Under Grounding of Power Lines 6

[ajeebmjeeb]

On one of my roadway design project we are being requested to incorporate future provisions for undergrounding 94.5 KV transmission lines. What does that mean? How do we size the trench and size the conduits for this almost 3 miles of length of roadway. Where can we find standards for such design. What about the Vaults? How many and what type.

Since I am a Civil Engineer by profession any advise in this regard will be appreciated. Thanks

 

[SlideRuleEra]

Suggest that you consult with an electrical (power) engineer. This is a very common situation; it will not be difficult for someone with the proper background to provide you the answers.

94.5 KV lines are not something for us civil "amateurs".

http://www.SlideRuleEra.net

 

[cvg]

undergrounding powerlines is always done by the power company, which I would assume is not your client. They must provide the plans.

 

[ACtrafficengr]

Eng-Tips does have an electrical transmission forum. I suggest you ask your question there. Also, I agree with cvg. Th4e problem if it's 10 years before the lines are buried, will standards change, making your conduits obsolete?

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"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail."

Wm. Phelps Eno, ca. 1928

 

[DRC1]

We have been installing 115 & 345 kV duct banks. Depth is 5 ft to bottom of trench, width on plans was 3 foot, but generally it will be wider to install conduit. (Depends on the ave. width of your local electrician). Generally power companies try to limit depth to 5 feet to reduce heat build up and reduce the need for shoring (OSHA limit). Typically duct will be 4 to 6 in. pvc poured with concrete to the top of the duct and backfilled with sand or flowable fill. In the old days cables ran at about 75 -80% of rated capacity and heat was not a problem. Today cables run much closer to 100% so heat is a big issue. To control this they use concrete and backfill with specific tested thermal properties. 94.5 is not a heavy line, but there may be some increase by the time it is installed as ACtrafficengr pointed out.
I would:

1. Ask the utility what they wanted as it will be there work.
2. Assume a 5x5 trench if you get no other information.
3. Keep crossing utilities 6 feet deep min. and parrallel utilities 5 ft away.
4. Accurately mark and map the utilties you install for this work. Monument above ground.
5 Cover utilites with a few inches of concrete, dye the top red and install narking tape. Be sure to use utility marking tape, as it partially metalic and will carry a tone for the mark out person.
6. Back chage the utility for the extra cost of the work. For example, Six foot of excavation will require shoring and additional excavation.

As for the vaults we are installing, they are huge. About 30 by 12 inside. We have also installed much smaller vaults so you really need to check with the utility on what they want. We are also istalling two redundant lines for the duct, complete with two sets of vaults at each location. Vaults are located about 1500 feet apart. This is due in part to the limitations on most cabe pulling equip.

 

[BigInch]

give them some additional working width so they don't have to park or move their equipment on the road surface when they construct. If you've got it to spare, give 50 feet for conduit right-of-way (plus some pipelines in future) and a 25 ft machine passage on the side of that. If you're making a grassroots roadway and have the width to spare, make a utility corridor as well. Why not?

 

[cvg]

50' would be quite generous. most electrical easements I have seen are about 10' wide

 

[BigInch]

With "(plus some pipelines)" its not even sufficient.

 

[AdmiralK]

We have some experience with this that may be helpful. In the last five years our power utility (Centerpoint energy) has begun to direct bury 79.5kva cable without using trenches and the usual red painted concrete conduit. They are careful about the location to avoid any area where digging might be possible (they ran under water to Galveston to avoid gantry service that is vulnerable to hurricane damage). You might call them or check out their web site:

http://www.centerpointenergy.com/about/operations/1,2776,100440,00.html

 

[BigInch]

I'm confused. How do you, "Avoid areas where digging might be possible"? How do you install this cable? Do they at least put red painted concrete tiles over the top of it?

 

[TDAA]

I have just worked on a project with a 115KV line running below the street. A portion of this one was directionally drilled to depths of up to about 50 feet, to allow the future (now a year from now) lowering of a road, and construction of a bridge. The reason for going so deep was to avoid conflicts with the bridge. I guess they did not realize that the depth would make locating it very difficult. Be sure to get good asbuilts and locating markers regardless of the depth installed. Any one who has to drill / dig near it will thank you.