthen spillway. The RCC spillway crest was set at an elevation of 0.5m below the existing earthen spillway crest. This elevation is approximately 30mm above the 100-year 24-hour storm, so as to not impact the FEMA mapped floodplain downstream of the structure.

Golder Associates Inc designed an arc-shaped chute spillway with an ogee weir. This design minimised convergence impacts and provided favourable hydraulic performance with uniform streamlines, preventing the formation of supercritical waves that could potentially overtop the chute walls. The arcshaped spillway also minimized excavation, encroachment on adjacent properties, and tree removal in the downstream abutments.

The RCC spillway steps were constructed by forming the front edge of the 0.3m high 3m wide steps. The chute steps were constructed with a 3H:1V slope; the training wall steps were constructed with a 2H:1V slope. The spillway’s arc shape was formed using 3m wide connected concrete forms. Bedding mix was required on the first four lifts, last two lifts, and on every cold joint having a joint maturity greater than 2000 °F-hours.

The contractor completed RCC placement in five weeks at a cost of $1.33M. The average price of constructed RCC (including cement and pozzolan) was $186/m3. Approximately 4650 cubic yards of RCC were placed.

The new RCC spillway safely routes the full Probable Maximum Precipitation (PMP) storm through the watershed, with a peak discharge capacity of approximately 453m3/sec and maintains the peak storm stage below the original dam crest elevation. Sixty-five percent of the total project costs were funded by the Natural Resources Conservation Service (NRCS) Watershed Rehabilitation Program.

Yellow River Watershed Dam No. 17

The Yellow River Watershed Dam No. 17 (Y-17) is located approximately 48km northeast of Atlanta, Georgia. The dam is part of a 37.2ha facility owned and operated by the Gwinnett County Department of Parks and Recreation.

For more than a decade Gwinnett County has been one of the fastest growing counties in metropolitan Atlanta. Like dam Y-14, the Y-17 structure was originally constructed to protect rural pasture and farmland but now serves an urban environment. This development caused the classification of the dam to change from low hazard to high hazard.

The Y-17 structure consists of an earthen embankment dam approximately 9m high and 274m long. Studies of the Y-17 structure determined that the existing 61m wide vegetated earthen spillway was inadequate for the design flows. An options assessment determined that the most cost effective solution was to armour the entire downstream slope and abutments of the dam with RCC.

The RCC forms a spillway approximately 152m wide that overtops the dam and allows the existing earthen spillway to be abandoned. The spillway is asymmetric and designed to correspond with the varying topography along the downstream abutments. The crest has three stages to control flow over the spillway and to provide flood protection during more frequent storm events, with the lowest stage crest elevation corresponding to the crest of the existing earthen auxiliary spillway. The crest was designed to maintain peak stage during the 1/3 PMP design storm. To prevent dam overtopping during the PMP design storm, the spillway sidewalls extend an additional 1.2m above the original dam crest and an earthen berm was constructed up to the PMP design storm peak stage in the existing auxiliary spillway.

Construction began in October 2004 and was completed by early April 2005. The total project cost was $2.1M. Approximately 65% of the funding for the project was provided by the National Resource Conservation Service (NRCS) Watershed Rehabilitation Program since the structure was originally constructed by them. ASI RCC, Inc. was awarded the job at a cost of $1.46M with an in-place RCC cost of $143.87 per m3 for approximately 5123m3 of RCC.

Early in construction, thick deposits of alluvial soils with high liquid limits and very low shear strength were identified along the downstream toe of the existing embankment. This discovery necessitated the removal of more than 6116m3 of additional soil. A re-design of the RCC spillway stilling basin was carried out after removal of the unsuitable material to correspond with the newly excavated site topography. The re-design resulted in an additional 688m3 of RCC. RCC placement began in mid December 2004 and was completed in early February 2005.

The RCC design compressive strength was 2,250 psi (15.5 MPa) at 28-day. The average compressive strengths of RCC cylinders molded per ASTM C 1435 were 2,300 psi (15.9 MPa), 3,400 psi (23.4 MPa), and 3,550 psi (24.5 MPa) at 28, 60, and 90 days, respectively. The moisture content during placement was controlled by the Vebe consistency time which was specified at 15 to 30 seconds.

The completed RCC spillway safely routes approximately 537m3/sec through the Y-17 watershed during the PMP design storm. Since the structure is located in a prominent park, the downstream slopes of the RCC spillway were covered with earthfill and vegetated to hide the RCC. The crest and crest sidewalls are still exposed. The spillway activates for storms at or less than a 25 year frequency. As a result, earthfill covering the RCC may erode during these storms requiring some future maintenance.

Yellow River Watershed Dams 15 and 16

Building on the success of RCC overtopping protection at Y-14 and Y-17, the Natural Resources Conservation Service (NRCS) and Gwinnett County, Georgia, recently completed upgrades to two more embankment dams. The dams are located approximately 48km northeast of Atlanta. NRCS built the dams in early 1960s; however, Gwinnett County currently owns and operates the dams.

The dams are located within 5.4km of each other. Golder Associates designed the upgrades to No. 15 while Schnabel Engineering designed No. 16 upgrades. NRCS and Gwinnett County elected to award both projects under one contract to realize the cost savings from setting up one instead of two RCC plants. The contract was awarded to ASI Constructors of Buena Vista, Colorado. Based on bid quantities, a total of 9600m3 and 2300m3 of RCC were used for No. 15 and No. 16, respectively. The average in-place unit cost of RCC considering the bids for both sites was $205.78/m3.

The RCC plant staging area was adjacent to No. 15 dam site. Dump trucks transported the RCC from the mixing plant to the hoppers of telescoping cranes positioned at the dam sites. The cranes then conveyed the materials to the points of placements. The projects were completed in March 2007. For aesthetical purposes, the RCC slopes were covered with soil and grassed.

Hickory Log Creek RCC dam and reservoir

Hickory Log Creek Dam is a 55m high, 291m long RCC gravity dam. The dam impounds a 166ha reservoir with a supply capacity of 44 mgd (167 mld).

Early planning studies identified that both an earthen dam and an RCC dam were potential viable options for the site. Because the type of dam would not affect the yield, the selection was based on economics and schedule. After considering all the various factors, it was determined that the RCC dam would cost about $4M less and could be built more quickly.

The project was separated into two phases. Phase I design and construction work covered foundation excavation and treatment, infrastructure improvements, and erosion and sediment controls. It also served as an extension of the geotechnical/geological field exploration program. Phase II work covered building the RCC dam and reinforced concrete training walls and ogee crest spillway. This phase design work progressed while Phase I construction was underway.

Phase I work was awarded to Thalle Construction Company, with Nicholson Construction Company, as a subcontractor performing the grouting programme. In Phase II, Thalle teamed up with ASI Constructors as a subcontractor to build the dam.

The main section of the dam was built using RCC. Geomembrane-lined panels, anchored to the vertical upstream face, achieved the dam’s waterproofing. carpi provided the liner. The downstream face of the dam was constructed with conventional concrete within the spillway area and grout enriched RCC elsewhere. The slope of the downstream face is 0.8H:1V.

The conventional concrete steps within the spillway section were built concurrently with the dam RCC lifts. However, the spillway training walls were constructed afterwards using self-consolidated concrete. The stilling basin portion of the spillway was built with reinforced conventional concrete and rock anchors anchored the stilling basin to the underlaying bedrock.

The drainage and monitoring gallery of the dam has exposed RCC walls with a conventional concrete floor and prefabricated steel reinforced concrete roof lids. Foundation drains, piezometers, and inclinometers were drilled and installed during the gallery construction. The gallery spans approximately two-thirds the length of the dam.

RCC was produced using a portable C.S. Johnson-Ross Bandit BTRR-600 batch type plant and an IHI HyDam 4500D mixer. Although the plant capacity is 800 tons per hour, it normally operated at a production rate of 550 tons per hour.

RCC was delivered from the batch plant to the dam via a nearly 2500ft (762m) long ROTEC conveyor belt. A ROTEC tripper and a 60ft (18.3m) long swinging conveyor delivered RCC to different locations of the dam. The swinging conveyor had an elephant trunk at the end to reduce segregation at the point of discharge.

Three different-size dozers spread the RCC in 12 inch (300mm) lifts. The dozers were equipped with laser-guided systems to obtain a level surface. Where space was limited or access was difficult, a small Komatsu D-21 was used. Elsewhere, spreading was accomplished with a larger size John-Deere 850J dozer and a mid size Caterpillar D5. The D5 was also used to finish grade the RCC lifts.

Horizontal surfaces exposed for more than 500 degree-hours were considered cold joints and required spreading an approximately 3/8-inch (10mm) thick bedding mortar layer just prior to placement of new RCC lift. The bedding mix contained 800lbs of cement with a water cement ratio of 0.59. Cold joints older than 36 hours required pressure washing before spreading the bedding mortar.

The project specifications required the RCC to be compacted to a minimum density of 97% of Theoretical Air-Free Density. A double drum Ingersoll-Rand DD-130 and a single-drum Ingersoll-Rand SD-100 were used to compact the RCC. Generally four passes with the double-drum roller were sufficient to achieve the required density. A higher number of passes was needed when using the single-drum roller. The areas adjacent to the upstream and downstream faces of the dam were compacted using a smaller Ingersoll-Rand DD-24. Sections where access was difficult were compacted using plate tampers and jumping jacks.

The RCC placement started in mid-December 2006 and was completed in early June 2007. The placement was performed with one shift up to the top of the gallery and then two shifts for most of the reminder of the dam. Typically, the afternoon hours were used to set panels and forms.

A total of 2000 full size [6 ft x 16.5 ft (1.8m x 5m)] as well as 500 half-size precast concrete panels were fabricated on site. All panels were 5-inch (125mm) thick and served as formwork for RCC placement. Those installed on the upstream face were lined with a flexible Carpi 40 mil synthetic waterproofing membrane. The panels installed on the downstream side of the chimney section have a decorative face for aesthetical purposes. Wood forms were used to form the steps of the downstream side of the dam.

Grout-enriched RCC was used to give the exposed downstream steps of the dam an improved appearance compared to typical exposed RCC. A grout mix was prepared using a colloidal mixing plant at the proportions of one part of portland cement to one part of water by weight. After the RCC had been graded and prior to compaction, the grout was manually poured over the top of the freshly placed RCC adjacent to the downstream wood forms. Workers would then internally vibrate the grout into the fresh RCC. Compaction of the RCC in this area was done using flat bottom plate tampers, resulting in a smooth, aesthetically pleasing exposed step.

A total of 167,000m3 of RCC and 6900m3 of conventional concrete were placed to build this 55m high dam, which is the tallest non-federally regulated dam in Georgia. The in-place cost of RCC was $97.66/m3, including the cost of materials, mixing, transporting, placing and curing.

The initial filling is scheduled to commence in January 2008. Initial filling is expected to take 18 months to reach normal operating level. Creating this water supply reservoir will help fend off future water supply crises for the rapidly growing City of Canton and for the wholesale customers of the water authority.

Taum Sauk Reconstruction

Built in 1963, the Taum Sauk Plant, located in Missouri approximately 90 miles south of St. Louis, is a pumped-storage hydroelectric plant. It stores water from the Black River in the upper reservoir, built atop the 485m high Proffit Mountain, and releases the water to generate electricity.

After a catastrophic failure on 14 December 2005 releasing 1.2 billion gallons of water and flooding the Johnsons Shut-Ins state park near Lesterville, Missouri, the owner hired Paul C. Rizzo Associates, Inc. (Rizzo) to investigate the cause of the upper reservoir earthen dam failure.

Rizzo determined that substandard construction and instrumentation problems were partly responsible for the failure. A repair of the existing dike was not technically feasible due to flaws in the original construction. Reopening the plant required a complete re-build of the upper reservoir.

In August 2007, the US Federal Regulatory Commission (FERC) issued a conditional approval for AmerenUE, owner of Taum Sauk Reservoir, to start preparing to rebuild the upper reservoir.

Rizzo evaluated several alternates for a re-build and selected a symmetrical (0.6H to 1.0 V upstream and downstream) RCC dam with conventional concrete facing. The existing rockfill dike material will be processed and utilized to create aggregate for the RCC. The RCC mixture is estimated to contain 100 pounds of cement and 100 pounds of fly ash per cubic yard. The fly ash will be excavated and processed from an existing AmerenUE facility. In addition to cost savings by not having to purchase commercial fly ash, this will create additional landfill space for use by AmerenUE.

The project is being built by Ozark/ASI constructors/Fred Webber joint venture. An RCC test section to evaluate RCC mix proportions, treatments at lift joints, forming for and placement of conventional concrete facing, control joints, and contractor methods of placing and compacting RCC was completed in August 2007. Testing of manufactured specimens and specimens obtained from the test section are underway.

RCC placement began in October 2007 and the project is expected to take two years to complete. Approximately 2.6 million cubic yards of RCC and 300,000 cubic yards of conventional concrete are needed to rebuild the dike.

McKinney Lake Dam

The first dam remediation project in North Carolina to use RCC for overtopping protection, the McKinney Lake Dam is located on Hitchcock Creek, north of Rockingham, North Carolina. Situated upstream of the McKinney Lake National Fish Hatchery, the dam’s primary purpose is to supply water to this facility.

Originally constructed in the late 1930s as a Works Progress Administration project, the 7m high dam consists of an earthfill embankment with a concrete corewall. The principal spillway is located in the middle third of the dam and consists of a concrete gravity overflow section and a rectangular concrete outlet channel. An unlined emergency spillway was cut into the left third of the embankment.

When state officials determined that the dam suffered from safety deficiencies, RCC was chosen as the material of choice for improvements. Plans called for increasing spillway capacity and raising the top of the dam.

The project’s design concept addressed inadequate spillway capacity by armoring a section of the embankment with RCC, and raising the crest of the embankment to contain the 1/3 PMP flood level plus freeboard. The armored section of the embankment serves as the emergency spillway.

The RCC mix design was as follows:

• Type I/II portland cement – 267kg/m3

• Modified NCDOT roadbase aggregate (1-1/2 MSA) – 2034 kg/m3

• Water – 156kg/m3

The specified 28-day compressive strength was 3000 psi (20.7 MPa). The unusually high cement content was required to compensate for poor grading of the sand fraction of the NCDOT base course aggregate. In addition the silt fines were on the high side of the specified range (3% to 8% by weight) for minus #200 (75 micron) sieve material.

The 1223m3 of RCC armoring was placed on the upper portion of the upstream slope, the crest, the downstream slope, and an apron area beyond the downstream slope. Sheet piling driven along the edges of the RCC serves as spillway training walls to contain the design discharge. The training walls converge to direct outflows up to the projected 100-year flood level through an existing box culvert under the downstream access road.

These improvements increased the spillway discharge capacity to more than 311m3/sec – a substantial improvement over the dam’s original capacity.

URS Corporation designed the dam improvements and is the engineer of record on this project, with Schnabel Engineering Associates as project manager responsible for owner and dam safety coordination, as well as construction oversight. Atlas Resource Management was the general contractor, with Gears, Inc as subcontractor for RCC mixing and placement, and S&ME performing the RCC mix design.

This article was compiled from information supplied by the Portland Cement Association (PCA)

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