Once a project receives its final permits, construction crews will begin clearing or trimming the transmission corridor where necessary. This includes clearing trees and brush to provide construction crews and their equipment safe access to the work site and enough clearance for the reliable operation of the line.
When construction is complete, disturbed areas will be restored. Native shrubs and ground cover are allowed to regrow.
To meet electric industry vegetation clearance standards, species of trees deemed non-compatible for transmission corridors must be permanently removed. These are trees that could become tall enough to grow or fall into the high-voltage transmission lines. Vegetation clearing is done in accordance with electric industry standards and without the use of herbicides. Read more about how Eversource manages vegetation on its transmission corridors.
Construction vehicles and other large equipment must be able to access the location of each structure that will support the transmission lines. Gravel roads approximately 15-20 feet wide will be built within the transmission corridor, and they will be made of gravel, timber mats or the native soils in the area. Large level work areas (generally 120 feet by 100 feet), called crane pads, are needed to stabilize equipment, such as drill rigs and cranes.
Timber mats may be used in or around wetlands to protect these environmentally sensitive areas. Silt fencing and other environmental controls are also used to stabilize the soil and protect wetlands during construction. With the consent of property owners, gates are placed across new access roads where these intersect with town or state roads. These gates help deter unauthorized access to the transmission corridor. By landowner request, gates are also installed where access roads cross agricultural land containing livestock.
Distribution lines are the lower-voltage power lines that bring electricity to customers' homes. Sometimes, these lines are in transmission corridors. During construction, Eversource carefully coordinates the removal of existing lines with the installation of new lines.
Where relocations are required, new distribution poles and wires are first installed in an alternate section of the transmission corridor. Once complete, the existing distribution line is de-energized so that power can be transferred to the newly-built line. The de-energized lines are then removed so that transmission line construction can continue. The old poles are taken to an off-site location and disposed of properly.
Eversource carefully plans the sequence of construction to allow for the safe construction of the new lines while customers continue to receive electrical power.
Existing structures that require removal are de-energized and the overhead wires removed. The wood poles or steel structures are taken off-site and disposed of properly. Concrete foundations are removed below grade and the area is filled.
The next step in the construction process is to drill foundations for the new transmission structures. This involves drilling holes, which are then typically filled with concrete for structure foundations. Drilling operations occur for a few days at each new structure location. Once drilling is complete, a steel rebar cage is placed in each hole and concrete is poured to create a secure foundation for the new steel or lattice structure. Concrete trucks are used to deliver the concrete mix for the foundations.
Once the foundation is cured, transmission structure installation can begin. Steel poles often come in sections that are assembled on or near the foundation. Cranes and/or bucket trucks are used to lift the poles and set them into position on the foundations. Construction crews will assemble or "lace” lattice structures at the site.
The structure components are delivered to the transmission corridor well in advance of this installation process. Generally, it takes one to three days to assemble and erect each new structure. After installation, the structure is grounded for safety purposes.
With the new structures in place, the next step is to install the wire ("conductor"). The wire-stringing operation requires equipment at each end of the section that is being strung. Wire is pulled between these "pulling sites" through stringing blocks (pulleys) at each structure. These pulling sites are set up at various intervals along the transmission corridor, typically one to three miles apart. Specific pulling sites are determined close to the time the stringing activity takes place. Once the wire is strung, the stringing blocks are removed and the wire clipped into its final hardware attachment. Helicopters may be used during wire stringing operations.
When construction is complete, disturbed areas will be restored. Native shrubs and ground cover are allowed to regrow. Environmental controls are removed, though some may remain until the area is stabilized.