Subgrade for MRI

[WesternJeb]

I have a project where we are putting a new MRI in an existing hospital on the ground floor. I have done all of the required checks for ferrous materials and our plan is to provide a 12" S.O.G. under the MRI on 4" of dga / csb.

The contractor told us that they can not machine compact any stone inside the existing facility because they can't run any gas powered equipment inside the hospital, which is understandable. They are telling us that the only thing they can do is hand tamp the base and soil prior to pouring concrete.

My question is, is there a stone that is self compacting or light enough to compact that it can be done with hand tampers, BUT withstand the vibration loads due to the MRI and have negligible settlement issues. Does anyone have a suggestion?

Thank you!

 

[EZBuilding]

My only suggestion is to package this question into a design RFI and send it to the project's geotechnical engineer for their recommendations.

 

[WesternJeb]

No geotech, unfortunately. There is an RFI in a room 30' down the hall and it has been deemed satisfactory, so it is assumed this room would be as well. We did the design of the original building, and there was no special detailing for the MRI room subbase compared to the rest of the structure.

 

[Brad805]

$1mil + for the piece of equipment. Geotech not optional.

 

[WesternJeb]

You sound surprised, Brad!

My only thought is that the base will be "confined" on every side, since it is in an existing facility with typical slab and subbase all around it.. So it wouldn't be able to truly settle, just shift around a tiny bit here and there. Does that track?

 

[azcats]

Pneumatic tamper?

Tamp the existing material and skip the dga / csb in lieu of a 4" 'mud slab' w/ 12" slab atop?

 

[Tomfh]

Why not just omit the base and mass concrete it? or some other sort of stabilised material.


What’s the existing floor construction? Regular slab on ground?

 

[WesternJeb]

The small pneumatic tampers are for the soil only, right?

The stone base is to help with spreading out vibrations a little bit more than just pure concrete. Our in house geotech didn't think pure concrete was the best idea either.

I believe we decided to go with (2) lifts of #57 stone, with a "substantial compactive effort" applied via hand tamps. We might have localized consolidation in the stone, but believe the 12" slab can spread the load out enough that we won't have more than 1/8" settlement globally, which is the tolerance of the machine.

 

[CarlB]

Like most tools, there are electric options for compactors:

https://www.wackerneuson.com/us/zer...Inw3CoOIplHHtSBcpt9b6Cem8pYkrcbRoCc-IQAvD_BwE

I'd think that for 4" thickness and cohesionless, no need for 2 lifts.

I don't know vibration propagation to understand how the aggregate can "spread out" vibrations any better than concrete.

 

[WesternJeb]

Unfortunately, that is not an option for a couple reasons.

I agree it is overkill, but would rather be safe than sorry for a million dollar piece of equipment given they won't buy the electrical compactors.

I don't know either, but I proposed it and our in house geotech said he would prefer the 57 stone. More options for energy dissipation.

 

[JLNJ]

Well-graded stone(s) will be easier to compact than #57s. Look into base materials your local ODOT uses.

 

[Agent666]

I'd just use flowable fill if your local concrete supplier has a suitable mix.

Let it spread out once it hits the soil.

https://engineervsheep.com

 

[ALK2415]

I think you should design compact (steel platform) with bolt connections to both; the existing concrete floor and machine support frame.

 

[WesternJeb]

Agent, we left it up to the contractor with the option of flowable fill or the system I described above.

Alk2415, they have to provide a lead shielding around the room, which is an entirely separate contractor. This system won't work due to their requirements.