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My Turn: Overhead lines require a lot of digging, too

Last modified: 5/22/2014 12:30:14 AM
Allie Morris’s “Burying electrical transmission lines not so simple” (Monitor, May 18) puts into perspective one controversial aspect of burying Northern Pass: cost.

Project sponsors claim that it would cost $15 million to $20 million per mile for underground cable vs. $3 million per mile for overhead wire. The former is grossly overstated, the latter understated.

Morris turns to TDI developer Donald Jessome to provide the commonly cited figure for underground HVDC cable, $5 million per mile. But no perspective is given for Sen. Jim Rausch’s charge that the digging process for buried lines could churn up new environmental issues: “You can cut trees anywhere, can clear the land; when you are digging it opens up a whole other can of worms.”

Morris quotes ABB Cable’s Roger Rosenqvist that underground lines must be buried at least 3½ to 4 feet below the ground. Forty-two inches is what other developers, including Hydro Quebec, say. But Morris – and Rausch – apparently forgot to ask whether overhead lines require any digging and, if so, how deep the excavations have to be.

How do the proposed Northern Pass steel monopoles 90 feet to 130 feet tall, the supports for the conductor wires, stay upright? Anyone who has put in a backyard fence knows the answer. You dig deep holes for the posts and fill them with cement or rocks.

The DIY rule of thumb is the post is buried to a depth half the height of the post above ground. Engineering for steel utility structures has its own complex rules, but here are some examples of excavation depths for monopoles that Northern Pass itself has provided.

A directly embedded 90-foot steel monopole for the relocated HVAC line would require a minimum 15-foot excavation. If the monopole were used for the HVDC line, it could require burial in concrete to a depth of 35 feet, Northern Pass says. (Extrapolating from these proportions, the project’s proposed 130-foot-tall HVAC monopoles in Pembroke would require a minimum excavation depth of 22 feet; its proposed 130-foot-tall HVDC monopoles in Northumberland could require an excavation depth of over 50 feet.) But compare only the figures Northern Pass itself cites, foundation depths of 15 feet to 35 feet for 90-foot monopoles, with the 3 feet to 4 feet required for direct burial HVDC cable and ask yourself which has greater impact.

And how deep would the foundations be for Northern Pass’s proposed steel lattice towers, which reach the height of 155 feet?

In fact, Northern Pass’s proposed overhead line requires far deeper excavation than direct burial HVDC cable does; structure foundations also require cement fill.

Direct burial in a pre-engineered highway right of way should require only trenching, not blasting, as Northern Pass’s structure foundations would.

Overhead lines require far wider rights of way to provide necessary spacing from other overhead lines than do underground lines; these wider right of ways fragment the land more. And if you think running a linear underground cable poses substantially more threat to wetlands, take a look at Northern Pass’s project maps and observe how many of the proposed 2,300 new AC and DC structures are located in wetlands.

And here is a partial list of transportation and construction vehicles that Northern Pass says it might use while building an overhead line: “automobiles, pickup trucks, bucket trucks, backhoes, tracked excavators, dump trucks, concrete trucks, tractor trailers, cranes (various sizes), bulldozers, tractors (various sizes), brush hog/mowing equipment, skid steer (Bobcat), log trucks/chipper vans, trains (some material might be transported by rail), helicopter (construction in some portions of the project may involve helicopters due to remote locations, rough terrain and/or presence of sensitive environmental resources).” They left out conductor stringing equipment.

No project the scope of Northern Pass is impact free. But, filed on May 20, New England Clean Power Link’s Presidential Permit application shows how low-impact a buried HVDC transmission line can be compared to Northern Pass’s overhead project.



(Susan Schibanoff, a member of Responsible Energy Action LLC, lives in Easton.)


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