Pole building foundation question

[adlertom]

I'm planning to construct a 48 wide x 40 deep pole building on a site that has a moderate slope. So, the floor level at the front of the building would be just above grade at that location, and floor level at the back of the building (40 feet back) would end up being be about 4 feet above the grade at that location. In other words, the grade slopes down as you go from front to rear.

I do want to have a foundation wall, as opposed to putting poles in the ground. Location is Wisconsin, so planning for a 4 foot frost depth is required.

I'm getting quotes from various contractors. One contractor proposes to pour taller foundation walls at the rear, about 8 feet total wall height (4 feet of wall below grade and 4 feet above). Sidewalls would "stairstep" to only being 6-ish feet tall (2-ish feet above grade), and front foundation wall would be 4 feet tall (mostly/all below grade).

Another contractor proposes to use a uniform 4 foot tall foundation wall all around, but also install crushed rock on the rear and sides, underneath the concrete foundation wall, to a depth of 4 feet below grade.

I'm sure the first method is "better" (and of course more expensive). But I'm wondering if the second crushed rock under concrete method is "adequate"? It would obviously save a lot of the concrete cost, but I want my building to be stable and long-lasting.

 

[Doughknob]

I have never heard of the crushed rock method up north. Water will get into the rock and freeze, thaw will make that part of the foundation move around. I'd not do the rock method.

 

[ponytug]

I'm not a concrete contractor, but I am doubtful that method #2 is going to work unless you extend the pad out some distance, like 15+ feet, and compact it up in small layers. I.e. build a 50x55' (l x d) compacted pad first, then put the foundations in. That crushed rock is only going be able to insulate and support the 4' foundation if you make it deep and broad enough that you are effectively back to native soil.

All the best,

Peter

 

[LD1]

If you don't want it to move the footer has to be below frost.

Sounds like you have one contractor that wants to do it right, and another that wants to take your money and run with no regard to how the building will be a few years down the road.

 

[BoylermanCT]

I just finished a 30x40 pole barn where the back elevation was 4' lower than the front. We brought in loads and loads of dirt fill and compacted it to bring the entire building site level. Dirt fill is pretty cheap.

The strength of the pole barn lies with the poles being buried 4'. If you want to stick with the concrete foundation walls, I would do normal construction of 2x4 or 2x6 walls. I don't think pole barn construction will be as sturdy if the poles are attached to the concrete walls vs buried.

 

[rademamj1]

There is nothing wrong with using compacted fill under the foundation, but it must be fully compacted, typically by using a skid steer. And that requires the fill material to have a high percentage (40%) of silt size particles. In fact many foundation crews use a "selected fill" for that purpose, because (1) its cheaper than concrete and (2) handles the annual winter frost heave stresses much better than concrete. Gravel does not qualify as a fill material for this foundation project, avoid that contractor who recommended it.

 

[rademamj1]

In the original post, I also didn't see any specifics on slab thickness, discussion of any rebar specs or even if this a canidate for a post tension slab. Most foundation crews will provide a summary report on their recommendations, including slab thickness, type foundation, support column depth if applicable, and specifications for rebar, and specs on any fill if needed.

It maybe better to use a specialized and experienced foundation crew. Than a general contractor who thinks he knows foundations.

 

[EddieWalker]

Just to be sure that we are talking about the same things.

Contractor One wants to build a solid retaining wall that will go 4 feet into the original, undisturbed soil, and extend 4 feet above grade. This wall will level off the building when backfilled.

Contractor Two wants to dig a 4 foot ditch and fill it with gravel. Then pour a 4 foot concrete wall on top of the gravel that will level off the building pad when backfilled.

Is this correct? Of the two choices, I would block the phone number of Contractor Two and never talk to him again.

If there is room behind the building, it would be cheaper to build up the pad with dirt first to have a level pad. Slope the dirt behind the building so you can mow it. If the soil is compacted properly, you only have to go down as far as your frost level demands from the compacted soil for your concrete footings.

If there isn't enough room to build up the pad with dirt, can you go back 6 feet or so and build a block retaining wall? or move the building forward so there is enough room to build up the pad with retaining wall blocks? Then you can build up your pad in lifts and compact each lift so that you can have a solid pad to build on.

Not knowing your land, or your situation, of the two choices you offered, Contractor One is who I would hire.

 

[adlertom]

Thanks for all of the replies/info.

@EddieWalker , It's possible that the 2nd contractor intends to build up soil against the concrete along the sides and in back. I need to verify that. There would be room to do so (although I'd have to take down a few more trees). I assume that his method would then be acceptable, since the concrete foundation would now be buried below frost level?

 

[LD1]

Footing below frost is the ONLY thing that is acceptable. And that is concrete.

Don't matter if it's a block foundation, concrete wall, or posts in the ground. They all should be sitting on concrete below frost.

Gravel below frost doesn't qualify. Sure you can have gravel under concrete footings....but the CONCRETE needs to be below frost