Foundation prep questions

[cerlawson]

I did a search like that and I see in one reference there is a nomograph for design of rigid pavement. You will see that one input is the Modulus of Subgrade Reaction. They don't there say much about how that value, k is determined, but it is a combination of both the soil below and any subbase or base course. You can see by making major changes (eliminate of add to the base) and very little thickness change of concrete slab is needed. So structurally, the thickness or presence of base course has little to do with the design.

Originally the inclusion under concrete pavements with base course was to keep from pumping saturated soil out thru the joints due to traffic. However, as I noted, the fines of that base course also can pump out. For a building slab, normally no need for such a filter under joints.

 

[cerlawson]

Hi: In trying to find the issue of Concrete Construction that po-poos use of sand,I came across this link.

https://www.facebook.com/search/str/sand subbase/keywords_top

A lot of sand going down.

It reminds me of the "tradition" in construction of removing to topsoil.

I ask those that go for that why. I get a bunch of answers, usually summarized as "We always have". I have seen lots of pavements and buildings built on "topsoil". If you removed all of it in some areas, such as Pontiac, IL, you would be digging down 3 or 4 feet, for no useful purpose. Forgetting about the color, there are other factors to consider. It ain't always something to remove.

Same thing goes for sand as sub base or other usage. They use it a lot in the link, but no one says why.

 

[cerlawson]

Back again.

For those interested in more discussion I suggest going to Eng-Tips.com and go to the room "Earthwork/grading engineering." Other rooms also will get the same audience of engineers.

 

[William1]

I have a few friends in construction. Big projects (100's of million dollar jobs)

They remove the topsoil, mounding it up to the side and charge for 'site prep' Then they spread the topsoil back down and charge for it by the yard. Then sell off the remainder, if any. When asked after (a few Johnny Walkers) it was explained that a 'smooth surface' is easier to work, charging to flatten and the sell it back was a big money maker. As long as the ground is compacted and if there are going to be large loads, good footings, that is all that is needed.
I'm no engineer but as I understand it there is a standardized test that is done. A rod of a particular diameter with a certain weight is placed on the ground and the amount of penetration measured. That determines the depth of the footing (when put in some sort of formula).

 

[m32825]

Oh yes, I disturbed the soil. I had a 60 foot water oak that I couldn't figure out how to work into my garage floor plan so it had to go. There were smaller trees as well and lots of shrubs/roots. I didn't want to do a foundation over all of that, so I raked it out. I'm happy with it now, it's just a question of whether the concrete guys can work with my dirt or need to bring in fill to achieve good compaction. I have plans for the current dirt if they bring in fill, so no loss.

I have a theory on why they scrape off the topsoil: because it is valuable and they can resell it. That helps boost profit margins on the job. After they price the job, ask them to leave the topsoil in a pile on your property and see if they wince.

cerlawson thanks for the links. I spent a while looking at the earthwork forum and learned some good stuff. I was excited to see a thread entitled "Mechanical Excavation vs Blasting" but disappointed to learn they weren't talking about removing trees!

-- Carl

 

[cerlawson]

Back again.

Well as to making money with topsoil, that is fine. Many times the site will be regraded and you willo want to plant grass on a nice organic topsoil. Fine.

However, lets say you are building a K mart in Janesville, WI. That parking lot is big. After removing sod, a small amount of site grading is needed there before proceeding. If you took that extra foot or 18 inches, it would set the grades lower, but still will work. But, that one foor or 18 inches is not replaced and costs money to haul off. I was in on that job about 1965. K Mart may be gone, but the pavement is fine yet.

When it comes to slabs on grade, check the topsoil as if it were not black and make your decision.

As to using a rod probe, fine we use it all the time to roughly determine the density of the soil. It is not scientific and if we really want to test for bearing capacity or other property there are other tests, usually requiring an undisturbed sample to be taken to a laboratory for tests. The cheapest of these is the unconfined compression test, usually for cohesive soil. Others are rather lengthy AND COSTLY.

On the job to check for density as compared to lab tests, replacing a given soil volume with known density stuff, such as water is one way. Also now a nuclear device that checks the ability of soil to slow neutrons or to absorb gamma radiation is the current procedure. No dependable use for pounding a rod with a 5 pound hammer, sorry. However, simple tests like that can tell roughly where the worst stuff is.

Put your questions on Eng-Tips.com and you will get a bunch of info.

 

[m32825]

Here's an update for those of you who are interested. The metal building guys design the building and specify the connections it should have to the slab, but foundation requirements vary from state to state so that's left for a local engineer to design. Florida's Building Code is tough, residential construction must withstand 140 mile per hour winds in Central Florida.

The design my guy put together is a slab on grade. The outside two feet of the slab extend downwards two feet and have reinforcing rebar along the bottom, at least 12 inches must be below grade. The slab is a minimum of 4 inches thick with 6 inch by 6 inch 1.4 x 1.4 welded wire reinforcement or fiber reinforced concrete. In the areas where the columns meet the foundation the two foot thickness of the perimeter extends inward an additional 1 to 5 feet. These areas have rebar hairpins to tie them in to the slab, anchor bolts will be drilled and epoxied from 5 to 8 inches deep.

1 inch deep saw cuts are called for within 24 hours of placement. The 30 foot dimension is divided in half, the 50 foot dimension is divided in fourths.

Termite protection is a big deal in Florida. You have to remove all organic material, stumps, roots underneath the foundation. After the footers have been dug the soil must be termite treated then a 6 mil vapor barrier is installed.

Footings have to be on undisturbed solid soil or on soil compacted to a density of at least 95% of standard Proctor maximum dry density for a depth of at least 3 feet below bottom of footing.

Any thoughts on fiber reinforced concrete versus welded wire mesh?

-- Carl

 

[sheepfarmer]

Just had that discussion...the person who did all the grading for my barn recommended fiber, said he'd taken out an awful lot of cracked and settled concrete with the metal mesh in it, largely because the metal ended up not embedded in the middle of the 4 inches but under it and rusted and did nothing to reinforce the strength. The concrete guy who was hired by Morton and was specified to put it in, didn't trust the fiber, but agreed that the trick was during the pour that the guys have to keep pulling the mesh up into the concrete as they spread it. The problem is they have to stand on it as they work and so it goes to the bottom again. So for the outside concrete I did both. Cost an additional $100 to add the fiber to two mixer loads. The concrete guy didn't like the older type of fiber because it messed with the surface finish, but the kind of fiber they put in mine didn't cause any problem (also we were not going for a smooth finish).

 

[skeets]

Cool beans,,now when are you going to have the unveiling of this little project

 

[sheepfarmer]

Cool beans,,now when are you going to have the unveiling of this little project

Well if it rains all afternoon I'll try to move some pics from phone onto computer for resizing. Almost done, alittle more grading to make a driveway and put the fences back, not a moment too soon.