National Grid

National Grid, one of the 10 largest utilities in the country, has the largest electricity transmission and distribution network in the New England/New York region. Moretrench served as general contractor for two drilled shaft foundation projects, one for the relocation of transmission lines to allow construction of a new highway right-of-way, and the other for upgrade of critical tower foundations.

The drilled shafts for the transmission line relocation involved six discrete construction sites extending over a total area one mile long and just 12 feet wide in places. Many of the sites involved working below live electric lines in low headroom conditions and adjacent to active traffic lanes. With the environmentally sensitive Providence River only 30 feet away, proper handling and disposal of spoil was also an important consideration.

At each location, a vibratory hammer was used to advance steel casing, ranging in diameter between 8 and 10 feet, to prescribed depths ranging from 30 to 60 feet below grade. Given the proximity of the river, shaft excavation below groundwater level was accomplished under supermud slurry to maintain a positive hydrostatic head within the shaft. Once excavation was completed, a full-length steel reinforcing cage and the transmission pole anchor bolt assembly were set. High strength concrete was then tremied in place. The displaced slurry was channeled to a frac tank for re-use at the next shaft location. Once all six shafts were completed, the slurry was allowed to settle out in the tank and water and sediment were treated and disposed of on site.

At each of the five Methuen area sites, two drilled shaft foundations were constructed adjacent to the existing transmission line pole in preparation for transfer of the H- shaped pole structures. Shaft diameters ranged from 5 to 6.5 feet and extended between 15 and 23 feet below grade. With the new foundations located directly beneath live electric lines, the use of special low headroom equipment, and heightened attention to safety, was required for all
stages of the work.

The permanent shafts had been designed to be constructed of galvanized thin-walled corrugated steel pipe that is cost effective and has a long service life. The holes were drilled and temporary steel casing advanced to design depth then cleaned out. The permanent corrugated shell was placed within the temporary casing and a concrete plug was poured to seal the bottom of the shell. The temporary casing was then removed. A reinforcing cage and anchor bolts, with extremely tight tolerances due to the H-shaped structure, were set and 4000 psi concrete was pumped to complete the new foundation.