28x50 Site Prep

[sledneck32!]

Hey all,
Been lurking for awhile, first thread. I been putting plans together for the 4+ car garage/workshop detached that I plan to get in this fall yet. Its a post frame and Im going to finish the inside with concrete and utilities. Insulated and heated. The size is what it is, max 1400 sqft for my lot size. Central MN.

Anyway, ive had two excavation contractors out to give an estimate.

First one says to scratch off the top foot or so and then fill with 18" of gravel. One corner of the building you hit clay about 10" down. The kitty corner you gotta go about 3 feet before you hit clay. So basically would be putting the fill on top of black dirt (was wooded up till this summer) on half the building.

The other contractor says you gotta dig it down to yellow clay on the whole thing. Fill back up. So about 2' of fill in one corner and almost 5' of fill in the other corner. Its at least 500 yards of fill this way.

Both contractors have been in business in my town well over 30 years.

Thoughts?



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The near post on the right is where there is only about 10" of topsoil. The back left (u cant see it cause its in that brushpile) is where theres like 30" of topsoil.

 

[8man]

Post frame is supported by the posts. The concrete is not load bearing for the building. So I don't understand why you would need that much pad (2nd contractor) since the concrete floor isn't load bearing.

 

[sledneck32!]

Post frame is supported by the posts. The concrete is not load bearing for the building. So I don't understand why you would need that much pad (2nd contractor) since the concrete floor isn't load bearing.

Yeah thats the debate... I dont want the concrete to settle or heave.... If theres a couple feet or 18" of class 5 on top of the black, am i asking for trouble?

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[matt_i]

I think best practice and probably most expensive is to remove all of the topsoil and get down to a clay-ish soil.

That said, you might be able to sell the top soil to a landscape material place via prior agreement. That might make up the difference in what you pay for fill stone.

 

[jvitez]

You need to get rid of all topsoil before making any sort of driveway/parking pad, whether gravel, asphalt, concrete, stone paver, etc. The top soil will absorb ground water differently than the subsoil and cause expansion and contraction and heaving during winter which will destroy your concrete pad. It's not a specific number you need to dig down to, it's whatever it takes to get rid of all the topsoil.

 

[sledneck32!]

Thanks all! Kinda what i expected.... Big $$$

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[jvitez]

Do it once, do it right.

When we built out house we had a gravel driveway put in, ~ 240 ft. The excavating contractor used a big old tracked front end loader, put 4" down limestone as a base and I swear it was over a foot thick as it sloped up to the house. He packed it down really well, and then the house was finished. Only then did he put a 3/4" down limestone top. I've had to add a a bit due to settling by the garage and topping off tire tracks, but the nothing has shifted or sunk in a major way.

In a hurry I dumped a load of 2" down limestone right on the top soil to make a parking pad for our travel trailer. It's nothing but a mass of weeds and holes embedded in the limestone now, about 5 years later.

Especially for a concrete pad, do it right!

 

[brownbagg]

they are both right.

 

[sledneck32!]

they are both right.

We've had some insightful response. Thanks!

Would u mind explaining a little further? Are you of the mindset that a foot or two of black dirt in the subgrade is no big deal?

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[machsnell]

it all depends on what you are using it for.

if it will take a heavy load then thicken concrete and add mesh and rebar and put control joints at max 12'.

You should never need to excavate to 5'. i can builld an asphalt road (which would need to be much more solid subgrade wise) on a 3 foot cut even if soft ground underneath.

i would say unless super soft and spongy you should be able to excavate a happy medium to allow a happy medium of 12-18" of stone grade (under slab).

do you have 4 to 6 inches of slab elevation to play with? if not go with 16 to 18"

install fabric (if soft and wet) and then 6" of #2 or #3 stone which is 3-5" stone then a 2" layer of crushed stone (roadbase names vary per state) compact and then install a layer of geogrid. grid is amazing. then another lift of 4" of crushed stone. at this time you should be solid. proof roll with loaded truck. if still soft and you can go another layer of grid and another lift of crushed stone.

put 3" insulation on the sides if you want/can and this will help with freeze thaw on slab lift or turndowns on your slab and istall insulation on outside.

or you could go with 6" of 3-5" stone, and a small layer of crushed stone so big rock doesnt cut grid and then a 6 to 8" lift of CTA (cement treated aggregate 7 percent portlland). water it well if it come dry. grade exact and roll the piss out of it and let it sit.

You have to remember a slab "floats" and doesnt have to be as perfect as a flexible product like asphalt. Particularly for a garage that doesnt see the axle count a roadway or p lot does.

thicker concrete slab and rebar and fiber (microfiber so you dont see in you finish) go a long way with bad subgrade.

in our area it is increasingly difficult to get rid of topsoil and dirt. if you can get rid of close by maybe not an issue and cut it and fill it but 5 feet is ridiculous.

6 inch slab should be plenty.

 

[rburke65]

Post and frame.....is this a pole barn type building that you are considering?

 

[sledneck32!]

Yes post frame aka pole barn garaage

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[joes169]

I think best practice and probably most expensive is to remove all of the topsoil and get down to a clay-ish soil.

That said, you might be able to sell the top soil to a landscape material place via prior agreement. That might make up the difference in what you pay for fill stone.

Aound here, topsoil is worth bout $1.00 per yard, assuming the buyer is supplying the loading and trucking. Gravel is close to $15-18 a yard, depending on how far the site is from the pit, so it's impossible to break even on this.

To the OP, have you considered getting estiamtes for a block or poured stem wall with a floor ledge? THese kind of scenarios are best for that style of construction, and it's far superior in the long run IMPO. I wouldn't worry about removing all the topsoil if the weight of the building is sitting on a separate foundation down to non-organic soil..........

 

[sledneck32!]

Yeah I used to do concrete so I pretty much know what it cost but it's more of a schedule type thing I don't think I could get the footings and everything in and get it completed in the time frame that I need to. It's also worth keeping in mind that this is not my final resting place I will more than likely be selling within 3 to 5 years.

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[joes169]

So, just to clarify, they're going to come in and build the pole building first, and you're going to pour the floor against the bottom girt after it's built?

 

[sledneck32!]

So, just to clarify, they're going to come in and build the pole building first, and you're going to pour the floor against the bottom girt after it's built?

Yeah.
Site prep to grade within 1 inch
Set posts and bottom girt
Set conduits for utilities an drains
Pour floor to bottom girt
Finish rest of framing and tin
Insulate 2' down on outside of columns

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[joes169]

Yeah.
Site prep to grade within 1 inch
Set posts and bottom girt
Set conduits for utilities an drains
Pour floor to bottom girt
Finish rest of framing and tin
Insulate 2' down on outside of columns

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OK, at first, when you said "post framed", I was thinking along hte lines of timber or post framing that would rest on the concrete, rather than sitting in post holes below hte frost line.


If that's the case, at the risk of being called a "hack" or something, I would go with contractor #1's plan, I can't see wasting that kind of additional money for a lightweight slab on grade. A lot of old time contractors around here would insist on pouring concrete directly over undisturbed topsoil, and it often worked well for them. Their mindset was that the less you disturb the surface, the better end product you'll get. ALthough I don't completely agree with that notion, if you're putting 10" plus gravel over it, and compacting the snot out of it, I think it will be just fine for decades.......