Direct buried vs Concrete Encased Ductbank 1

[rockman7892]

Wanted to hear experience or opinions from others when its typical to install direct buried vs concrete encased duct bank? As I understand it, this is not really a code issue but more of a design consideration for given application.

I'm looking at a project where new 34.5kV cable will be installed in conduit and run from a substation through an industrial site. Existing 34.5kV and 5kV duct banks on site are all concrete encased so plan was to follow-suit with new installation but was also interested in common approaches for applying each.

If conduits aren't crossing any road or drives with heavy vehicular traffic (IE inside of a Substation) is it typically to just install direct buried duct bank?

Do most industrial sites where there is vehicular traffic and other underground utilities ran typically have concreate encased duct bank?

 

[7anoter4]

In a power station, for instance, we always design duct bank and manholes all about 40 m and 100%[at least] reserved ducts for power cables.
You cannot know what will be built here in the future.

 

[davidbeach]

If your concern is first cost, go direct buried. If your concern is life cycle cost, put it in duct. The addition of concrete is more case specific.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[rockman7892]

Davidbeach

When u refer to direct bury I’m assuming your taking about direct buried conduit as opposed to direct buried cable?

Also based on your last sentence what would you consider a “duct” without the addition of concrete? Is there a typical differentiation between ductbank and concrete encased ductbank?

Thanks for the response.

 

[davidbeach]

Direct buried cable vs. in conduit. An organized array of buried conduits is often referred to as a duct bank. Duct banks may be buried using a variety of backfill. Native backfill being the cheapest/easiest, and sometimes unsuitable. Sand is common. CDF is also common. Concrete is stronger and more expensive than CDF, over kill in some applications but insufficient without reinforcement in others.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[bacon4life]

One important distinction is thermal constraints. For large power cables, thermal considerations are really important. Depending on the thermal properties of the native soil, we may require the use of Fluidized Thermal Backfill (FTB) for installations with thermal constraints. FTB is very similar to CDF but is specifically designed to have good thermal transfer. Sand and pea gravel have horrible thermal properties, so these materials are only used for conduits carrying instrumentation cables or power cables with negligible thermal loading.

FTB and CDF both provide some protection again accidental dig-ins, while still allowing some ability to expose conduits if needed. In some applications red dye is also added as an attempt to discourage dig-ins.

I have heard various discussions from civil engineers about which duct protection approach is best for handling movement from earthquakes. There seemed to be some concern that inadequately reinforced concrete duct banks could facture during earth movements. Duct banks in loose back fill or low strength CDF/FTB may flex instead of fracturing.