Frost Protection Pit slab/foundation

[jchi]

I have a 100'x160' Pre-Engineered building sitting on mass column piers 5'x5'x3'-6" deep and a 8"x3'-6" frost foundation around the perimeter. I have a conveyor pit 10' deep 18' wide 150' long (open top - no heat) going out the side of the building. Question... What do I do under the pit slab/footing which is 12" thick reinforced concrete to prevent frost lifting the pit which is attached to the building foundation. To make it worse at the end of the pit I have 100' tall conveyor support tower with the legs sitting on top of the pit wall. I am considering 2" rigid polystyrene under and up on the side of the footing... Reportedly, 1" is equvalent to 4' of earth cover. My total bearing preasure is around 600psf and reportedly blue board insulation's compresive strenght is around 2,150 psf.

 

[CarlB]

What's the design frost depth in the area, what type of soils under the pit, how much movement can be tolerated...

But yes I think placing some insulation under and around the pit would be more economical than digging out 4 feet to 6 feet of soil and replacing with NFS material. The normal rule of thumb I've heard is 1" insulation = 1' earth cover, but modeling shows it worth more like 2' to 3'. In solely thermal resistance it may approach the 4' number but soil also slows frost with the latent heat of fusion in freezing it. Would be good to provide some NFS material under/around pit if available, say 6" min or 1' better, along with insulation. And you can get insulation with up to 60 psi compressive strength so 600 psf should not be a concern.

Good info at this link:

http://www.dow.com/styrofoam/na/

 

[jchi]

CarlB,

My thoughts exactly... I just wanted to make sure I had not overlooked anything, or anybody had any better ideas. I will be using 6" of granular under the slab. Thanks for the response!

Jeff