Foundation Frost Line 2

[Engineer4321]

I am designing a foundation for a 150kW diesel generator weighing about 9kips. Area dimensions for the pad are 6'-0x13'-6. I developed the depth of 1'-6 thick mat to obtain a mass of 2x the generator weight. My question is about frost depth. Can I just put 6 to 10 inches of crushed stone to prevent upheaval or do I need to turndown the two 6'-0 ends to 4 ft to get below frost line. I don't think it is necessary to pour a solid 4ft thick mat but do I need to turn down 2 edges or can it just be a 1'-6 mat on a crushed stone base.

 

[dik]

Can you use vibration isolators in lieu of mass? Normal slab dimensions for that sort of loading would be 6" slab with 16 or 18" edge stiffener. In these environs if slab heave were a problem, it would be on piles or load bearing extruded polyurethane insulation, that would extend 6' around. 6' is our frost depth.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Engineer4321]

The problem is the generator is not bought yet and there are a couple different models. So I don't have all the information about the generator only they approximate dimensions and weight. So I was just looking at a worst case scenario sort of deal. I'm fine with thickness I just am second guessing if I need to go below frost line.

 

[YC611]

If the diesel generator is not moving due to some vertical movement, I would say you are fine without going down the frost line.
Or you could provide the rigid insulation in accordance with ASCE 32.

Check out the IBC Section 1809.5.

 

[JAE]

Clean compacted rock works fine.
Also a lean concrete fill works too (1,500 psi concrete- unreinforced) with your mat on top of it.

 

[dik]

drained "Clean compacted rock works fine."

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JAE]

I should have said "uniformly graded" rock.

 

[dik]

well graded Gran A, in these environs, is nearly impermeable... water will sit on it for hours...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Eng16080]

I would use ground insulation below the footing (as YC611 mentioned above) and 6-10 inches of crushed stone below that.

 

[Engineer4321]

Thanks all for the information. Looks like nobody feels turning down the 6ft side ends 4 feet below grade is necessary.

 

[TLHS]

It's honestly just not that helpful unless you also replace interior fill with something granular. Your interior can heave. If you're excavating you might as well just replace with clean granular material.

Your options are:
-Replace with non-heave susceptible material to frost depth
-Insulate under and extended past the foundation to prevent freezing
-Install on a deep foundation of some sort with the supported foundation using a void form/frost cusion
-Justify that heave isn't going to occur based on the water conditions at the location, or the type of existing soil
-Accept the possibility of heave and detail to suit. Hopefully this is done with some understanding of other items in the area and how they seem to have reacted over the years

I'd likely insulate at

 

[dik]

You only need to provide that type of foundation if movement is not possible. Connections to the unit have to be capable of undergoing some movement.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik