Underground conduit in a landfill cap

[webuildum]

Have a project, constructed on top of a former municiple landfill. Env Engineer has classified anything under building slab as Class I Div 1, due to Methane. Only reference I can find in code concerns filling stations and is concerned about flammable leaking into the ground from above... but in a former landfill, it's coming up from below?

The architect has nice infloor outlet boxes and other things that will require underslab conduits - which now I'm thinking all need to be installed per Art 501, which could be problematic - aesthetically speaking. But what about site lighting fixtures and the branch wiring to them, ground based illuminated signs or parking control equipment - all of which may exist in the project.

With old brownfield sites being "reinvented" I was wondering if anyone else has come across this situation at a landfill and what the approach was? Not sure the local AHJ has enough experience to give a good read.

 

[natfuelbill]

Have a read of 501.20(A)(1) Exception:...
Would the inspector permit running the conduit in (as in cast inside) the concrete slab? Might need to coordinate with general contractor.

 

[petersonra]

Have a project, constructed on top of a former municiple landfill. Env Engineer has classified anything under building slab as Class I Div 1, due to Methane. Only reference I can find in code concerns filling stations and is concerned about flammable leaking into the ground from above... but in a former landfill, it's coming up from below?

The architect has nice infloor outlet boxes and other things that will require underslab conduits - which now I'm thinking all need to be installed per Art 501, which could be problematic - aesthetically speaking. But what about site lighting fixtures and the branch wiring to them, ground based illuminated signs or parking control equipment - all of which may exist in the project.

With old brownfield sites being "reinvented" I was wondering if anyone else has come across this situation at a landfill and what the approach was? Not sure the local AHJ has enough experience to give a good read.

"Env engineer"????

I think the only thing under the slab that is in a classified area is the conduit. The underground conduit is passing through the classified area on its way from one non-classifed area to another non-classifed area.

I don't see how you can classify everything under the slab D1 and not have a D2 buffer area above it.Are you sure he did not say everything under the slab is D2, rather than D1?

 

[rbalex]

I?ll confess I hesitated responding to this one because there is no good simple answer.

To begin with, the underground should probably not be classified at all; although, doing so is not entirely without basis.

Only applications described in Articles 511 to 516 should use the NEC to directly determine electrical area classification. The OP description does not appear to be one of them. The determination of electrical area classification from one of these Articles is largely extracted from other NFPA documents that are controlled by Technical Committees (TC) that are not under the guidance of the NEC Correlating Committee nor Code Making Panel 14. The TCs that control the other NFPA documents are not entirely consistent in their opinions.

Determining the electrical area classification for other applications should use one of the Standards listed in the Fine Print Notes (FPN) of Section 500.4(B). NFPA 497 and API RP 500 are the two most general and neither of them would call for classifying the underground described in the OP.

Having said that; methane generation in old landfills is a fairly serious issue. One would certainly not want the electrical system to be a literal ?conduit? of methane into the building. Conversely, there are far more means than the electrical system that would permit gas to enter the building. So my initial question would be. ?What are you doing about that?? Any thing that was truly effective in preventing methane migration into the facility would also mitigate against requiring the underground to be classified for the purposes of electrical installations.

Otherwise, there is no good answer.

 

[webuildum]

Some more info...

Some more info...

Thank you for the reply's - they have been helpful.

Env Engineer = Environmental Engineer. Sorry for the abbreviation. This engineer has designed a very sizable methane removal system consisting of underground piping and exhaust fans along with monitored well heads. Those rooms - with the exhaust fans, are rated Div 2 - all the monitoring system wiring is Class 1.

I did see the concrete encasement option and this may be "considered", however, there seem to be some very tricky structural issues at play with building on top of a landfill. 1) There are pilings - and lots of them, 2) between all pilings there are grade beams, and lots of them too, 3) The slab is designed as a structural slab. So we might embed in the slab, but it will require coordination with the structural engineer, on how big and where. Typically would not want to imbed in the slab, for fear that future reno work might "saw-cut" a conduit everyone forgot about.

I think the classification is more an issue of being overly cautious, but I was unable to find anything that either supports or refutes the concept... and I've heard from field superintendents, that they have experienced 100% LEL reading in testpits. This dissapates rapidly typically - but it's there initially.

All of the design professionals involved have been keen to methane infiltration, hence the removal system. Which is why I too see this as having some merits - and maybe see the benefit of the sealoffs serving their primary function as to limit the methane infiltration within the boundary of the slab.

However, outside - in the parking lot areas... that's where I start to wonder more. There will be illuminated direction signs and parking control gates, with piping from underground to bring power (120v) to the sign or gate operator... should we use sealoffs ? Again, with little more than an overcautious gut - one might say yes... it might prevent methane from accumulating inside the housings, even though there appears to be no clear reason to "classify" the subsurface - or the boundary area above it, without any "scientific basis" in good conscious you can't ignore the known conditions?

 

[rbalex]

As I said, this is a tough one.

MY personal inclination would be to keep the building is at positive pressure. It would probably integrate fairly well with your regular HVAC system and mitigate other potential entry paths also. Depending on the projected methane generation I?d probably use a ?Type X? or ?Type Z? approach although I?d avoid Type X if at all possible. This would take some study.

In the parking lot and similar areas, I?d most likely go with a direct burial MC-HL cable.

The reason underground areas are not typically classified is they usually do not have sufficient oxygen to support ignition. The hydrocarbons present displace the oxygen. This isn?t the case though with underground ?generated? methane. While it would still most likely have an oxygen deficit, you couldn?t depend on it.

You basically have two concerns, preventing or containing ignitions and reducing volatile material migration. See 501.15 FPN No. 1. Keeping the facility properly pressurized should do the trick.