draining underground pull boxes

[cvirgil]

Anyone know of methods for draining underground pull boxes. Woudl one need to install a drain pipe tied into a storm system or could the drain pipe just extned beyond teh pull box and termninate a few feet away in the ground>

 

[dpc]

It depends on the expected level of the water table during wet conditions.

For shallow boxes and vaults where the water level will be below the bottom, you can provide a little sump in the bottom with a drain hole. Put a layer of crushed rock under the handlhole prior to installation.

If you try the above where the water level is high, the hole just allows water to enter the box. If you can hard pipe a drain line to a storm sewer, I guess that might work, but I've never seen that done. Normally for high water areas, we would specify a sealed sump in the bottom of the vault and then use portable sump pumps to pump them out if necessary.

It is important to slope the duct banks and conduits away from buildings to avoid flooding of basements. But you pretty much have to assume that anything you put underground is going to get wet. Standing water in manholes and handholes is more the norm than the exception from my experience.

 

[itsmoked]

dpc; Do you ever have trouble floating a box?

 

[dpc]

Yes, that is a concern and must be considered during design, especially for really high water tables. Normally, we used pre-cast concrete vaults connected to concrete-encased duct banks, so there was enough mass to keep them in the ground. Or maybe the cables held them down .

But manholes and vaults have definitely been known to float - that's a really bad day for all involved.

Good comment.

 

[itsmoked]

Thanks. Yeah, I can just see, that manhole float in the middle of the night on that dark country road in the pouring rain..

 

[Electic]

I previously worked for a utility in a wet area with much rain and high water tables. The conduit system was not sealed and vaults were not drained. Water was allowed to seep in and out of the vaults to match water table within soas not to cause floating.

When working on vaults, often a hydraulic pump was used to lower to water table inside. This could take considerable time as the conduit system might drain a bigger area than expected.

All hardware in vaults was water proof, and seemed to survive submersion, including medium voltage load break elbows. Of course the vaults would need to be pumped down prior to operating the elbows.

Other nearby utilities installed above grade J-boxes.

One problem encountered was exposed concentric neutral corrosion, this was helped by specifying overjacket. Transformers and secondary connections were generally above grade.

I have seen consultants specify: drain vault to daylight, assuming there is sufficient drop within reasonable distance to allow the water to drain. This requires some sort of rodent screen/protection.

I've not seen a vault drained to sewer. This would require a P-trap to prevent sewer gas entering the vault, and sounds like a whole new set of headaches.

Finally, I have seen vaults with permanent sump pump, and float switches, with pump discharge drained to sewer, but with an air break in the line.

 

[alehman]

In a few cases where nearby grade allows, I've been able to run a drain pipe from the bottom of the box to daylight some place.

 

[rcw retired EE]

We put sumps in the bottom of vaults directly under the manhole to allow a pump to be dropped in.

On some sites we used a drain fitting with an integral "check valve". A floating ball would block the drain outlet if the exterior water level was above the drain level. This supposedly prevented water from coming back up the drain. It didn't work that well due to the almost constant high water level at that site. (One of the large manoles floated to the surface during construction prior to conduit installation.)

 

[Dumbo2929]

It 15 years of utility work in the Northeast US, I've never been in a dry manhole, that is a fact of life. For small handholes we usually spec open bottom with crushed gravel to allow water to drain away and not stand. Manholes have sumps and utilities use a portable pump to drain them. Even manholes that get coated on the outside with waterproofing get water in them. Open bottom manholes get used in coastal areas where the water table is high. Floating manholes = bad.