Backfilling Existing Steel Pipeline

[Nutsforbolts]

I am developing a backfill specification specifically related to how to properly support and backfill a welded steel pipeline. I am facing several challenges. It is not always feasible to bring in aggregate or other engineered materials for fill under the pipe to provide support. Pipelines are typically miles off of an easy access road. Since its hard to compact native soils when a pipe is in the way I require some other support to hold up the pipe, contents, and soil loading and keep the stresses on the pipe within acceptable levels. I have looked into sandbags supports at certain spacing, and other supports like PipePillo or poured concrete. Woven plastic or burlap sandbags have been used in the pipeline industry but I have doubt on how much actual loading they can take. PipePillo seems like a good product but there isn't a long history of how these hold up over time. Concrete is not desirable in many ways, due to getting concrete onsite and waiting for the support to cure before backfill. Any advice for what others in the pipeline industry may do to support existing pipes prior to backfill? Any studies preformed to validate sandbag strengths? Anyone have difficulties backfilling existing pipes?


"Give me pipelines, or give me death!"

 

[BigInch]

It is not always feasible to bring in aggregate or other engineered materials for fill under the pipe to provide support. Pipelines are typically miles off of an easy access road.

Unless you're doing city work, or very close by, trucking in engineered materials will never be economically feasible. Look for sand sources nearby, or bring along a rock crushing machine to make your own. That is expensive enough. You may have to dig down below min specified depth, enough until you encounter firm soil layers, then sand pad with 8-12" above that level and continue around the pipe to about 6-12" above top of pipe. Usually there is enough fines, or sand in lots of soils so all that is needed is to screen out any rocks and chips larger than about 3/4" from the soils placed immediately adjacent to the pipe.

Since its hard to compact native soils when a pipe is in the way I require some other support to hold up the pipe, contents, and soil loading and keep the stresses on the pipe within acceptable levels. I have looked into sandbags supports at certain spacing, and other supports like PipePillo or poured concrete. Woven plastic or burlap sandbags have been used in the pipeline industry but I have doubt on how much actual loading they can take. PipePillo seems like a good product but there isn't a long history of how these hold up over time. Concrete is not desirable in many ways, due to getting concrete onsite and waiting for the support to cure before backfill.

You can do wonders with sand-cement bags and some rockshield. You an use them wet.

Those other solutions are only for limited problem areas and use where you might need ditch breakers on slopes. You will not be able to afford to pay for any of that stuff for the entire length of a real pipeline. Plus you want uniform loading and support from the padding below. Spaning from sand bag to bag, or anything else, is not what you want to be doing. Pipe needs uniform support. Sand bags can be used for temporary support of short spans while lowering, so the pipe doesn't lay on or against exposed rock, or the natural soil below while waiting for sand or soil padding. Sand bags are for holding pipe placement within the trench until a sand bedding can be placed and only act temporarily as the bedding soon placed around the pipe should overtake their function. Know where your rocky areas will be and bring along crushers if needed and/or a lot of rock shield.


Any advice for what others in the pipeline industry may do to support existing pipes prior to backfill? Any studies preformed to validate sandbag strengths?
You can waste money writing a spec for sand/sand cement bags or those woven plastic bags if you want, but you'll just pay more to have the contractor provide those, rather than what he always uses. Best method is if they break, whatever they are, just put another one down.

Anyone have difficulties backfilling existing pipes?
Finding appropriate bedding materials from approved sources can at times be challenging. Be resourceful.

 

[bimr]

Review the discussion in the Pipe Handbook

http://www.steeltank.com/Portals/0/pubs/Welded Steel Pipe 10.10.07.pdf

 

[LittleInch]

Ok,

Lets go from first principles here:

The general requirement for construction is to build what ever it is you're building for the lowest possible cost whilst meeting the technical specifications / requirements of the project.

For pipelines that normally means using what ever it is you've just dug out of the ground and placed locally to put back into the hole again. I've seen many a cost estimate which is inflated by 30% or more because someone, with no skin in the game, has decided to import sand or some other such material from long way away. Now if money is no object then sure, its a fool proof way of obtaining a comprehensive way of supporting a buried pipeline. But is is necessary or actually any better than the loac stuff - very rarely (IMHO).

You give no details so I really don't understand your quote " Since its hard to compact native soils ..." Why is it hard? I'd love to know so I can argue the opposite.

Also your quote " I require some other support to hold up the pipe, contents, and soil loading and keep the stresses on the pipe within acceptable levels. I have looked into sandbags supports at certain spacing, and other supports like PipePillo or poured concrete. Woven plastic or burlap sandbags have been used in the pipeline industry but I have doubt on how much actual loading they can take. PipePillo "

Errr no you don't. Buried steel pipelines require a continuous support and to have backfill surrounding them on all sides, but they don't need the types of support you're talking about, other than in some very specific locations. In the main all you need to do is dig your trench, smooth out the bottom f the trench with material free from rocks, sharp stones etc, which can be graded or crushed natural material. If you want to you can send an unmanned compactor along it, but often an excavator bucket will doo a good job. Then you lay the pipe into the trench on top of said initial "bedding" layer. Then you place higher quality backfill which may need grading or crushing, depending what is is and compact around the pipe and in layers as you go.

I looked up those pipe pillo and they seem to me to be an answer to a problem which doesn't exist I failed to understand their benefit, other than increasing the profits of the company selling them and in some cases could end up like a load of buried supports and hard spots, which is counter to what a buried pipeline needs which is good continuous support.

A sandbag can take a lot of load, but why are you lifting the pipe off the bottom of the trench in the first place??

Big Inch talks a lot of sense in his reply also.





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

 

[Nutsforbolts]

I wasn't clear in my previous post. This application is for existing pipes. For example a pipeline laid in 1950, then is inspected or repaired today. I do not have any problems with new construction, aka laying a pipe today.

You have a hard time compacting because your pipe is in the way. For example what equipment can compact soil under a 48" pipe when it is 3 feet off the bottom of the trench? Trackhoe buck is about all I have seen, and sometimes it's not enough to prevent damage to existing flange connection. I have seen up to 2" of deflection on 50' excavations on 20" STD wall pipe with 5' of cover. That's enough load to cause flange connections to leak. It's just not easy to get 90% standard proctor in a clay soil with a trackhoe bucket, let alone when a pipe is in your way.

Also all engineering specs on backfill and compaction are relevant to new construction. There is no guidance specific to maintenance. I agree that compaction and full support are the best practice but have found this hard to achieve during maintenance.

Nutsforbolts

"Give me pipelines, or give me death!"

 

[cvg]

I'm more confused now. how is it that an existing 60 year old line ends up being 3 feet off the trench bottom? you dug it up to repair, than for a small repair, just use CLSM backfill with temporary support at reasonable spacing using sand bags. For a larger replacment section, treat it as new construction.

 

[dhengr]

Nutsforbolts:
At the very least, you should not leave large voids under the pipe when backfilling, over which the pipe must span. I would be more concerned with reasonable uniformity of bedding, backfilling to mid pipe dia. and then the remaining fill, all with some reasonable effort at compaction, than I would about some elaborate compaction or backfill soil spec. Under most conditions, I don’t see much wrong with using the excavation spoils or decent local material for final bedding and backfilling. However, this backfill should be reasonably flowable and able to be compacted to some extent. This pretty well eliminates/precludes the use of clods of hardened clay the size of basketballs being pushed back into the pipe trench for backfill. It won’t fill/flow under the pipe and it will take a hundred years to begin to settle and infill. I don’t quite understand 3' of void under a 48" pipe in a trench. That trench should have been brought up to a reasonable elevation before the pipe was placed or replaced.

I can imagine a more controlled bedding, fill and compaction at a location where there will be a heavily trafficked service road, or some such, crossing the pipeline. Also, I believe I have seen some equipment which fits on the end of a backhoe boom tip which scoops/digs loose soil, or is filled be another backhoe; it pulverizes the loose fill (boom hydraulics, etc.) and drops it down into the trench; and can of course be positioned with the boom. There is also vibrating and jumping jack type compaction equipment which fits the end of a backhoe boom and can be nicely manipulated, under and around pipe.

 

[robyengIT]

steel pipes are considered "flexible". May be the attached literature can help

 

[BigInch]

That makes a big difference.

If you have pipeline segments that are in such bad shape today, you've got a lot more problems than how to properly do backfill. I wouldn't even be thinking about backfilling until I corrected whatever problem brought the pipeline into such deplorable conditions. It is likely to be as much or more of management's problem rather than engineering's. You should be able to handle the engineering part of the problem by following dhnegr's suggestions. For repair of short segments of pipelines, just getting out there to make the repairs is usually far more costly than whatever little things you need to do to compact some soil around the pipe.

Thanks for clarifying. Sorry I wasted your time with previous answers..

 

[dik]

BigInch... no need to apologise... they were good informative replies, and appreciated by others...

Dik

 

[LittleInch]

Nuts for bolts,

I answered on the basis it was new pipelines which wasn't a bad thing (similar to BI).

I think in order to move this forward, we need to understand what it is exactly you're doing with these existing pipelines as it isn't clear to me or, reading my fellow posters responses, clear to them either.

The only thing I can think of is that your are excavating an existing pipeline including a depth underneath it, to presumably replace the external coating and fix any external defects?

Is this what you're doing? If so for what sort of length are you exposing at any one time?
I don't understand the bit about flange connections though.

Short distances will be a different story to long lengths.

The type of soil is also important.

I agree compacting underneath will not be easy. Granular material if local available ( crushed rock, sandy soils) can be poured in then compacted by wetting thoroughly, at least over short sections.

You could also look at the portable foam systems they now use for trench breakers - like this stuff

http://www.aimex.com.au/en/Exhibito...hane-AUE307P-Polyurethane-Trench-Breaker-Foam

Not the cheapest method, but quick, simple and strong.

Otherwise it is a fairly manual operation to compact material by hand rammers from the top surface. There are few more effective methods for compacting under a pipe than person hand ramming material under the pipe. You can use things like foam concrete for the final section, but filling under the pipe to as high as you can go with local material is still the most cost effective method in most locations.

Let us know what it is you're actually trying to do and we can take it from there.

LI

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

 

[Nutsforbolts]

Thanks for the post LittleInch. The purpose for digging up an existing pipe is generally to investigate and repair the pipe. Typical repairs consist of installing metal sleeves and recoating. Recoating involves sandblasting 360 degrees of pipe and applying liquid epoxy. Generally only about 25' of pipe is exposed and dug out but sometimes these investigations may span 100' or more. It depends on the extent of indications requiring analysis and coating condition.

We routinely inspect pipe. Average installation year of hazardous liquid pipelines is in the 1950s. Just because some parts need more attention does not mean the lines need replaced or are unsafe. We try to find small problems before they become big ones.

Thank you all for the comments.

Nutsforbolts

"Give me pipelines, or give me death!"

 

[BigInch]

As far as digging up and repairing coatings, etc. try not to wind up doing more net damage to the system than you are managing to fix up by repairing the coating. IE fixing the coating will all be for naught, if you leave the trench exposed to erosion, washouts, undermining, etc. These kind of repair works can have a heavy consequence and actually increase potential damage, if not done properly. Concentrate in fixing it all very well, rather than looking for fast and cheap solutions. Fixing it well now will pay off for many years in the future. Nothing worse, or more expensive, than having to fix something again that you thought you had already fixed before!

 

[dik]

Do you have to worry about the 'corona effect' with corrosion? Forcing greater corrosion to the outside of the repair area? Or is corrosion the issue?

Dik

 

[LittleInch]

Ok, knowing what we know now ( the known knowns) it seems you have a set of pipeline repairs on old, large diameter pipelines in ground which is difficult to place back under the pipe which you've just excavated ( maybe by hand).

Leaving aside the issue of whether these should be repairs or replacements ( my usual ROT is anything more than two pipe lengths exposed at one time should be replaced), you apparently have clay soils which do not readily make easy soil with which to back fill a gap under the pipe of 3 to 4 ft.

You seem to have an issue with sandbags which I can't quite work out - "Woven plastic or burlap sandbags have been used in the pipeline industry but I have doubt on how much actual loading they can take" and "Any studies preformed to validate sandbag strengths?"

Sand bags can take the same load as their contents so long as they have support. If you work on the basis that the contents of the sandbag exert pressure equally in all directions it doesn't take long to find out how much load a bag will take before it splits or rips.

There is a difference between free standing sand bags used for support and those buried in place. I can easily imagine a set of sandbags in a triangular section under the pipe on a continuous basis providing the required level of support to a pipe with the remainder of the space filled in with native soil duly compacted around them. You can usually get access to either side of the pipeline given you've excavated a 3 foot diameter in order to do your repairs..

The issue I have with those pipe pillo things is that you need to analysis your pipe as though these are supports with extra soil load on top. They create hard spots along the line which pipelines don't like.

If you want to reduce the amount of new material to insert, look at using geo bags and only leave a small gap then fill them with wet sand or foamed concrete -

http://www.eabassoc.co.uk/uses-of-foamed-concrete.php

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

 

[BigInch]

There is a spec or two for woven polypropylene fabrics

http://www.enviro-pro-us.com/wp-content/uploads/2016/08/SandbagGeotextileSpecification.pdf

You can also check your local state highway department specs.

 

[LittleInch]

Nice,

one of my points is that for a repair type operation, your cost for backfill is going to be a small fraction of the cost of the repair compared to the cost for a new pipeline.

considering you need to gain access for an excavator, men, repair systems, I can't see how getting a couple of tipper trucks there would be that difficult.

as said - if you have a lot of repairs then the cost of replacement starts to get close to or cheaper than hundreds of repairs.

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