I am sure this has been discussed many times, but I wanted to get some opinions on my scenario.
Two years ago when I built my home, my yard was cut down about 4 feet in some areas to give it a good grade. I had the excavator use what we both thought was good fill dirt to build up an area for a future garage. The depth of fill ranges from about 4’ to 7’. He put it down in 4-6” lifts and compacted with his 40k lb track loader.
Recently I had a sample taken was told there was considerable topsoil mixed in and was encouraged not to build anything without significant engineering, except maybe a pole barn on 7’ piers. Primarily my soil is clay with moderate shrink swell. At least visually, when the garage pad was being built up, didn’t see what I think of as topsoil.
My builder friend who watched the whole excavation process with me thinks I am fine to build a simple 24×32 garage on a monolithic slab, but the engineer is freaking me out.
I would love some opinions…..
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That's a lot of fill and tracks don't compact really well. Maybe you could have another sample tested?
I am going to talk with the soil scientist again. He didn't mention that he had an issue with the compaction, but that it was a mixture of clay and topsoil. I did ask if I waited a number of years to build if if would change anything, and he said no. Everything that went in was red, so apparently topsoil means something different in scientific terms. More than likely I can build with my original soil report. I paid for another report for peace of mind, but it made it worse:)
If the test is telling you the ground is not sufficient for building do not build on it. Or take proper steps with engineering to make sure you don’t loose a ton of $ later. I’ve seen settling issues happen before and it is never an easy, cheap fix. Why risk it? A second opinion from soil tester might be a good option as well. If the engineer firm(s) work together or are same company they may be trying to get more business from you. I typically pay around $300 for my test, it’s really not much for the peace of mind. I know where your friend is coming from, but if you asked him to guarantee there will not be settling issues later I doubt he could. He may be 95% certain, but the other 5% is what you need to worry about. A colleague of mine built his $500K home on trucked in compacted dirt (no soil test, bad move). It ended his career as he spent all his $ repairing the home and burned all his bridges with subs as he could not pay them. I was the lucky one and only got hit with a $2K lose for work I did for him. The last time I spoke to him the repair bill was $130K and on going.
I would never even consider risking it, except its a cheap building. Pole framing might be the best option. At least then settling would only effect the slab.
If the soil was tested properly then the issue is certainly that there are too many organics in the mixture not that it's "topsoil" per se.
4 to 6 inch lifts is good practice but compacting with the track is not. The soil should be compacted with a compactor not tracks!
Get yourself a decent engineer and design the slab to span deep footings and you will be good to go.
Let's start by saying the most right thing to do is to listen to the engineer.
But I think part of the answer you're looking for lies in what you're going to use the garage for and how much you'd be bothered by some incidental settling and cracking. If the structure is lightweight construction and you don't have heavy equipment constantly going in and out, I think you and your friend are probably right that a monolithic slab spreading the load over a large enough surface area is not going to be a problem. Maybe significantly upgrade your reinforcement plan in the slab for added peace of mind.
The next safest option without going too crazy would be the more traditional approach to excavate a perimeter trench down to solid bearing below frost depth and pour footings and stem walls. Then your assured that the structure is good and it's just the slab that will be questionable, tho with reinforcement tied into the stem walls a lot of that risk can probably be designed out too.
If you want to know you're doing it right, then you have to fix the problem. One option would be to just scrape out the fill and bring in engineered fill that's properly compacted, tho admittedly that's a lot of fill to buy when you're not 100% convinced it's accomplishing anything. Or this situation is a good candidate for a grid of helical piers and a slab reinforcement design to span accordingly.