Georgia Goes Nuclear With First New Reactors Since 1970s
CSP provides storm drainage for 3,100-acre power plant.
Towering 548 feet above the east Georgia landscape stand the mammoth cooling units of Plant Vogtle. Vogtle is a 3,100-acre nuclear plant along the Savannah River and is the largest construction project ever undertaken in Georgia. Planning on Plant Vogtle began in 1971 and continues today; groundbreaking on two new cooling towers began in 2009. Miles of storm drainage and underground utilities had to be installed before construction of the reactors could begin.
A matter of concern in the site work for Plant Vogtle was the storm drainage pipe in the Laydown Area. There were several natural streams on the predeveloped site. This pipe is designed to have a constant flow of water, and the finished elevation of the Laydown Area varies between 34 feet and 56 feet above the top of the storm drainage pipe. There are also several elevation and directional changes required in the pipeline, as well as a pipe junction that brings the flow of two springs together within the storm drain. This storm drain line was originally designed to be built with reinforced concrete pipe (RCP), but the bidding process and product characteristics quickly changed the vision for this site.
The storm line consists of a series of 60-inch and 66-inch pipes that eventually converge into a 72-inch pipe to carry the flow. Several problems presented themselves after the original design. The most prominent issue was that, due to the height of cover above the drainage system, the RCP would require a special-design pipe in order to be structurally capable under these conditions—neither cost-effective nor readily available. Standard RCP pipes would not sufficiently support the dead loads. Secondly, the series of changes and pipe junctions in the storm line would require large and deep junction boxes in order to redirect the RCP. The design engineer, Jack Palmer of The Shaw Group, also the project manager, was pressed to find a more economical and functional option.
Flexibility of CSP Allows Greater Height of Cover
After some research, it was determined that the original intent of the design could be accomplished using standard corrugated steel pipe (CSP) products. Palmer turned to Southeast Culvert Inc., a manufacturer of CSP in Winder, GA, for some needed expertise. After evaluating the conditions of the Laydown Area, Southeast Culvert determined that standard gauge and corrugation profiles could be used for these storm drain lines. The height of cover dictated that the 60- and 66-inch pipe be 12 gauge and the 72-inch pipe be 10 gauge. All three pipe diameters needed 3-inch by 1-inch corrugation profile to supply enough stiffness to the pipe to support the fill. These combinations of pipe size, gauge, and corrugation profile can support well over 80 feet of cover according to the National Corrugated Steel Pipe Association’s design manual.
Corrugated steel pipe can support deep covers for a counterintuitive reason: It is flexible. The flexible nature of CSP allows for the pipe to share the load with the surrounding backfill. If the pipe is installed correctly and the minimum compaction requirements are achieved in the backfill, then there will be a load split with the pipe carrying 60% and the soils accepting 40% of the load. With that issue resolved, Palmer and Southeast Culvert moved on to the more difficult problem of the pipe junctions.
|An example of the pipe used for the project
Southeast Culvert suggested eliminating the large junction boxes by incorporating specialty shop fabrications. Corrugated steel pipe can be easily fabricated into elbows and junctions, but under situations of such extreme cover height, the fabrications needed to be structurally evaluated to ensure that they were capable of carrying the soil dead load. The elbows were of no major concern, but there was a junction in storm drain line where the 66-inch and 72-inch pipes would come together. This required a 72-inch by 66-inch wye fitting. Due to the full-flow characteristics of these pipes, standard pipe fabrication reinforcement methods, such as manway cutouts, were not viable options because the manway would act as a weir inside the storm drain line. A manway is an industry standard for fabrication reinforcement where, instead of cutting out a full pipe diameter of steel at a pipe junction, the fabricator welds the pipes together at the outer edges of the pipe and cuts only a small window from the trunk line, removing less steel and bolstering the structural strength.
The only option was a full 66-inch wye cutout into the 72-inch pipe to ensure that full-flow events could be accommodated. Southeast Culvert used finite element analysis to analyze the wye junction and provide adequate structural reinforcement to the cutout. When the analysis was complete, 6-inch by 6-inch by ½-inch structural angle was added to the junction design to ensure that the fabrication would stand up to the cover requirements. These angles added stiffness to the wye fabrication and eliminated the need for a large, square junction box. The pipe fabrications also increased the economic feasibility because the cost is minimal and they can be constructed and shipped with only a few days notice.
Matching the Product to the Site Conditions
With the structural requirements of the project met, Southeast Culvert turned to the issue of pipe durability and supplying a drainage pipe that would last for many years. The most important factor in choosing a pipe coating is to know the environmental conditions in which the pipe will be installed. The pH of Georgia soils is typically greater than 5, and they have relatively high resistivities. These conditions make Aluminized Steel Type 2 the ideal coating for corrugated steel pipe. Recent studies have shown that a 75-year service life can be reasonably expected for 16-gauge Aluminized Steel Type 2 pipes installed in soils with a pH between 5 and 9 and with a resistivity higher than 1,500 ohm-cm. This service life increases with greater gauge requirements. The Shaw Group requested that the pipe also be bituminous coated, just for some peace of mind. Laboratory certifications were provided to validate the physical properties of every piece supplied.
One final benefit of the corrugated steel pipe choice was time savings. Because of the change in design and the time required to work out the details, The Shaw Group was in a time crunch to get that piece of the site work accomplished. Because all of the products were standard, Southeast Culvert was able to produce the pipe and have it ready to ship to the job site in one week’s time. The pipe was run in 25-foot lengths to maximize the loads to the site. The long lengths reduced the number of trucks on the job site and deliveries taken. This also aided in reducing construction time, because fewer pieces were needed for the entire storm drain system. The fabricated elbows and junctions joined directly to the pipe lengths so no connections to manhole structures were needed. The pipes were backfilled to the spring line with gravel in order to achieve the desired compaction levels and to hasten the process. A filter fabric was placed over the stone to keep soil fines from migrating into the stone’s void space and losing compaction in the native soil that was used to finish the backfill process. The forethought put into the storm drain system by The Shaw Group allowed for quick, efficient construction.
When asked why he decided to change the storm drainage plan, Palmer stated that it was purely an economic and service decision. The physical and fiscal attributes of steel won the day. The lesson taken from the process is that open lines of communication between all parties involved and the sharing of ideas can lead to a superior result for all. The construction of Plant Vogtle is proceeding, and soon two new chiller towers will dominate the landscape.
Author's Bio: Mark McCord is a civil engineer for Southeast Culvert Inc.
Author's Bio: Mike McGough, P.E., is chief engineer for the National Corrugated Steel Pipe Association.
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