The amount of underground transmission on this project makes it one of the most complex projects being constructed today. The following are some of the underground issues being addressed.

• Splice Vaults

  Splice vault locations are determined by cable-pulling calculations, cable reel lengths, existing utilities, system grounding method, and the locations of rock or water or railroad crossings. The vaults have 8 ft by 8 ft by 30 ft (2.4 m by 2.4 m by 9.1 m) inside dimensions and will be spaced about every 1700 ft (518 m) along the 18.5-mile (29-km) total length of underground line.

  Each splice vault is precast, multisectioned and watertight. The vaults are approximately 30,000 lb per section with two sections per vault. The splice vaults are grounded and contain predrilled holes for the electric and communication ducts that will be racked on one side of the vault. Cable manufacturers specified the inside vault dimensions based on construction, maintenance and National Electric Safety Code requirements. The splice vaults will be supplied by regional precasters using custom structural designs and will have 10-inch to 12-inch (254-mm to 305-mm) thick walls. (Vault design was the subject of a T&D World article “Design and Test 345-kV Cable Vaults” in May 2006.)

• Duct Bank Installation

  The civil contractor is installing 50 ft to 150 ft (15 m to 45 m) of duct bank per day per crew and will run 10 to 12 vault and duct bank crews at the peak of construction. Two to three supporting crews will be employed for utility relocation, repaving and rock coring. Minimal rock is expected and blasting is not being used.

  The minimum size of the duct bank is 3.5 ft by 2.5 ft (1.1 m by 0.8 m) and includes horizontal and vertical curves to avoid existing utilities, buildings and landscaping. These curves increase pulling tensions, which decreases cable cut lengths.

  Installation includes saw cutting the road, excavating the trench, exporting the spoils, placing 10-ft (3-m) lengths of 8-inch (203-mm) PVC conduits in spacers, encasing the ducts in 2500-psi (17-Mpa) concrete, backfilling the trench with a thermally tested flowable fill and laying a temporary bituminous patch. The concrete and flowable fill will undergo thermal testing and slump testing in the field. Fluidized thermal backfill (FTB) is a “diggable concrete” of 100 psi to 150 psi (690 kPa to 1034 kPa) that allows heat to transfer from the energized conductors inside the duct bank, minimizing cable ampacity losses. After the FTB is placed 19 inches (483 mm) below the surface, the civil contractor applies a 10-inch (254-mm) layer of base rock and a 9-inch (229-mm) layer of class 1 and class 3 asphalt concrete hot mix as a temporary patch. The permanent pavement restoration occurs later.

  Upon completion of the duct banks, the civil contractor will “proof” the integrity of individual polyvinyl chloride (PVC) ducts by a robotic video check, swabbing and mandreling. The civil contractor then installs a “mule tape” to show the exact length of the in-place individual duct bank.

• Cable Pulling

  Burns & McDonnell prequalified and selected, through competitive bidding, vendors to manufacture the four 345-kV circuits. They are Silec Cable, LS Cable and Prysmian Cable. The manufacturers are responsible for furnishing and installing the cable system. The cable for the MNP is a 3000-kcmil copper conductor with 345-kV cross-linked polyethylene (XLPE) insulation. The cable is jointly specified by CL&P and United Illuminating (New Haven, Connecticut). Three cables, weighing approximately 29 lb/ft each (209 kg/m), will carry up to 860 MVA.

Maximum allowable pulling tension on the cable is approximately 21,000 lb (9500 kg), and average tension during pulling will be between 5000 to 15,000 lb (2300 to 6800 kg). Conduits are lubricated with a soap-like slurry product to lower the coefficient of friction from 0.3 (dry) to values as low as 0.1.

Burns & McDonnell hired GZA GeoEnvironmental Inc. (Norwood, Massachusetts, U.S.) to conduct geotechnical investigations along the proposed underground transmission line route. Work began in early September 2005 and was substantially completed in April 2006. The geologic and hydrogeologic properties that would affect the project costs and schedule in the design and construction phases were determined. Depth to groundwater, depth to rock and rock coring to determine subsurface cross-section profiles were completed.

More than 150 borings ranging in depth from 14 to 100 ft (4 m to 30 m) were completed during the investigation. Draft boring logs were e-mailed by GZA to Burns & McDonnell within two days of drilling activities to facilitate decision making as the project data became available. Geotechnical engineering reports were issued in phases to comply with the program management schedule in time for RFPs issued for civil/construction activities.