Pipe Zone Compaction 1

[bank]

We designed an underground detention system using 30" HDPE pipe. According to our details and ASTM 2321 the trench is supposed to have a minimum clearance between the pipe and the trench wall of 10", backfilled with compacted Class I or II material. The contractor instead set the pipe in a trench almost exactly the width of the pipe OD. In-situ soil in this area is generally fat clay.

When we told him he had to take the pipe out of the ground, widen the trench, and provide the required backfill he asked if he could leave the pipe in place, widen the trench as required, and backfill and compact the Class I or II material. My question is can this work? Can they widen the trench 10" on either side of the HDPE pipe without damaging it, and achieve the required compaction such that the pipe is adequately supported laterally?

 

[BigInch]

I hope you know the answer to this already. If what you say is were true, then why the heck was it specified to have adequate bedding under the haunch in the first place.

Adequate bedding is important UNDER the pipe. The 10" (I'd say that's insufficient for a 30" diameter pipe BTW) clear to the trench wall was so that you'd have enough room to compact the bedding below the pipe centerline. Placed in clay, is even worse. It will tighten up on the pipe. It should be sand placed around the pipe. Anyway, now you've got nothing there. Vertical load from above on the HDPE pipe will eventualy deform it and try to fold it into the now empty corners below the pipe. There is nothing to support it.

It will be your fault, because you knew about it, but did not object and make the contractor follow the installation specifications you gave him.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[bank]

Sorry BigInch. I forgot to mention that in widening the trench they would also place the Class I or II material in the void and compact it.

 

[BigInch]

It may be too late for that.
With hand excavation, maybe they can expose it without mucking it up even more.
Keep a constant eye on them.
If you find even a hint of any damage anywhere, ... anywhwere, they should pull it all up.


"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[BenJohnson]

The 30in. HDPE is a big pipe to be filled with water for a detention system. Without proper backfill support on both sides of the pipe, it may want to flatten to a oval shape. That is big problem.

However, if the contractor can widen the trench and the pipe can be properly supported with good quality, compacted backfill, I think that process meets the specs. You may which to tighten the backfill requirements to Class II SW so the contractor is able to easily meet any compaction requirements.

Also, you want to make sure the backfill is well drained. You don't want any standing water (ie, saturated soils) in your backfilled trench or it will cause the clay sidewalls to swell and put unwanted lateral pressure on both the backfill and the HDPE pipe when the pipe is empty.

 

[stanier]

There is a solution that may involve considerable cost. You could jet in a sand water slurry. The trench would need to be drained of the water. This technique has been used to achive acceptable support for a PE pipe.

If you are not going to do this instruct the contractor to lift the pipe and do it properly.

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

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

 

[dik]

I'm not familiar with the use of pipe for detention. Is the 30" HDPE pipe for conveyance of the water to a detention pond? or somewhere? For pipes larger than 12", I typically have the side of the trench equal to the pipe diameter (I've not used piping greater than 3' dia...

I usually use bedding stone up to the diametre and then good granular material for the backfill. I'm not sure what Class I and Class II material are.

Dik

 

[dicksewerrat]

Is this a group of parallel pipes? Or just one long pipe? If a group of pipes, what is the center to center distance?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[jgailla]

This is why I use RCP whenever possible. It is much more resilient to poor installation.

 

[bank]

Richard: The design called for three parallel barrels of 30" HDPE pipe in a common trench, spaced 55" center to center. The contractor instead dug three separate trenches that appear to be spaced almost ten feet C-C. He claims that he provided six inches between the pipe and the trench wall, but I and my construction manager agree there is virtually no space between the pipes and the walls.

Dik: The pipe is solely for detention. The outfall pipe is 12" HDPE with a restrictor pipe at the downstream manhole. I prefer underground detention because I think those big holes in the ground are a waste of good real estate in most cases. Plus, that's the only option when there's no room for a pond.

Ben: I put an underground detention system using 60" HDPE beneath the driveway and parking lot of a fire station back in 2003. It was installed per the plans and specs, and has continued to perform with no problems through one hurricane and despite the heavy vehicle loads.

 

[dicksewerrat]

Tell the contractor to remove, redig and replace properly. Or call his bonding company and tell them to fork over the cash to install correctly.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[stanier]

Direct the contractor to perform the contract. If the contractor fails to do so use the contract conditions and have the work performed by others and backcharge the contractor. If you dont be strong on this matter in years to come someone is going to sue you!

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

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

 

[cvg]

hold on now, the owner can call in the bonding company, but not a design engineer. And I believe it might be a bit premature for that step. First of all, start with more frequent inspection. It should have never gotten to this point and the contractor should be told in no uncertain terms that deviation from the plans is not allowed without prior approval. Hopefully the owner is not doing this approving?

CLSM could be used to backfill with only minor vibration required (if any). but this requires some experience by the contractor to not float the pipe, maintain alignment and roundness. doesn't sound like your's has that experience. I think it would be far easier for them to just pull the pipe, fill one trench and re-dig the single wide trench per plan.

 

[stanier]

The more time spent avoiding the contractual side of things the more difficult this will become to resolve. Direct the contractor to dig it up and follow the pseccification or provide an altenative solution backed up by an engineer with Pi insurance to cover the job if it is stuffed up. Dont get involved in solutions. Remember that you shouldnt have to take the risks on this one.

What are you going to do if in the future a truck goes over the pipe, it collapses and someone is killed? What will you say tot he coroner? What will you say to your wife when you tell her "we have to sell the house darling to pay the lawyers".

Stick to the script its far easier in the long run.

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

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

 

[dik]

The pipe is for storage only with a control flow outlet to a manhole and then to a storm sewer system?

Dik

 

[BigInch]

Right. It's a detention pond without the pond.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[cnstengineer]

Check with the pipe manufacturer on their backfill requirements. They usually require a stone envelope and the contractor should have known better. I've installed many of these underground systems and one of the big dollar items is the stone backfill. My guess is that the contractor dug 3 seperate trenches to save on stone backfill in the area between the pipes.

 

[dik]

Thanks BigInch... learned something else today... could likely place it under a parking lot or someplace that will have little likelihood of future development.

Dik

 

[BigInch]

Ya. It's seems to be a good idea when short of space.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[rconner]

To add to what others have said, I am aware there is at least some minimal guidance in ASCE MOP No. 117, "Inspecting Pipeline Installation" (2009). I believe this manual defines a term referred to as "Side Clearance" (and a variable "b" designating same?) and also states, "Operations to compact backfill in pipe trenches require minimum clearances between the outside of the pipe and either the undisturbed soil in the trench wall or members of the trench structural support system. The nature of the compacted material and the efficient operation of compaction equipment govern these clearances. For typical backfills using sand-clay mixtures, 8 in. (200 mm) of clearance is commonly specified for pipe diameters less than 24-in (610-mm). Pipes greater than 24 in. (610 mm) usually require at least 12 in. (305 mm) of clearance."
I'm not sure what is wanted for right at 24" size, but guess it may be in general about 10 in. minimum for that size! I think somewhere else it is also stated this rationale basically also applies for multiple pipes in the same trench.
Hope this information also helps - everyone have a good weekend!