Low Clearance of Underground Pipes (Crossing) 2

[MarshfieldTimC]

I have an existing 24-inch sanitary sewer that will be crossed by a new 54-inch concrete storm sewer pipe. The new 54-inch storm sewer will cross over the sanitary sewer with minimal clearance (1 to 2 inches, max.). The 24-inch sanitary sewer is old but in good condition (I have no reason to replace it, and no way to relocate it to avoid the conflict). My concern is that the new storm sewer will damage the existing sanitary sewer. How can I install/support the new storm sewer to 'bridge' the existing sanitary sewer? How can I protect the existing sanitary sewer pipe to prevent damage, including construction and long-term affects?
Thanks in advance for your comments.

 

[DWHA]

You could form up around the sanitary pipe and pour concrete up to the elevation of the bottom of the storm sewer pipe.

 

[dicksewerrat]

DWHA has the right idea. You need some pillars, supports for the storm drain that rest on virgin soils and do not touch the sanitary line. I would build them a half inch higher than required. When you place the 54 inch on them they will settle a little. Look at using a CIPP liner in the sanitary line, using a live load for the storm drain.

You could also build manholes on either side of the the sanitary and use a plastic pipe over the sanitary to lighten the load on the sanitary, Lamson makes a pvc profile pipe or use HDPE. Just remember to tell your equipment operator that the sanitary is there, so it stays there, not in the bucket.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[gbam]

Have you considered eliptical pipe with equivalent 54" diameter and a concrete cap to dispurse the loading on the sewer pipe?

 

[PELS]

I have seen crossings where styrofoam was used between the two pipes and I have also seen concrete caps with supports but that design also involved structural analysis and good geotechnical data of the existing soil and existing trench backfill and bedding material.

Is the 54-inch storm drain flowing under pressure at times? That would mean the weight of the pipe and the weight of water both have to be taken into consideration.

 

[GoldDredger]

That's a tricky one you have there. The one or two inches worries me the most. Has the sewer pipe actually been exposed and shot in, so you are certain? If based on as-built or design drawings, who knows what your separation really will be, might be even less, or worse, conflicting.

The City of Dallas (Water/Wastewater Department) has a detail for exactly this situation. You can find it in their Standard Drawings for Water and Wastewater Construction, detail 'Encasement Protection for Wastewater Main', pp 414.

Basically it calls for a steel or D.I. pipe encasement of the sanitary line, keyed in 3' minimum past storm trench, everything backfilled with crushed rock. The wastewater line within the encasement is replaced, and reconnected with a pressure rated coupling at the new connection points.

Another thing you might look at is a 'Utility Support' (Same reference as above, pp 121) Basically it proposes vertical colums, supported by footers, and connected at the top by a horizontal beam. About 12" square concrete with plenty of rft all around.

Essentially you would create two supports either end of the storm pipe to hold it up, then backfill around it. You'll have to look at your soils for bearing pressure and potential settlement with the tolerance you have.

You could create your own type of design too. Install two concrete monstrosities either side of the sewer and set the new storm pipe on them.

Be prepared for your backup plan if it turns out you really have a utility conflict. After all is said and done, I would call for a TV inspection of sewer to verify no crushing or belly caused by settlement.

Hope this helps, good luck.

 

[Hoagie]

Encase the sanitary line with concrete.

I like the elliptical section idea...this should gain you a few more inches in separation.

Spec out some sand as cushioning under the storm drain and above the SS.

I would also recommend a plate of vulcanized rubber (in lieu of styrofoam) between the pipes to protect the pipes when the bedding is compacted.

Call-out some bold font warning on the plans and in the notes.

 

[CPENG78]

I agree with Golddredger, the fact that you are describing it as 1 or 2 inches would make me nervours. As-built drawings or record drawings (the difference between the two is debateable for a different forum) will not necessarily show exact locations. If rough calculations show a 2 inch clearance, then exposing the pipe and having it shot or potholing is a must to get an exact number. As someone mentioned, not only changing pipe materials for both pipes but also providing a concrete cap may be necessary or build the monstrous custom support structures that may just end up been your final option.

As Hoagie mentioned, your plans need to call out THIS SCENERIO IN BOLD LETTERS SO THAT EVERYONE CAN SEE IT (protect yourself basically), not only in plan and profile view, but also in general notes, project specifications if applicable, and even provide a details of how the contractor will make that crossing.

 

[DRC1]

I like Diksewerat's idea about the two manholes. The manholes can bear below and away from the existing pipe and the new pipe elevation can be rasied to clear the existing pipe at a better elevation. If cover is an issue, the manhole could be a drop mamhole and the new pipe could cross under the old pipe.
use flowable fill to backfill excavations
Although it is good to make sure everyone understands all the important issues on a project such as this one, I am not sure how printing this in bold will protect the designer.

 

[GoldDredger]

I'm not so sure about the two manhole idea.

All your doing with that is arbitrarily raising the storm pipe elevation at the sewer crossing. That would surcharge the storm line as far back as coinciding new invert and reduce the overall capacity. Not to mention there would be standing water in the pipe and manhole until it could dry out.

If you went under with the manholes, you'de basically have an inverted siphon with horrendous headloss. I would imagine you would see stormwater surcharging out of the manhole lid as well.

Would be curious to know what they end up doing.

 

[MarshfieldTimC]

We have not constructed this crossing yet, but I am planning to do the following:

Build spread footing/block type supports on each side of the old 24" sanitary sewer, without disturbing the existing granular backfill material around the 24" concrete pipe (hand excavate). Concrete (ready-mix w/reinforcement) to cure prior to laoding it.
Place 2" insulation (compressible) between the sanitary and storm sewer pipes.
Center a piece of Class IV 54" RCP storm sewer pipe over the existing sanitary sewer, cradled and supported by the concrete footings/blocks (probably set on shims/blocks with the annular spaces grouted).

Thanks to all for the comments and discussion. I would appreciate further comments/discussion on the proposed method.

 

[civilman72]

You mentioned insulation - do you have concerns with freezing. If so, with only a few inches between the pipes, you will need a lot more than 2" of insulation...

 

[rconner]

I don't know enough about your design to comment much. However, just a cautionary note -- it doesn't appear that you have herein explained either the specific strengths of the pipes involved, the depths of cover, nor any dynamic or surcharge loadings, settlements etc. that might eventually be imposed on the composite structures -- while I don't know that this proposal will not work, I don't believe that ordinary "54" RCP storm sewer pipe" is necessarily designed as a beam in the axial direction (with substantial beam shear/bending strength e.g. longitudinally), as it appears it might have to function in this installation.