Guide to Typical Underground Transmission Line Construction

Subject to permitting requirements, underground transmission line construction typically, but not always, occurs in the following sequence:

Site Preparation

In the beginning stages of construction the underground alignment will be surveyed and existing utilities will be demarcated. Traffic control measures will be implemented to provide a safe working area. In areas where the installation is in a paved road, the pavement will be saw cut before excavation.

Duct Bank Installation

The duct bank trench will be excavated to the design depths and PVC conduits will be installed. These conduits will provide a sleeve that the electrical cable can be pulled through. The conduit will be encased in thermally approved sand backfill. A layer of concrete, called a concrete cap, is placed above the backfill, encasing the conduits to provide mechanical protection; marking tape is used to provide a warning in case of future excavation. Above the concrete cap the trench is backfilled and compacted with a granular material or a high slump ‘dig-able’ concrete. After the trench is backfilled, disturbed areas will be restored. Temporary surface restoration will occur by backfilling the top 4 to 6 inches of the trench with road surface material similar to the type of existing roads (asphalt or dirt).

Here is an example of an open cut trench installation of a duct bank:

Trenchless Crossings

During the process of installing the conduit system there are some cases a trenchless crossing technique is utilized, such as horizontal directional drilling or a horizontal bore. Some typical applications for these types of uses are when crossing under rivers, streams, large culverts, highways or railroads.

Cable Pulling

Once the splice pits or splice vaults are installed, the cable is pulled through the conduit system from splice point to splice/termination point. This means that during cable installation, the only locations that will have disturbance shall be the splice locations. After the cable has been pulled into a pit or vault it is spliced to the next run of cable. Once the splicing operation has been completed, the splice pit or splice vault is restored to grade and the splicing operation moves to the next splice location.

Splice Pits / Splice Vaults

The underground conduit system will terminate at splice pits, splice vaults or the underground to overhead transition stations. Pits and vaults are precast concrete structures and spaced about 1,500 to 2,500 feet apart (the typical length of a cable segment). Splice pits are filled with a granular material, a precast lid is placed on top, and the area will be restored. Once restored, there will be no visible evidence that a splice pit exists at the installed location. Splice vaults will not be filled with any granular materials, they will have riser rings and manhole covers. Once restored, there will be no visible evidence that a splice vault is installed at the location other than the manhole lids installed at grade. 

Road Restoration

Upon completion of all underground construction work, the roads will be restored. In general, surface restoration shall be done to meet or exceed the pre-construction conditions. Typically, a twelve foot wide section of the pavement (equivalent to one travel lane) will be milled and paved during this process. Where construction occurred in the shoulders of the roads or off road areas, the surface will be covered in a layer of loam and seed.