Acidity of Rock Backfill

[cher122976]

I was recently asked what provisions are being made for the acidity in rock backfill and it through me for a loop. Anyone have any experience with this? We are designing a 24" steel gas pipeline that will have corrosion protection. In the roadways it will have rock backfill. I think this is a non issue but interested to hear if anyone has dealt with this before.

 

[cvg]

Yes, some soils and rock do have the potential to cause corrosion. Testing is done to determine soil pH, sulfates, chorides and minimum resistivity.

For a high risk project such as a gas pipeline, you should have a corrosion engineer do an investigation and give recommendations. This will require material sampling along the alignment and if you are importing bedding or backfill material, you can sample and test that also.

 

[cher122976]

In my experience that testing is not normally done. You will get a proctor from the quarry but that's it.

 

[Hoaokapohaku]

I have no idea if this would be acceptable in your application, but if it is, it's simple and cheap,

http://www.northtowncompany.com/polywrap.html.

 

[cvg]

proctor for rock?

 

[BenJohnson]

I have never heard of rock having acidic properties. Do they mean acidity of the soil?

I would make a call to the local Public Works Director and see if this has been a problem before. Also, experienced contractors can give you some advice on local conditions. If the pipe already is going to be protected from corrosion with a special coating or cathodic protection, then this probably is a mute point. If you have a hint you might have a problem, then put the condition in the specs that the pipe must be corrosion protected according to ASTM xxxx.

But over design is not the mark of a good engineer. Make sure you need the protection before incurring the extra costs.

 

[cvg]

suggest just a little bit of snooping around will yield ample evidence that some rock (and consequently soil also) can be acidic. It is a real problem, but easy to avoid by just choosing your material source wisely.

7th International Conference on Acid Rock Drainage

http://technology.infomine.com/acidminedrainage/welcome.asp?i=amd4

Acid Rock Drainage (ARD)

http://bioteq.ca/services/solutions-for-mining/acid-mine-drainage-amd-acid-rock-drainage-ard/

 

[dicksewerrat]

Repost in the Pipeline forum.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[fattdad]

fresh rock with pyrite will oxidize and liberate sulfuric acid. Acid-base accounting will provide insight into this problem. Zenah Orndorf, Virginia Tech, has a Ph.D thesis on this topic (2001). It's worth reading the front part of the thesis as it explains these chemical processes.

I just read it!

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

just happens to be the very type of rock found in a lot of metal mines. The more highly oxidized rock in the overburden layer which is typically considered "waste rock" is the most likely to form acid. This waste rock is also commonly used for other purposes such as riprap and rock fill for tailings dams.

 

[bimr]

If you are backfilling with rock from an existing quarry, you do not have to worry about acid forming rock because the quarry site will have been selected such that the quarried rock is not acid forming.

Add this sentence to your riprap specification:

"Material used shall be free of acid-forming material and toxic-forming material and riprap shall comply with the durability requirements."

Acid forming rocks are normally present at mine sites because the same metal ores that are mined for metals are the sulfides that have the potential to form acids. Waste rock from mines may be acid forming and is not the same material as quarry rock.

See the linked article discussing rock.

http://aptalaska.com/upload/pdf/Acid_Rock_Drainage_Evaluation-2009.pdf

 

[bimr]

Please ignore the word "riprap" in the preceding post.