CONSTRUCTION OF ROLLER COMPACTED CONCRETE SPILLWAY PROJECTS

A CONTRACTOR'S PERSPECTIVE
BILL FRAME JR.


  • ABSTRACT AND INTRODUCTION

Gears, Inc. was established in Colorado in 1975. Project construction history includes highways, utilities, dams, golf courses and airports. In 1986, we successfully bid on the Colorado Division of Wildlife's Spring Creek Dam modification. The project was Colorado's first earth embankment dam with inadequate spillway capacity to be modified utilizing RCC and also the world's highest dam to be rehabilitated using this economically attractive solution. The project was an unqualified success and marked the beginning of our venturing into specialty mixing and mixing and placement.

Since 1986, Gears, Inc. has been involved in 32 RCC projects throughout the United States and Canada. We have mixed approximately 1,224,725 tons of RCC to date. Gears, Inc. has been involved in mixing and placement on 9 projects, mixed only on another 18 projects and been the prime contractor on 5 overtopping projects. These 5 projects will be utilized in presenting a contractor's view of the construction of RCC spillway projects.


  • RCC SPILLWAY PROJECTS

Spring Creek Dam     Owner: Colorado Division of Wildlife
Gunnison County, Colorado            Engineer: Mk Engineering
RCC Quantity: 4,860 CY            Completion:1986
RCC Mix - maximum size aggregate - 2 inch, type II cement, no admixtures
            Aggregate         3,698 lbs./cy
            Cement               225 lbs./cy
            Water                 160 lbs./cy
Price including cement      $36.92/CY

Goose Pasture Dam                  Owner: City of Breckenridge
Breckenridge, Colorado                Engineer: Tipton and Kalmbach, Inc.
RCC Quantity: 4,315 CY              Completion:1991
RCC Mix - maximum size aggregate - 11/2 inch, type II cement, no admixtures
            Aggregate         3,750 lbs./cy
            Cement               325 lbs./cy
            Water                 150 lbs./cy
Price including cement      $48.39/CY

Horsethief Dam Rehabilitation     Owner: USDA Forest Service
Keystone, South Dakota                 Engineer: Black Hills National Forest
RCC Quantity: 6,450 CY                 Completion:1992
RCC Mix - maximum size aggregate - 3 inch, type 11,10w alkali cement, no admixtures
            Aggregate         3,700 Ibs./cy
            Cement               325 lbs./cy
             Water                  l70 lbs./cy
Price including cement      $50.00/CY

 Ponca Dam Rehabilitation                  Owner: Rosebud Sioux Tribe
Bonesteel, South Dakota                       Engineer: Harza Engineering
RCC Quantity: 7,700 CY Formed RCC     Completion: 1993
RCC Mix - maximum size aggregate – 1 1/2 inch, two piles, type I-Il cement, class C fly ash, air entraining admixture
             Coarse aggregate          2,115 lbs./cy
              Fine aggregate              1,275 lbs./cy
              Cement                           205 lbs./cy
              Fly ash                           175 lbs./cy
               Water                              l75 lbs./cy
               Air entraining admixture       1 lbs./cy
No Specified Strength, Anticipated Strength 3,000 psi at 28 days
Price including cement & fly ash      $66.46/CY

Left Hand Valley Dam Rehabilitation       Owner: Left Hand Ditch Company
Boulder, Colorado                                       Engineer: Rocky Mountain Consultants
RCC Quantity: 4,685 CY                             Completion: 1998
RCC Mix - maximum size aggregate - 11/2 inch, type 1 PF cement - 77% cement & 23% Class F fly ash, no admixtures
             Aggregate             3,550 lbs./cy
             Cement                   325 lbs./cy
             Water                     214 lbs./cy
Specified Strength 2,000 psi at 56 days, Test Section Cylinder 2,930 psi at 7 days
Price including cement           $66.44/CY


  • THE BID PROCESS

For each of the above projects, Gears, Inc. was involved in the competitive bid process. The process begins with a thorough evaluation of the technical specifications. Next a site visit is made to evaluate pertinent site conditions such as site location, site elevation, site access, staging area locations, proximity to residences, utility availability and general site layout. Using the information gathered above equipment is selected, risks are identified, a preliminary schedule is developed and a construction plan is drawn up. During the final pricing phase, the market is assessed, local labor force is evaluated, materials are priced and the bid is developed. All of this is done in a short time period under a great deal of pressure. The lowest price is not always the best price when considering claims and overall performance, in order to increase the chances of a successful project the owner and engineer may want to pre-qualify potential contractors and lengthen the time allowed to submit the bid package.


  • CONSTRUCTION

In general RCC spillway construction utilizes conventional construction methods and equipment. This use of on site equipment is what makes RCC an economically attractive alternative. RCC itself is unique and the methods utilized to mix and transport it are what make it a specialized technology. The actual RCC construction is usually achieved in a very short time period. An experienced crew makes all the difference, for the inexperienced the learning curve is too steep, by the time they know what is going on the RCC is complete. A test section is an important facet of the construction, it not only demonstrates the important construction features but it gives those unfamiliar with the process the opportunity to witness it first hand prior to the actual construction. It is also an ideal opportunity to work out any bugs or concerns prior to full production. During the test section an approved placement procedure is developed and a roller pattern is developed in order to achieve the desired densities. The test section also gives the testing firm and engineer the opportunity to develop a testing procedure, something that is often overlooked yet can have a huge impact on construction in a tight placement area.

Gears, Inc. utilizes Aran continuous mix pugmills for RCC mixing. These plants are highly mobile, self-contained units which are state of the art. Their twin shaft pugmill thoroughly mixes a homogeneous product and their size and space requirements are perfect for smaller zites vet their production capabilities can meet production requirements on larger projects. RCC transport is either done with conveyors, trucks or loaders depending on individual site constraints. Spreading is normally done with a laser guided small track type tractor. The equipment is chosen to meet specified lift widths and is normally modified to alleviate placement problems. A single drum roller in the 25,000 lb. dass has been found to achieve the required densities while allowing a little more mobility in tight areas. Plate and jumping jack compactors are used for inaccessible areas as well as unformed steps. Water trucks and pre-installed sprinkler systems are utilized for curing. If required, visqueen and concrete blankets are utilized for cold weather protection.


  • IMPORTANT FACTORS - A CONTRACTOR'S PERSPECTIVE

1.   Proper mix is provided, specified cement and moisture contents.

2.   Minimize segregation of RCC.

3.   Adequate compaction effort in a timely fashion in order to realize specified densities.

4.   Minimize contamination of lifts and have adequate cleanup if needed.

5.   Sufficient cure period and cold weather protection if necessary.

6.   Test section - helps learning curve, develop roller pattern and establish a relationship between the lab and the field.

7.   Incorporate constructability into the design phase.

8.   Engineer's need to clearly describe RCC to potential owners and prepare them for the actual product, RCC is placed with standard construction equipment, not trowels.

9.   Partnering, an open and mutually respectful relationship between the owner, engineer and contractor.

10.  Price considerations of specification requirements.


  • PROBLEMS ENCOUNTERED

1.   Specified materials availability. (i.e. cement)

2.   Complicated RCC mixes with difficult proportion requirements.

3.   Lab work done with materials other than those being utilized in the actual construction, including water.

4.   Inexperienced testing labs and/or engineers.

5.   Rock/sand drain on the entire dam face, material may not stay on face depending on slope, material can prevent the use of preferred equipment. (i.e. spreader was unable to operate on sand)

6.   Specified backup equipment adds potentially unneeded expense and may not be available.

7.   Requiring specific equipment to be on site for what seems to be arbitrarily determined time period prior to test section.

8.   Requiring unrealistic down time between test section and construction.

9.   Time required for RCC to set for adequate support of form stakes on short structure determines RCC placement rate.

10.  Training walls at a 90-degree angle to the steps, hard to maneuver a roller at a 90-degree angle.

11.  Stepped abutments with no access, need to have one side sloped at an angle that allows roller access. Size of abutment lifts need to consider size of equipment being utilized.

12.  Unrealistic expectations in regards to aesthetics. Unformed RCC is less expensive than formed RCC, which is less expensive than formed RCC with conventional concrete facing for a reason. Owners need to be shown early on what they can expect from each alternative.

13.  "Boiler Plate" RCC specifications that do not apply to specific projects. Changes need to be made as the technology develops.


  • SUGGESTIONS

1.   Develop a consistent mix design standard to avoid confusion.

2.   Simplify RCC mixes whenever possible.

3.   Consider the reasons RCC is less expensive than conventional concrete and accept the product for what it is.

4.   Utilize partnering when developing plans and specifications.

5.   Consider pre-qualifying contractors prior to the bid process.

6.   Encourage contractor ingenuity by utilizing more performance-based specifications.

7.   Share information with others in the industry.


  • CONCLUSION

RCC construction is perfectly suited for Spillway projects in most cases. It is an economically attractive and an aesthetically pleasing solution to the inadequate spillway problem that exists at many high hazard dams. A successful project for all requires an understanding of RCC and an open, respectful relationship between owner, engineer and contractor.

© Gears, Inc. 1997-2017