The underground infrastructure of Cambridge, Massachusetts, U.S., is a tightly orchestrated web of water pipes, storm drains and sewer lines, which, combined with electric distribution, and telephone, natural gas, cable and fiber-optic services, allow the city to continually build on its storied history.
Lights burn long into the night at Harvard University where young minds shape the nation's political, social and economic agendas, and scientists at Massachusetts Institute of Technology (MIT) map the human genome. At the same time, Rebecca's Cafe brews coffee for weary patrons on a Saturday morning. Clearly, Cambridge would like as little disruption as possible to its way of life.
NSTAR (Boston, Massachusetts) knew this well when it sought to bring the output of a new 187-MW natural gas-fired combustion turbine generator at Mirant's Kendall Generating Station to the New England power grid. While the project promised to strengthen the energy security of Cambridge via a more reliable power grid, engineers would have to weave the transmission line into the already crowded maze beneath the city's streets. The city wanted the added reliability but without a disruption and any visual blight. Under the direction of Project Manager Y. Oktay, NSTAR engineers proceeded to find a suitable route for an underground transmission line installation in a concrete duct bank from the Kendall Station on the east end of Cambridge to the Putnam Substation, 2.6 miles (4 km) to the west.
Getting the Permits
As is the case with all such projects, permits were required from the various state and local agencies. NSTAR's first hurdle along this road was obtaining project approval from the Commonwealth of Massachusetts' Energy Facilities Siting Board (EFSB), which requires electric utilities to submit an application for the construction of any electric transmission project over 69 kV and 1-mile (1.6-km) long. The application must present the board with the purpose and the need for the project, identify a preferred and alternate route, and explain the construction process along each of those routes.
To prepare the application for submittal to the EFSB, NSTAR hired consulting firm EarthTech (Concord, Massachusetts). At the same time, NSTAR also hired POWER Engineers Inc. (POWER, Hailey, Idaho, U.S.) to provide preliminary engineering for the selected routes. The application identified the purpose and need for the project as a necessary interconnect to the New England power grid for the already approved additional generating capacity at the Kendall Generating Station. The application then detailed the selection process of an interconnect route that satisfied the EFSB mandate from the Commonwealth “to provide a reliable energy supply for the Commonwealth with a minimum impact on the environment at the lowest possible cost.” NSTAR considered interconnects between the Kendall Station and several regional substations that had existing links to the New England grid before settling on the Putnam Substation, which has 115-kV ties to the grid via a 345-kV step-up transformer at the Alewife Substation.
With a connection point selected, EarthTech, in consultation with POWER, considered various routes before finding one that was viable and could be completed in about a year. The route selected ran through a few streets in Cambridge and along the four-lane Memorial Drive, which skirts the Charles River on the west side of town.
POWER determined that the circuit should consist of three 115-kV cables installed in a concrete-encased duct bank, 3 ft by 3 ft (1 m by 1 m), having nine 6-inch (15-cm) PVC ducts. The majority of the route would consist of a duct bank constructed within a trench beneath paved roadways or sidewalks.
The selected cable consisted of a 3200 kcmil segmented-copper conductor with 690 mils of extruded cross-linked polyethylene (XLPE) insulation, an extruded lead sheath having a thickness of 125 mils and a 110-mil polyethylene jacket. Rated ampacity of the cable at 90°C (194°F) is 1400 A with an emergency rating of 1600 A at 105°C (221°F).
While all the up-front time and care put into the permit application allowed the project to pass the muster of the EFSB, routing the duct bank along Memorial Drive required approval of the Metropolitan District Commission (MDC), which oversees the Metropolitan Park System.
The MDC gave preliminary approval to the project, but required review and acceptance of the detailed design before construction could begin to ensure that the duct bank did not mar the scenic and historic qualities of Memorial Drive. The city of Cambridge also had to give its approval for that portion of the duct bank in the city.
Following POWER's preliminary engineering for the preferred and alternate routes, NSTAR retained the firm as project engineers to perform the detailed engineering upon approval of the project by EFSB. In addition to its dealings with EFSB, the MDC and the city of Cambridge, POWER had to adhere to requirements of the Massachusetts Water Resources Authority (MWRA, Boston), which oversees the sewer, water and storm drains.
Although each underground utility in Cambridge has easements to allow for maintenance and repair, the MWRA was concerned that the proposed duct bank would impede access to its sewer pipes. The MWRA raised this concern because, inevitably, the duct bank route traversed much of the existing underground infrastructure.
For example, the duct bank traversed large sewer pipes at three locations. Two locations were relatively short 40-ft (12-m) perpendicular crossings. The third location was a much longer 94-ft (29-m) “curve intercept” type crossing. The presence of a newly constructed office building 4 ft (1.2 m) west of the duct bank centerline compounded the difficulty at this location.
Since the support system for this type of transmission line is just the underlying soils, the MWRA wanted to have the ability to excavate anywhere within its easement without shoring and supporting a duct bank that weighed 1100 pounds per foot.
In order to permit unimpeded access to the sewer pipe, the duct bank bridged over all of the MWRA easements via by an underground overpass. The initial designs considered either a built-up steel member or a cast-in-place concrete for the bridge system. Fabrication lead times and lifelong exposure to corrosive soils both contributed to the decision to use concrete.
The next challenge was to squeeze both the duct bank and a support beam into the tangle of existing sewer, gas and water pipelines, as well as the other duct banks. The space restrictions forced the engineers to minimize the beam depth by designing a fixed-end beam that would connect to the foundation. The beam ended up with a 38-inch (97-cm) overall depth and weighed about 1850 pounds per foot. One of the drawbacks to the fixed beam was that the large moments transferred to the support required a stouter foundation.
The foundation loads and a weak soil top layer required consideration for a pile group and cap foundation design. For this phase of the work, the engineers called upon the services of the New England Foundation Co. (NEFCO, Quincy, Massachusetts), a company that specializes in various types of pile installations in the Northeast. NEFCO recommended installing pressure-injected footings (PIFs).
A casing, 12 to 16 inches (30 to 41 cm) in diameter, was inserted down to a competent soil layer, which was about 40 ft (12 m) below the surface. Zero-inch slump concrete was then pounded into the casing to form a bulb at the base, and reinforcement was inserted into the shaft and tied to the pile cap.
The length of the span required a large number of PIFs and, because of the proximity to the building, it was necessary to optimize the pile layout to reduce the pile cap footprint. The final design consisted of six compression and three tension piles for each support, which were tied into a 3.5-ft (1.1-m) pile cap. With this system in place, the MWRA would only need to insert sheet piling at its easement line to excavate. While the cost for such a transmission support system is high, the MWAR expects there will be greater use in the future as underground transmission lines are routed through urban centers where utilities will compete for the limited space available.
Bridges Over Canal and Tracks
Clear of the sewer line, the next requirement was to route the duct bank across the Broad Canal, which feeds into the Charles River, and then across a railroad track owned by the CSX Corp. Though engineers considered a directional drill beneath the canal and tracks, environmental and ease-of-construction constraints mitigated against that option. NSTAR decided, instead, to use bridges that crossed over the canal and the tracks.
The bridge over the canal is a 75-year-old drawbridge that is no longer operational because it is reinforced with fixed steel beams. To cross the bridge, the transmission routing used a fairly typical design that consisted of attaching the duct bank with stainless-steel brackets to the underside of the bridge.
The bridge over the railroad tracks represented a different challenge because the tracks go under Memorial Drive and fall under the jurisdiction of the MDC and the CSX Corp. Any design had to meet with their joint approval. The MDC stipulated that the duct bank not mar the landscape and CSX required that the duct bank not obstruct trains, which already had a tight fit going under the bridge.
The railroad bridge, which was built in 1932 and widened in the mid 1960s, is 50 ft (15 m) long and four lanes wide. The engineers created four designs to cross the bridge before satisfying all parties, including the MDC. Side and underside attachments were ruled out as visual blights and required ripping up the sidewalk. To go under it was thought too disruptive. The final design did go under the sidewalk, but from the underside of the bridge, using a cavity created when the bridge was first built.
To squeeze all the ducts in the tight space, the engineers designed a five wide by two high duct bank configuration. The duct bank was attached by chipping away at the concrete fill on either side of the cavity to reveal steel I beams, which served as bridge supports. A stainless-steel rack mounted on a stainless-steel beam configuration was bolted to the I beam on each side of the cavity every 4 ft (1.2 m) along the length of the bridge to support the weight of the conduits and cable. The completed duct bank design poses no space restrictions to passing trains and is out of immediate sight, thus preserving the historic character of Memorial Drive.
With bridges crossed, attention focused on concerns that were raised regarding potential electric and magnetic fields (EMF) produced by the high-voltage circuit. EarthTech, in application to the EFSB, documented negligible EMF for the circuit, but questions raised by high-tech scientists at MIT and parents of school children encouraged the engineers to take appropriate steps to minimize these concerns. The scientists, engaged in sensitive research, wanted assurance that their work would not be influenced by EMF fields. Since some of its laboratories abut Memorial Drive, the transmission line routing went further out into Memorial Drive to increase the distance between the line and the labs as insurance against possible interference.
The duct bank also passes by an elementary school on Memorial Drive in an area where the application to the EFSB indicated a rise in EMF levels. POWER designed an aluminum shield to go around the duct bank to reduce EMF levels, which were well below acceptable standards.
Construction and Completion
With the detailed designs approved, construction proceeded with traffic consultants from EarthTech and off-duty policemen controlling the public as the trench was dug in 150-ft (46-m) sections along the prescribed route. In addition, an archaeologist examined the trenched up soils in search of historic and cultural artifacts. None were found. The line was completed on time and the circuit was energized in April 2002.
William Hansen received the BSEE degree from the University of Missouri Rolla. Hansen has special expertise in underground transmission lines that includes high-pressure fluid-filled pipe-type, high-pressure gas-filled, self-contained fluid-filled and extruded-dielectric cables from 69 kV through 400 kV. His cable installation experience includes trenchless technology involving transmission cable installations utilizing horizontal boring and horizontal directionally controlled drilling. His cable design experience also includes submarine cable installations and bridge and tunnel installations. Hansen is presently vice president at POWER Engineers.