Concrete pipe trench 1

[cher122976]

We are designing a concrete pipe trench on an industrial site that will hold 2 24" Gas lines with 2 smaller gas lines running above them. 4 pipes total in concrete trench. One 24" gas line will be laid in the trench initially will the others to be installed with other phases. What would be the best design for this trench? I am thinking a grated top for the length of the trench (it will be partiall underground and above ground). If there is not a grated top and instead a fully enclosed trench it would be difficult to install the future lines. A grated top will pose some problems where the tench crosses the roadwAy with drainage considerations. Anyone have experience with this? Thanks.

 

[stanier]

Why not install a large diameter pre cast concrete pipe tunnel where the lines can be installed in the future by men working inside the pipe? Road crossing would be realtively standard design. This could be used at least for the road crossings or in difficult areas.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[cher122976]

That would be a little extreme dont you think for 2-24", 1-6", and 1-12" pipes? A huge tunnel vs a 8'w x 6'd concrete trench is significant. But, its an option, thakns. I think the road crossings are the most significant part of this design as a grate will work in all other areas. Maybe in the roadway areas put a water tight steel top on the trench.

 

[dicksewerrat]

Use precast box culverts under the road.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[cher122976]

How would they install the future pipe under the road within the culvert?

 

[racookpe1978]

I don't recommend a grated top: too weak to drive over, lets all the critters and dirt in anyway. Still needs all of the same support and prep work that a solid concrete top does. Plain plate is too expensive.

Strongly recommend against any "covered" "solution" such as a precast pipe or culvert or the like except where road crossings are needed and planned. You really, really want to be able to lay the pipe in the ditch/trench with a simple crane and tools. Long runs of covered pipe are a slow nightmare of moving the pipe, welding, supporting it, cliimbing arond it, insulating/cleaning/painting it, etc. Did I mention "slow" very expensive as well?


Use a simple pre-cast or pre-fab'ed (poured-in-place only if absolutely necessary!) rectangular pipe trench with a cast lip and simple lift-off concrete covers and lifting plugs/lugs/wires. Or put the retaining lip on the pre-cast conc covers. Rate them for a pickup truck or forklift loads where needed, or best case, for full traffic: you will want the access later.

First pipe in the first one planned: 24 inch. Above is room for the next 24 inch, then the third and fourth. Now, this means you can't get to the first 24 inch for mods or repairs, so consider a shallower but wider: 2x 24 on the bottom, then the two smaller. The first option means you need a less expensive cover plate, but a deeper trench. Second option gives you more flexibility to add more pipes later.

 

[stanier]

You could install individual 30" pipes under the road. When the gas lines need to be installed they are installed with "spiders/ rollers" to keep them centralised. 30" pipe could be steel or concrete. If steel you may need to cathodically protect them.

For the rest of the site why not install the pieps aboveground as in a refinery? Piperacks are invariably cheaper than trenches and much easier to maintain.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[cher122976]

Thanks rakook, I think that would work great. We have one roadway crossing where we will design to H20 but the remainder will have minimal loading. A lot of this trench will be level and there will be some water penetration into the vault so I think 4" drain pipes will probably be necessary at certain intervals.

The only downsize I see is not being able to maintain the two 24" pipes at the bottom of the trench.

 

[cher122976]

Thanks stainier I think the vault is a much cleaner solution then several casings. We do have tresses on site that will also be used.

 

[BigInch]

If using an open trench, or possibly a buried casing, those pipes will most likely float should they, or the casing, be flooded with water. Be assured that the drain will plug, or be too small, and the trench will flood one day. You should use concrete weight coating, screw anchors, or clamps, etc. to hold the pipe in place.

If installing in a casing, one would weld up a pipe string equal to the crossing length and then pull it through the casing.

Multiple strings would need specially fabricated spacers to hold the pipes in position inside the casing. For installing future lines, you may need guide trays between spacers. Supposedly you could weld and test the assembly, then pull through, if the length was not too long.

Now you know the reason that each gas pipeline is customarily placed in its own trench, or conduit.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[BigInch]

Not to mention that any future maintenance on the 24" pipelines will be extremely difficult.

In light of all the above, I would be considering laying those lines above ground, above water level, on pipe racks, "T" type supports, or sleepers.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[DRWeig]

Hi cher122976,

Just a prcedural comment on your post --

You have two threads running on this question, each with a bunch or thoughtful answers. If you feel the need to post in more than one forum, put your question in one of them only, and post a link to that thread in the others. It's good to consolidate all responses so that the experienced folks can see what all the others have said. Read the forum policies, especially number 4. There's a link below under my name.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[cher122976]

Big inch, you really think a 6'w x 6'd trench is going to float? I don't like the casing idea for multiple pipes at all, too many issues with that. We are going to utilize a concede trench with an removable top.

 

[racookpe1978]

Well any (open, pipe-filled) trench will try to float more than a filled ditch with gravel and cover put back in. Put then again, you'll face that with even a simple open culvert.

For supports, assuming a pre-cast series of conc units are going to be used as the trench liner, I'd cast into both walls of the trench units support niches (slots) for WF members crossing the trench perpendicular to the pipe centerline. Then on each WF cross member, pre-fab a pipe support for BOTH 24 inch pipes. (It's easier to shop weld two supports at once in the shop on jigs waist high 240 times rather than a separate welded joint 240 times in the field inside the bottom of trench next to a filled and pressurized 24 inch gas pipeline.) In the field, drop in the pre-fab'ed WF members, drop in the first pipe, cover it with the blocks. Weld at each joint where needed before putting the cover on.

For floating, check total relative weights, check the results of the movement (it may be negligible (your plant is in dry gravel or rock or hard dirt, or major if in wet soil or crossing a quicksand/marsh environment) Add weight if needed.

 

[BigInch]

Actually I was only suggesting that the pipe in the culvert would float, should the culvert become full of water, BUT, now that you mention it, the culvert might float as well, if the ground water became high and you didn't have it properly weighted for buoyancy control. If the walls were 8" thick, or less, positive buoyancy could be a problem.

If it ain't broke, don't fix it. If it's not safe ... make it that way.

 

[racookpe1978]

Re-reading your earlier comment, I should have seen that implication before I wrote. Shows my bias towards pipes that are most often either indoors, overhead, or both.

Thank you.

 

[BigInch]

Given the possibility that nuclear plants can be affected by a tsumani from time to time, it's a good thing to keep in the back of your mind.

If it ain't broke, don't fix it. If it's not safe ... make it that way.