backfill using flowable fill and other options to fill soil void/pocket 2

[delagina]

I have a construction issue where soil erosion happened under a pipeline.
contractor back filled using cement stabilized fill but just dumped it above the pipeline.
Now we have a huge void under the pipeline that we need to backfill.

removing the backfill above pipeline is not an option anymore per contractor.

what are the best options here.
I was thinking using a hose and pumping flowable cement fill to fill up the void.

what else can you suggest including the type of flowable fill that is best to use in this case.

 

[msquared48]

If there is nowhere for the air in the void to go, flowable fill (CDF) may not work. Otherwise it should. Do you have the ability to verify visually that the void is filled?

You may have to go with a high pressure grout, but not so great that you collapse the pipeline.

Mike McCann, PE, SE (WA)

 

[TehMightyEngineer]

If I understand you correctly you have a long void under a section of pipe. If you can run a smallish PVC pipe to the back of the void and then fill from the front of the void with flowable fill then the pipe will act as both a vent for trapped air and an indicator that you've filled the length of the void (once you get fill coming out of the pipe you're pretty much done.

Will this work for you?

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[cvg]

grouting or flowable fill could damage the pipe. It might work but would require a qualified, experienced contractor which it sounds like your contractor is not.

remove the pipe and replace would be my option. otherwise, contractor should be suggesting his method to fix his problem.

 

[dhengr]

Delagina:
The flowable fill should work. You’ll still have to do some hand excavating around the pipe to prove where the voids are, and to make them visible from both sides. The one side for pumping in the fill, and the other side to be sure that the fill is traveling/moving to the same elevation as that at the pumping hose. Then you should be able to determine when to quit, visually, on both sides.

 

[cvg]

be very careful, pipes will float on flowable fill grout. I have seen it done and it is not a pretty sight...

 

[hokie66]

I agree with cvg. I don't like your chances of filling those voids without damage to the pipeline. The contractor is just trying to avoid rectifying his stupid mistake.

 

[LittleInch]

"soil erosion" doesn't just happen, it happens for a reason. You need to find out what that reason is and fix it before anything else, but t with the very small amount f information you've provided, not removing the top backfill doesn't sound right. Virtually anything is possible, it just might be expensive and time consuming.

One thing pipelines hate is differential settlement and shear - that's what breaks most pipelines so it sounds like you're not in great shape.

Some diagrams, pictures or a bit more info might help.

Filling the void might help but as noted, you need to find a way to fill slowly and remove air and allow from some settling before filling it up. all sounds a bit strange to me.

Something like bentonite might be good as it flows then stiffens up but not the same as concrete or foamed concrete

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[oldestguy]

The use of bentonite is not good. If it has not expanded like it "wants to", it will further expand or later drying will cause significant shrinkage. Can you fill the pipe or place suitable surface load while allowable fill sets up?

 

[TehMightyEngineer]

LittleInch has a good point. I've seen a sewer pipeline broken by as little as the wooden dunnage supporting the pipe during it's construction wasn't removed when they buried the pipe. These created hard spots where the pipe had little settlement and it kept breaking at these points.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[delagina]

@dhengr, this is what happened.

Contractor dig a pit for a new pipeline right beside existing pipelines.
When it rained soil eroded into the pit.
I told them to backfill with cement stabilized sand.
I was expecting contractor to do this per lifts and compress each lift but they just dumped everything on top of the pipelines,
creating a huge void under the pipelines.

My question is maybe my original suggestion was not good in the first place.
I'm now thinking how could they compress the soil under the existing pipelines?

Maybe they should have put flowable fill below the pipeline then backfill with cement stabilized sand.

What would have been your recommendation to the original problem?

 

[cvg]

not sure what the difference is between "cement stabilized sand" and "flowable fill". If you are trying to fill below a pipe, you can't get compaction equipment down there and you need a self consolidating material such as flowable fill. but you need to make sure it does not expand and that you do not exert excessive pressure on the pipe which might move it, displace the joints or break it. that can be done but requires an experienced contractor.

never expect a contractor to read your mind. you need to request a detailed work plan and don't start work until you are in agreement with it. you should not be making the recommendations, since this is the contractors liability to repair. but you should be looking out for the owner and do a critical review of the contractors repair proposal.

 

[LittleInch]

Delagina,

It's not clear what the relative responsibilities and contract you're working with here, but lets get real.

The contractor has made some serious errors in his construction by first off allowing the hole to develop and secondly making a complete mess of the required re-instatement.

It sounds like he is trying it on to get out of his responsibilities under the contract and you're trying to help him!

NO. The issue lies in the hands of the contractor and it remains his responsibility to deliver a workable solution to the issue to you and your clients satisfaction /( unless you are the client). Stop messing around and tell him to remove the incorrectly placed backfill material and then place the correct fill below the pipe, compacted and stabilized in layers.

either that or get someone else to do it and send him the bill and spend the next 2 years in lawyers discussions.

You might need to escalate this up the line to get some heavier weight involvement to help you.

Good luck and let us know how you get on and next time don't trust a contractor to do anything without making it clear in the first place. - "I was expecting contractor to do this" is more than a little naïve. Did you not have any inspectors or people on site when they were doing this?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.