Site Prep and Foundation

[sinr98]

Hey all,
I will be beginning my site prep and excavating in the near future on my new detached 26x28 garage. I have a few questions on the layout of the site.
At the front of the site where the thickened edge (monolithic slab) will be sitting the left front corner is the highest point so I will call it location "A". Location "B" is the left rear corner, "C" is the right rear corner, and D is the Right front corner.

I shot the locations tonight and came up with my slope.
"A" was the highest point which is raised from a tree that was previously in that location and has a mound in the area. The stump was ground down and roots were chased.
So from the location of "A" to "B" there is a 5" drop in grade. From "A" to "C" there is a 13" drop in grade, and from "A" to "D" there is a 14" drop in grade.
I am sure I can get away with a 4" cut from location "A" to bring the surrounding area to grade and keep runoff away from the foundation.

I originally planned for a pour of a 12"wide footer with a taper of 12" to a 4" thick slab and form wall of 16". I also planned to have the slab 4" above grade at the highest point at location "A", and everything else would be backfilled and graded according.

Would my best bet be to bring the grade down to my lowest point and compact and fill to my desired height, or remove 6" or so of the top soil in the highest areas (A and B) place the soil removed from there to the areas that are the lowest (C and D) then compact the earth, then bring in some crushed limestone and compact 3" then add 3" more crushed limestone or some other base and compact that also? The soil is a great sand/dirt mix and has great drainage. I have had concrete trucks and heavy equipment run over the area several times 2 years ago when I put an addition on and had very little ground penetration. The front will run to the road which has a 30" drop in slope in a run of 80'.

Also would it be ok to run a 12" sidewall on the high side and taper it to a 18"-20" sidewall on the low area or keep it the same around?

I have had a few companies come out and look at it to give me an estimate but have not had a estimate returned to me. I have a pool of 8 concrete guys that will be helping me place and finish the concrete but none of them do the digging. The will also be helping me do the reinforcement.
That will consist of 3x 1/2" rebar around the perimeter of the footing, and wire mesh through the slab area. This will also have poly sheeting over the compacted base.

Let me know what your thoughts are. I am trying to keep this as low on the budget as possible since this is my temp garage and plan on building a house and selling this one in the next two years. It will be un-heated and un-insulated but will have it's own 200amp electrical service brought in so I can power my welders and compressor.
This is located in Ohio too for what it's worth.
Thanks

 

[ConCretin]

There really isn't a right answer here. Either the cut or fill method of leveling your pad would work and I'm not sure there would be a significant difference in cost either.

The fill method will probably require you to import material (don't use 'topsoil') If you cut, you'll import less but will need a higher curb wall in back and would probably be wise to do something about damp proofing and drainage. It really just comes down to aesthetics.

Not sure I understand your question on sidewalls.

 

[sinr98]

Here are a few pics for better understanding.
As for sidewalls I was referring to the exterior height of the foundation. The above and below grade height.

 

[nolimits76]

LLWilly is probably right, there probably isn't a huge cost difference between the 2 methods on a project your size. Generally speaking, it is usually better to cut/fill with your own material vs bringing in imported materials and possibly having to haul off excess spoils...from a cost perspective. Trucking can add up pretty quickly. Sometimes it's recommended, regardless of cost, if your soils are bad.

Some general calculations for your own use (adjust as necessary for your location). Keep in mind, these costs below are true material/haul/royalty costs. They don't include labor or other incidentals (fuel, water, equipment, etc) to install.

Earthwork Quantities (assume 2' extra from garage dimensions)
--> Max cut (worst scenario) = 28' x 30' x 14" = 36.31cy
--> Average cut (0" to 14") = 28' x 30' x 7" = 18.14cy

Earthwork Costing:
-->Average cost to haul/truck small aggregates or earthwork spoils = $80/hr for 8-10cy truck

-->Average cost to dump wasted spoils (some can be used for final grade) = $1.00/cy, depending on land owner you find that will accept the materials. The closer the better as you save on hauling since it's an hourly rate. So maybe $1.25/cy is a better for a site 1/2 mile away vs $.75/cy for one 10 miles away.

-->Aggregate fill material for compaction = $10/cy +/-

-->Haul aggregate material to site = Can use hourly rates or talk with haulers that will quote on a CY basis. Varies greatly on distance. $3.50/cy is low side. $20+/cy would be high side. Probably $7-10/cy is more average.

 

[theoldwizard1]

There really isn't a right answer here. Either the cut or fill method of leveling your pad would work and I'm not sure there would be a significant difference in cost either.

The fill method will probably require you to import material (don't use 'topsoil')

"Topsoil" contains organic material that will decay over time and could potential leave void underneath your concrete. Most codes require a later of inorganic fill (sand and/or gravel) to be the top layer underneath any slab.

Many codes now require a curb around the perimeter of at least 6" above the final slab height. This is easy to do if the footing/foundation side wall are poured separate from the floor.

What ever you choose, make sure the site drainage is correct after the final grading !

 

[nolimits76]

Wizard makes a valid point. Even if you had a perfectly level piece of ground (yeah right) and you wanted a 4" slab, then you would strip 10" off. Then put in 6" screenings or similar material that can be compacted properly, and then do your slab. At least that is common here, soil conditions might vary in your location.

I might also note...anytime fill is required, compaction becomes critical. Your slope variances are pretty minor on this project, but I'd still want it properly compacted.

 

[sinr98]

Tons of great info from you guys. Thank you!
I should not have an issue with drainage as this area is about the highest point of around 200yds all the way around. I do plan on running a tile around the perimeter to the back of the lot.
The lot was graded 2 years ago and slopes away from the house on my property and slopes away from the neighbors house to the center of where the drive will be. The grading runs down hill from the garage location to the road just over 30".
The whole yard was compacted at that time with a road compactor and has had some heavy equipment run through it (tree trucks, crane flatbeds etc) with less than an inch of compaction. The sub-base and base will be compacted with a plate compactor, and I planned on using 4" of #1 crushed stone at the high spot and level it off at the low end, and 2" of crush and run on top both being compacted.

 

[bczygan]

Balanced earthwork (The same amount of cut, as fill) is the cheapest as you don't have to export or import material.

But to create a pad and building that is high and dry, I like to do it this way. I pick the highest corner of the building and set the finished floor about 8" above existing grade at that point. If surrounding grade in that area slopes toward the building, I raise it another 8" and regrade so that grade slopes positively away from the building until it meets the opposing existing slope, creating a swale. This keeps water away.

Then I cut down the existing grade under the pad until firm, undisturbed earth is reached.

This area is then filled with compacted material as required to reach the underside of the proposed slab and insulation.

Grading adjacent to all these built up pad areas, requires the pad to slope down at an angle to meet existing grade at an angle that will accommodate compaction.

 

[Kevin54]

sinr98......If you don't mind, could you put what State you live in, into your profile? The State of Denial, may or may not work , but it does make it easier to answer questions. Knowing the state you live in, just makes it easier to answer various questions about frost lines, insulation, and such.



With that being said....Don't use any soil for a backfill. Use crushed stone or sand. IIRC, I think in Ohio on monolithic slabs, the edges are around 18" down and about the same width. You will want to scrape off your sod and then some so you are down to firm dirt. Most that I have saw done have at least a minimum of around 6" of crushed stone. If it were me, I would find the high point or high corner, then come out level from that leaving at least 12" plus above the high point for your top level of your pour. Then once you fill in around the area, you will have adequate fall of your ground to keep water away from the building. Once everything is backfilled and feathered out, 12" will not really be that much even though it will be deeper at the low side. And you will only be looking at maybe an extra $100 for stone and form boards. If you are planning on moving in 2 years, you still don't want to cut corners. At least make it nice for the next possible gearhead that may become the lucky owner.

I've never been real fond of slab foundations or slab floors for the fact that if the soil is too high up on the outside, the water has a chance to wick in at the bottom plate, or if one goes to finish the inside, then any water from washing a vehicle, melting snow, or washing out the garage, the water is up against wood.

Another thing to consider that I have saw quite often is having a level slab, then add a course of cement block, or have a curb poured on top. This keeps any water from wicking up on your wood or finished drywall or whatever other suitable wall covering material.

 

[sinr98]

Kevin, I also live in Ohio. North Central just south of Cedar Point actually.
Out of the 3 contractors I talked to they all said that they would skim the top 4" of soil, and add 6" of base then the finish grade would be at 6" above current high point.
But I just talked to my neighbor and asked him if he would mind if I graded the high section of his property that joins mine to the hump out and he said he didn't mind one bit. I take care of mowing his lawn as it is and I will plant seed through out the area anyways. So I took a look and found that the high spot is actually 7" high in that spot. So that leaves me with a 7" slope to deal with now.