By Guy Dickes, Constellation Group | December 2014, Vol. 69, No. 12
Competitive Power Ventures is currently building a new dual energy (primary- natural gas; secondary- ultra-low sulfur diesel) high-efficiency generating plant in north central New Jersey.
Called Woodbridge Energy Center, it will produce 700 megawatts of electricity for the New Jersey metropolitan area. It will generate enough electricity to power more than 600,000 homes, helping the area meet its growing demand for energy while also increasing the reliability of the state’s electrical grid.
Situated in a Brownfields Development Area in the Woodbridge New Jersey area, the power plant needs to tie into the state’s electrical grid. Situated roughly three miles from Jersey Central Power and Light’s Raritan Substation, overhead transmission lines are being built above ground for roughly half that distance, conventional open cut for ½ mile and trenchless for a little over one mile.
Undergrounding for these segments of the transmission lines were necessitated by wetland restrictions to construction. Other permitting issues for overhead towers and wires across the river prevented their installation. Furthermore, towers would have obstructed the view in a great natural habitat of the state.
Undergrounding of electrical cable is more expensive than overhead lines so it is imperative to maximize electrical efficiency (ampacity) for the underground portion of transmission lines. Best practices include the use of plastic casing and conduit in place of steel for elimination of stray currents and the use of an engineered thermal grout for the dissipation of thermal loading.
Grout requirement
Thermal grout is necessary to remove heat from the conduit bundle and transfer to the ground. Overhead cables are naturally cooled by the atmosphere. Underground these wires would be surrounded by trapped air in the casing- an excellent insulator. Thermal grout allows the heat to be transferred to the surrounding soil.
The underground portion of the transmission lines (230kV) starts about ½ mile behind the New Jersey Convention Center in Edison, NJ, and crosses roughly 1.5 miles of wetlands and the Raritan River. Transmission line work was awarded to Ferriera Construction/EJ Electric and the horizontal directional drilling (HDD) work was subcontracted to Carson Corporation.
Carson Corporation started drilling in late spring 2014 with a tight construction schedule. It was necessary to complete four pairs of two parallel HDD bores totaling over 11,000 feet by the end of summer. Further complicating this was working under/around/over other contractors to complete their portion of the work. This work was all Design-Build for the Owners- Competitive Power Ventures (CPV).
The first two parallel bores reached 1,600 feet across environmentally sensitive wetlands to a point where Ferriera/EJ continued with a ½-mile run of open-cut trenching. Open-cut conduits were covered by thermal concrete and specialized thermal sand. At this point, twin HDD bores continued 2,400 feet under the Raritan River with very tight right-of-way restrictions. Parallel bores had be 20-feet apart. Further complicating the construction, electric phases had to be kept aligned between bores. Across the river, the third set of parallel bores intersected with the second bores for another 1,500 feet. Right-of-way and permitting required this jog in the transmission lines.
Due to fused PVC’s greater strength Carson Corporation selected it over HDPE for the HDD bored casing and the bundled conduit slipped inside. This allowed reduced wall thickness and weight of both the casing and the conduit which, in turn, lowered risk and cost by requiring a smaller bore than HDPE. The HDD bores were cased with Underground Solutions Inc. fusible PVC 30 inch pipe and the internal bundles were comprised of 8-inch fusible PVC (for cable) and 2-inch HDPE (for fiber optic lines). Underground Devices Inc. (UDI) supplied the spacers for the project. In order to expedite the project and maximize productivity, the primary HDD rigs were not used to pull bundles. A Vermeer rig was used to pull all the bundles, proving that UGSI/UDI system works well.
Bundle
Noteworthy is the bundle design. By using the 30-inch and 8-inch fusible PVC, while this allowed for a smaller bore than HDPE, it also reduced the internal space and provided for only two, 3-inch thermal grout pipes from either side of the bore. There was no room for error. Strategically placed, sacrificial grout pipes can only be used once before becoming filled with grout. In the typical HDD/thermal grout scenario, the grout is pumped to the lowest part of the bore (center) and allowed to fill the bore in both directions. Secondary grout pipes are located much further away, up the bore profile. In the event grouting must be halted, then by knowing the locations of these pipes grouting can be controlled.
Constellation Group LLC was brought on board early in the project and worked with Carson to develop a thermal grout that could be easily pumped the distances required and then flow through the tightly spaced conduit bundle. Numerous mix designs were formulated and tested. By utilizing a locally sourced supply of very fine natural sand, an economical grout was formulated. As should be standard on all thermal grouting projects, samples were sent to Geotherm Inc. for thermal resistivity testing. Final result ranged from 49 degrees C-cm/W for wet samples to 95 degrees C-cm/W at zero percent moisture. These results exceeded the requirements of the project.
Carson self-performed the grouting operations with Constellation Group. The use of very fine sand dictated a hybrid batching plan. The selected ready mix supplier (Clayton Companies) could not handle the sand at their plant. It was decided that Clayton would supply, premix the cement, water and the first two admixtures. Carson’s crew then introduced the sand into the trucks onsite and thereafter the third chemical admixture. Within the first two truck loads, the operation looked like a finely tuned ballet. Each truck required five 3,000-pound supersacks of sand. Over 1,680,000 pounds of very fine sand was handled.
Supersacks of sand took less than 90 seconds to lift, unload and reload. An admixture pump supplied by Sika Corporation took less than 2 minutes to add the Sika stabilizer. Truck drivers were then instructed to add a specific amount of additional water. This was required for three reasons: to avoid splashing as the trucks went down the road; to wash all the sand and admixture into the barrel; and to maintain the ability to monitor and adjust the final mix.
QC
Quality control at this point was to check the truck tickets and unit weight of the thermal grout. The unit weight is checked against the lowest allowable weight as tested by Geotherm. If the unit weight (also called slurry density or specific gravity) is above the minimums, it will produce thermal grout meeting or exceeding the performance of the test samples. As a general finding, nearly all field tests showed weights 1 to 2 percent above minimums. Although not a requirement, efflux (flow) using a standard ASTM C-939 flow cone showed results less than 20 seconds which is a very fluid grout. This work took place during the summer. It was decided to substitute ice for some water during the hottest days to lower the initial temperature of the grout. Even on the hottest days, initial temperature of the thermal grout did not exceed 90 degrees F as tested with a laser thermometer.
Thermal grouting proceeded in an orderly fashion. On the first bore, it was necessary to utilize all three grout pipes (two from the primary side; one from the far side) because of delays in delivery. This bore took two days to complete. Thereafter, grouting was done in one day per bore, utilizing the primary (centrally located) grout pipe and the secondary (near side short pipe). Operational parameters determined it was necessary to leave a short section (less than 50 to 75-feet) ungrouted from several bores so the casings and conduits could be excavated, lowered and tied into other underground structures.
Roughly 1,260 cubic yards of grout was pumped over seven non-consecutive days. Scheduling included working around deliveries of cable spools (up to 100,000 pounds) and ready mix trucks delivering fluidized thermal backfill for the open-cut portion of the work.
Several records were believed to have been set on this project:
• Thermal Grout: largest thermal grouting project: 1260 cubic yards on one project;\
• Fusible PVC: largest fusible PVC conduit project;
• Longest HDD thermal grout project, total footage 11,000 feet
FOR MORE INFORMATION:
Carson Corporation, (201) 230-4686, www.carsoncorporation.net
Underground Solutions, (858) 679-9551, www.undergroundsolutions.com
Underground Devices Inc., (847) 205-9000, www.udevices.com
Vermeer Corp., (888) 837-6337, www.vermeer.com
Constellation Group LLC, (410) 484 0672; www.cgllc.us