Thick mat foundation and Frost depth

[kodstruct]

hello,

I am designing a block foundation for outdoor compressor and H-pumps for Oil companies in North Dakota.
My analysis gives me about 24" thick block foundation (24" thick x 7'-0" wide x 40'-0" long). Do I need to consider the 6'-6" frost depth for this 24" thick block foundation?
We do a lot these foundations in TX where frost depth is not an issue.

Thanks.

 

[KootK]

If your equipment is at all sensitive to movement, I'd be very tempted to support it on piles. I've seen open field frost depth geotech recommendations in that area pushing 9'. I do work near by and we can get a helical pile installed for ~$700.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[kodstruct]

Thanks KootK.
So basically I need to design drilled piers or helical piers to resist the tensile force due to the Frost Heave forces under my 24" bloc foundation.

Thanks.

 

[BAretired]

An insulating blanket under the mat foundation may be worth considering, but it would need to extend beyond the perimeter of the mat in order to protect it from frost heave. Your geotechnical engineer may be able to provide recommendations.

BA

 

[JAE]

kodstruct - you normally wouldn't design helical piling to resist uplift from frost - I believe that the frost is a significant force (irresistible force?).

Typically you would include some kind of void under the mat (or with piling perhaps a two-way slab that isn't as thick). These can be the degradable cardboard carton forms.

The only other issue then is once the cardboard rots away you have a void under the concrete - but potential sluffing of dirt into the void from the perimeter of the slab....thus
some kind of fill retainers around the perimeter. All this adds up to extra cost - compare that with determining if the piping can flex or be designed to flex with anticipated frost heave deflections.

 

[kodstruct]

Taking in account all the above comments, I think I will use and design for the following:

- Perimeter grade beams about 3'-0" deep
- Void form under the 24" thick concrete block.
- Drilled piers to support the perimeter grade beams.


Why this approach? Because, if the void form rots under the concrete block, the 1'-0" extension of the grade beam that extend below the concrete block will act like the fill retainer JAE mentioned. And the drilled piers will control any settlement of the foundation system.

 

[KootK]

From your previous oil and gas experience, you surely know that energy clients will often consider the costs associated with structural work to be trivial in comparison to their other concerns. In my experience, they are predominantly interested in:

1) Speed (design and construction).
2) Durability.
3) Future flexibility.
4) Ease of construction in remote locations.

My northern O&G clients generally avoid concrete like the plague. If it were me and it were feasible, I'd be looking at a raised steel platform on helical steel piles. Your only frost concern would be upwards drag on the helical piles which they can certainly handle so long as your helices are below frost depth.



The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[kodstruct]

KootK, you are absolutely right. SPEED is always a big factor with O&G firms. matter of fact the client pulled this project from another company this morning and they want us to issue construction drawings of 7 structures by Friday.
Elevated steel platform is an option but I prefer concrete blocks because they absorb vibrations better since I am dealing a H-pump with more than 4,000 rpm.
Seems like you mention helical piers a lot. Are they the preferred pier/pile system in ND?

 

[KootK]

I have an airtight patent on helical pile technology. Just kidding. Helical piles are popular and preferred for some applications. The main reason that I've been pitching them in this thread is that, of the possible piling solutions, I feel that helical piles are of a scale that suits your situation. To bring out a big pile driving / auguring rig for a machine base seems excessive to me. The equipment for installing helical piles can be mounted on a bobcat or the back of a truck (I think).

I didn't realize that your machine base was part of a much larger complex. That may make it more feasible to use other technologies that will be on site for other reasons. With regard to vibration control, is one better off with a pad footing or pile support? My gut tells me pad footing but I really don't know.

O&G clients present some challenges but they're also exceptionally good when it comes to paying their bills. That's more than I can say for some architectural clients.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[SteelPE]

Just for the record, I will agree with JAE that frost is an irresistible force and should never be messed with. I have also seen methods described by BA in regards to frost protection although I have never personally done it. I think there is an ASCE document (ASCE32?) that addressed this type of frost mitigation.

 

[BigH]

Question: Is the foundation soils frost susceptible? I would presume so in the Dakotas but check this out. For frost heave you need (1) a frost susceptable material,(2) supply of water and (3) freezing temperatures (which, of course, you have). I've done projects in northern Ontario where the "frost depth" is about 3 m+ but the foundations were to be on a clean sand and the water table was about 5 to 6 m deep. Hence, we put the foundations down about 1 m.

http://www.pavementinteractive.org/article/frost-action/