Underground conduit requirements

[eeprom]

Hello,
I have been debating with a fellow engineer the need to use a concrete encased duct bank, as opposed to just running PCV, packed in dirt, and covered with a 2" layer of concrete. A duct bank reduces conductor ampacity and costs a lot of money to build. I don't think it's necessary except in locations where damage is very likely. The code makes no mention of needing concrete encasement, even beneath road crossings.

When and why is a concrete duct bank a necessity?

EE

 

[HCBFlash]

I've been told, whenever I've asked, that it's simply "what's been spec'd" but these locations have always been where other utilities were nearby, and heavy industrial conditions and occupancies were present.. I would definitely advise considering possible damage in the area, and would also say that 2" of concrete wouldn't even be worth the pour! I admit this is only my opinion, but 2" wouldn't bear any weight or impact, or prevent any flotation. A multi layer formation would make compaction of dirt difficult, I'd recommend something similar to "pea gravel" in the conduit formation with a denser cover layer and, if flotation is possible, at least planks of concrete, no less than 4" thick with some re-bar in the planks.

Consider your soil conditions carefully. If your area has flooding or other factors present even rarely, PVC conduits in particular can float and move enough to damage your "duct bank", which could nullify any initial savings.

Still, I say this only as an experienced electrician (37 yrs), and can't refer you to any papers or studies to support my recommendations, although I'm sure some exist. I'm not in the IEEE anymore, but their IAS used to have such stuff.

.


Me wrong? I'm just fine-tuning my sarcasm!

 

[7anoter4]

I agree with you, HCBFlash. The duct bank is destined to support heavy traffic. Usually, in a new power station one has to design at first the duct bank and manhole web before the building are completely located and in certain situation a new building-or part of this-could be located above the duct bank line.
The duct bank is provided with manholes to facilitate cable run and the duct are big enough to accommodate a lot of cables for further development or as on the initially design the cable amount is unknown.
And –by-the-way-for a power station the entire electrical installation-equipment and cable and cable raceway represents less than 10% of the investment. It is more important to put the power
station in operation as soon as possible.

 

[GroovyGuy]

Howdy eeprom,
Are direct buried cables a consideration in this instance?
GG

"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[waross]

It may depend on the number of conduits and also on the amount of real estate available. I wouldn't install a duct bank for one or two conduits. Probably not for three or four conduits depending on site conditions.
I saw four circuits leaving a substation underground with direct buried PVC. One conductor per conduit plus spare conduits.
The ditch was over twenty feet wide.
You don't always have that much space available.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HCBFlash]

Assumptions are the stuff of forums and long-running threads, as well as catastrophe,,, as we all know. The 20' wide formation with single conductors per conduit must have been single phase or DC, and even then, "atypical". I don't know about the OPs system characteristics.

.


Me wrong? I'm just fine-tuning my sarcasm!

 

[waross]

25,000 Volts three phase. A fairly large utility. 111,500 customers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[eeprom]

In this particular case I have nine conduits, running through an area that will not get any vehicular traffic. The soil is very dry. I was going to spec that the conduits have 3" spacing between them, and the spaces are to filled with fine back fill or sand. I just don't see concrete as adding any value.

EE

 

[ScottyUK]

What benefit are the ducts adding? Your cables will probably need to be larger than if direct buried, which adds to the cost of the ducts themselves. Is there a risk of damage from other influences, or is it to allow cable puling once other works are complete, or something else entirely?

 

[bacon4life]

An additional option is thermal flowable backfill (FTB). It is hard enough to provide some protection, but still soft enough to remove if needed in the future. FTB has better thermal properties than pea gravel or sand.

There is question as to the seismic performance of duct banks, particularly duct banks that have just enough rebar to be very rigid but not enough rebar to fully withstand the earthquake forces.