Trenching 2

[cher122976]

We need to take a 24" pipe under a good size electrical duct bank sitting with a bottom of 5' deep. Is there normally issues with supporting the duct bank during trenching activities?

 

[zdas04]

I'm not certain I understand your situation. "under a good size electrical duct bank sitting with a bottom of 5' deep" doesn't seem very clear (what has a bottom of 5' deep?).

Whenever I have a bit of trench that I have questions on I call a Geotech Engineer immediately. Those guys have saved my bacon any number of times. They are generally worth whatever you have to pay them.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.

 

[BigInch]

There are many kinds of electrical ducting. Some are relatively short segments joined together piece by piece. They may just have some mastic for water stop at the joints. If these segments are short, you may have a joint in the trench, so for sure it will need support. In any case, you should plan on providing support no matter what kind it is, because underground ducting is basically designed for continuous support by soil contact, just as your pipeline is, and you will remove that soil.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[dicksewerrat]

Does your client own the duct back? If not, call the owner before you remove any dirt.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[dicksewerrat]

Duct bank. sorry

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[stanier]

It depends on the native soil that you are excavating. Generally att such depths you would use trench shields as workers could be hurt if the wall collapses.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[cvg]

trench for a 24 inch pipe will only be 4 - 5 feet wide. Most electrical ducts can span that easily. However, you have not indicated if the duct is perpendicular to the pipe, whether the ducts are encased or not, how many and how large of a ductbank, what type of soil was used to backfill the duct bank etc, all factors which could make supporting the ductbank and shoring the trench more critical.

 

[bimr]

An exploratory excavation is necessary. Higher voltage duct banks are installed with unreinforced concrete. The concrete is installed to protect the duct bank against a potential hit from an excavator. The duct bank may not be able to support the concrete.

 

[BigInch]

Good bimr. They don't want to create inductive current in and losses into the bars. I've seen these cement duct banks as short as 1 m long sections.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[StephenA]

Higher voltage cables are sometimes oil filled after installation which may require continuous support.

Stephen Argles
Land & Marine

http://www.landandmarine.com

 

[BigInch]

Wooh, I didn't know that!

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[Kuhuh]

What we usually do, if pipe must cross the duct banks projection, is excavate all soil above ductbank throw an I-beam across the trench and put some straps with 2x4's under the duct to disperse the forces on what is most likely slurry encasement. This is more than enough to hold it up. We only do this if we know what is actually in the encasement.

 

[BigInch]

StephenA, why are they filled with oil? Do the electrons move faster? Seriously, I would actually like to know why the oil.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[dicksewerrat]

The oil helps dissipate the heat from the wires. And if the bank is old it may still have oil with PCB's, like the old transformers.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com