Allowable pressure on an 11KVA cable

[FHRebuildEng]

I am planning some temparory works to repair a bridge.

Part of these works require a 180t crane and there is an 11KVA cable running though the middle of the site.

The cable owner has stated that we are not to damage it (no ....), however has left it up to us to do this and will not provide any useful info on allowable stress on the cable.

I plan to protect the cable using compacted earth.

So I need to know what is the maxium allowable settlement and pressure on the cable.

Further more what else do i need to know/consider?

regards

 

[7anoter4]

First of all I think your cable is 11 kV rated [not 11 kVA what could be the power[apparent] if 1 A will flow it through]
The NEC Table 300.50 indicates the minimum required cover-for direct-buried 30 in is required, for instance.

 

[Jebb]

I recently had a project with a 250T crawler crane working on top of a 230kV cable (direct buried 36 inches deep). This cable would be much harder to fix than a 11kV cable. We had the contractor put down timber crane mats to distribute the ground pressure where the crane was to be set up on the cable. We also had them place 1 inch steel plates down whenever they were required to travel over the cable. It might have been overkill, but there has been no cable failures yet.

 

[DTR2011]

@Jebb - With the material costs alone on the cable, I don't blame your caution.

 

[ScottyUK]

Much depends on the quality of the backfield material. Poor backfill will cause problems if a light truck drives over the cable, a properly installed cable will likely survive a severe but uniform pressure. I second the use of road plates to spread the load.

 

[PHovnanian]

Scotty is right. Lay the cable in about 6" of sand. Backfill a foot of sand on top of it. At this point, you can lay wood planks on top of the sand backfill (about 6" wider on each side than the cable(s)). This will stand in for the crane mats, should someone forget or lose track of the cable location. Then finish the backfill with native soil.

For something the size of a 250 Ton crane, I'd still lay down timber mats.

 

[ScottyUK]

'Backfield' - damned mobile phone spellchecker!

 

[7anoter4]

I am not a structural engineer but if I interpreted well the Boussinesq equation
For "SOIL PRESSURE CAUSED BY POINT LOAD":
SigmaZ=3*P*z^3/2/pi()/R^5
where
SigmaZ=soil pressure at Z [ft.] depth lbs./ft^2
P=point load [lbs.]
R=sqrt(Z^2+LENGTH^2)
for a depth of 30 inches a force of 350000 lbs. [160 metric tons]
will produce 8376 lbs./ft^2
If the cable diameter would be 2 inches the force exercised on a foot length could be:
8376*2/12=1396 lbs.
Compared with maximum permitted side pressure of a cable around a pulling wheel
it is less than permitted as side-pressure: 1500 lbs./ft. for a tape-shielded cable.

 

[7anoter4]

Sorry! I used kg instead of lbs. in the above formula.
The resulted pressure has to be more: 3075 lbs./ft^2
that means only 160000 lbs.[73 metric tons] is the point load in order to produce 8376 lbs./ft^2
If the truck of 180 tons stays on 3 wheels[at least] and we may divide the force by 3 then 30 inches depth could be fair.

 

[ScottyUK]

7another4,

Unless this is a paved roadway or concreted apron it is unlikely that the soil itself will be able to support the ground loadings you suggest. In softer ground the cable will be subject to tensile stress as the surrounding soil compresses and the cable will stretch as it deflects vertically. Most manufacturers publish maximum pulling tensions for applications where the cable is installed in a duct; this might be a starting point for calculating the maximum loading if the soil conditions are known.

 

[7anoter4]

Thank you, Scotty.I agree with you.

 

[bacon4life]

PHovnanian, Is burying planks in trench widely accepted? I have seen our civil department go to quite a bit of work to exclude organic matter from backfill.

 

[PHovnanian]

Use treated planks if the cable installation is going to be permanent. Wood poles usually rot fastest near the surface, where air and water can get to them. One foot deep and I've seen the bottoms of poles last 20 or 30 years.

 

[PersonalProfile]

AS2566 provides guidence for polyethylene pipe, which may be of interest.

Regards,
Lyle