Concrete Pipe Trenching

[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

 

[beej67]

How do you plan to keep runoff out of it?

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[cher122976]

about half the trench is going to be above ground. Rather then a grate I am thinking of just a watertight steel lid along the length of it that can be removed for future installations. But, yes, that is one of the critical design issues I am trying to get figured out.

 

[cher122976]

Is it feasible to even put removable steel plates along 700' of trench?

 

[Ron]

Use a concrete vault along the entire length....support the upper pipes by cross-supports

 

[cher122976]

Ron, I think the concrete vault would work well but it will need to have concrete removal lid along the entire length to install the future piping. I dont think that is a big deal to design from a strucural standpoint.

 

[BenJohnson]

In the past, for large utility trenches, I have seen moveable concrete covers that are pre-cast on-site. Normally an extra piece of re-bar is bent into the shape of a loop. The bottom of the loop is tied into the concrete, and the top of the loop exposed.

To lift the covers, you put a chain or cable through the loop and attached to a backhoe bucket, forklift tongs, etc. and guide them out using this mechanical equipment.

You will need to look at some details, such as how the pre-cast covers can properly fit on top of the main concrete trench. If cars or pedestrians are to travel over the top of the trench, then the lid design must be strengthened and the lifting loop imbedded below the travel surface.

If the top of the trench is buried below ground, it is a good idea to tie some nylon rope to the top of the cover. That way, the backhoe operator who is excavating for the trench in the future will will have some indication when the concrete cover is nearby. I have seen backhoe drivers come down hard on something he is looking for - and break whatever he is trying to uncover. A rope above your concrete vault will stop that.

 

[Ron]

BenJohnson...the pre-casters do it the same way. Concrete vaults are readily available and can be configured to essentially any size and load requirement. Traffic loads can also be included.

 

[gbam]

Rather than provide a lid the entire length can you push or pull the new pipes from specified lidded locations?

 

[Kuhuh]

Steel plates, Precast Vaults are not cheap, either are supports for the upper pipe. I think your best bet would be to install the pipe and back fill it loosely above the phase 1 pipe (use wheel on excavator to compact it). For the road section ,depends on time (days, months, years) between phases. If days then steel plates, months vaults?, years compact and asphalt it. From experience its cheaper to just back fill it and re excavate it most of the time. But it really depends on the time between phases for both areas.

 

[Hoaokapohaku]

I agree with Kuhuh. Though all of the ideas presented by others sound workable, I think it would be more cost effective just to place the pipes intended for future use at the same time as the first one is installed. I can't imagine how a 24" concrete pipe and couple of smaller gas lines could cost more than 700' of concrete vault, unless there is some reason the first 24" line must be in an accessible vault anyway.

 

[BenJohnson]

Just a guess, but many types of industries need to access the pipes on a fairly regular basis to make modifications. Something like a petroleum processing plant is constantly being modified. I'm guessing that is why they want all the pipes exposed. If this is for a permanent gas transmission lines, then better to just bury the pipe like most other gas transmission lines.