compact subgrade with weight of machine 743?

[marko13]

I'm grading my yard in prep for a garage slab. Very cohesive rich black dirt in this area. I spread some broken concrete chunks in low areas and covered w ~6" soil. Drove over w wheeled bobcat 743 several times till it looked tight. The high areas I'm taking down and spreading over the rip rap were soil mixed with gravel to a good degree.
Once I get the site fairly flat I'll bring in crusher run for 2 to 6" probably and run over that as well.
Wondering if the weight of the machine is enough for this or if I should get some kind of wacker to pound it down?

 

[brdgbldr]

I'm grading my yard in prep for a garage slab. Very cohesive rich black dirt in this area. I spread some broken concrete chunks in low areas and covered w ~6" soil. Drove over w wheeled bobcat 743 several times till it looked tight. The high areas I'm taking down and spreading over the rip rap were soil mixed with gravel to a good degree.
Once I get the site fairly flat I'll bring in crusher run for 2 to 6" probably and run over that as well.
Wondering if the weight of the machine is enough for this or if I should get some kind of wacker to pound it down?

It depends on how thick of a concrete slab you are putting down and if it will be steel reinforced or not. A thick enough slab will "float" on the sub-grade.

I worked in road construction for 30 years and the only thing that got the base rock compacted enough was a dedicated compactor Or vibratory roller. Depending on how big your slab will be, a plate compactor is normally cheap to rent and will do a pretty good job. Just make sure to wet the gravel (not too much but it can't be dry). It acts kind of like a lube to let the pieces of gravel slide in tight to each other.

One low tech way we would check for good compaction was to have a fully loaded ten yard truck drive over the area. If the tires sunk in at all, it needed more compaction.

 

[marko13]

Thanks for your response.
i know more about carpentry than concrete/site work so I appreciate your expertise.
The slab will be 25 x 60. 4" thick w 1/2" rebar 24" oc. ew.(or I could space it closer) ... I have a few questions rolling around in (what's left of ) my brain ... would appreciate any comments/insights.

1. I may have 1.5 to 2 ft of fill at the low side. Will ab3/crusher run be ok if done in 6in lifts and compacted between or should I use larger rock first then ab3 on top?

2. Do I want to ...
A. over fill the footprint a couple feet and slope down to grade then the slab is just on grade with a thickened edge... or...
B. put a wall around perimeter and just fill inside then the slab sits on the wall and inside fill.
I'm thinking the fill might be cheaper than the wall idea but not sure.

3. I'm coming off an existing drive way that slopes 6" in the 25' front width of the building. I plan an 8' approach that will twist from the slope to level at the face of the garage.
A. I thought about sloping the approach down from the driveway to the face of garage and using a trench drain in front of garage in order to make the fill less tall at the back (site slopes down about 2' from front to back.)
B. or put the top of slab level with the high side of driveway and slope the approach down to driveway at the low side. That adds about 1' of fill overall.

4. The driveway is only 4" thick w fiberglass rebar. I'm guessing I cant' drive a concrete truck or dumptruck with the fill over that to get to the pour site, is that right?

 

[brdgbldr]

One thing you may want is to add microfiber reinforcement to the concrete. It will help prevent cracking and is normally pretty cheap. There are a lot of admixtures that you could look into, depending on your climate and budget, that can make the concrete stronger, more durable, better with freeze thaw etc.

1: In my experience if you fill with larger rock first you need to use a road fabric on top of it before putting the smaller fill on top. If you don't use the fabric, over time despite compacting, the smaller fill will eventually fill in gaps in the larger fill causing voids and eventually cracks in the concrete.

2: This would be an esthetic choice really. The wall would look cleaner but probably be more expensive. It is also hard to compact gravel inside of a concrete wall without damaging or destroying it especially with green/new concrete.

3: I can't really make a comment about this without actually seeing it. Trench Drains are great but do need to be maintained.

4: A concrete truck would most likely completely destroy a 4" thick concrete driveway unless the sub-grade under it is extremely well compacted and stable.
Your best bet is going to be pumping the concrete. The last time I had to pour a slab at my house (30+ years ago) it was pretty cheap to pay a guy to come up with a portable pump. Talk to your concrete supplier, they should have people that they have worked within the past that they can recommend. You also have to order the concrete mix with pumping in mind.

 

[Atilathehun99]

When you have fill with a mixture of sizes (silt/sand/stone) a little to a moderate amount of moisture is a great help when compacting to achieve density. Actually that's true for homogenous fill too.