Compromised soil compaction in foundation prep

[subarctic_moose]

Disclaimer: I take full responsibility for my current situation and the contractor did exactly what I was asking him to do.
After a few rough winters here in the Sierras and no garage on the property my wife and I are finally pulling the trigger on a 30x40x12 metal frame shop. It's being built to our snow load spec of 80psf and will have studs/trusses 24" OC to accommodate the loads. For the foundation, the plan was to first do a 8"x16" footer below frost depth of 18" and have a 24" stem wall on the perimeter, 6" above grade, with the exception of 18"/grade level at the doors. I was hoping to get this part done in the next month so the building could be constructed on a foundation when it's delivered end of October. The building will be delivered with the frame and roof and our plan is to mill up the siding and frame in the ends with vehicle/man doors next year and tie in a slab as well. My understanding is that this foundation type is typical in our area.

The shop will be in a flat area behind our house in a zone where I've had to drop a number of small and large pines to make room. I hired a local (and recommended) excavator to come out and handle the earth work. My very naive instructions were that we needed the stumps removed and a building pad leveled and graded for a shop. I also explained that because of this was for a foundation the stumps had to be completely dug out and not just ground down below grade. Using a medium sized tractor/backhoe they made pretty quick work of some of the smaller stumps but there were a few larger ones where they dug down about 24" and then cut the taproot below the root ball. There's also a 28" stump that's still standing as of now due to a massive taproot. It's one of the trees I cut down just a few weeks ago and is pretty fresh.

So here's where things have gone off the rails. After talking more with a neighbor, I realized I was woefully unaware of compaction requirements with organic material removal. I also did not think to have that conversation with the contractor to gauge his level of understanding of earth work specifically for foundation requirements. After removing each stump, they backfilled the holes with the surrounding dirt and graded over it. No base gravel, no compaction layers, nothing. I now have a nice looking pad with no way to tell where stumps were previously and what dirt is original or filled. I don't know if this is a big problem for the footers as those would be at a total depth of at least 24" and there were no stumps in their path. But in terms of a future slab, my understanding is that the whole area is now compromised.

1. What are our options at this point?
2. How much dirt will have to be excavated and replaced now that fill dirt has been introduced?
3. How much of a problem are we looking at if part of a tap root still exists below 24"?
4. I'm thinking we'll need to delay delivery of our building to Spring and gut it out one more winter. Even if we can do the footers now, I'm guessing it would be horrendous to then have to try and remove large quantities of material inside the perimeter at a later time?

Again, this is my fault. My wife had a talk with me recently about finding more help as I've been doing significant amounts of tree work/snow removal/road maintenance on our property by myself the last few years and it's getting tiring. So I thought I was doing the right thing by hiring some professionals to at least handle most of the prep work, but unfortunately I didn't know the right questions to ask them. We also live in a rural area and finding the right people to ask is surprisingly difficult out here. On the bright side I'm glad we at least caught this now instead of later. And I got rid of another clump of trees in close proximity without nailing the house, so there's always that at least...

Edit: Originally we were not going the permitted route as we have an ag exception in our zoning laws. But since we're already planning on having everything engineered and will be building to code regardless, we might as well (and maybe avoid these kinds of situations).

 

[wssix99]

For the foundation, the plan was to first do a 8"x16" footer below frost depth of 18" and have a 24" stem wall on the perimeter, 6" above grade

Easy solution, go hire an engineer and have them re-visit your foundation design. You may be good or you may need some additional reinforcing in your footer or you may need a wider spread footer. They could also prescribe a special fill underneath the footer you are pouring. It should be a simple exercise. The footer spreads the loads in a way to match the bearing strength of the soil. They just need to be matched up.

When we dug the foundations for our 4 story house, we encountered a 100 year old wooden cesspool of horse manure right underneath a critical area of our foundation. A call to the engineer (and a relatively reasonable fee) got us straightened out with some proper fill and some additional reinforcing in the footer.

^ Your problem should be much easier to deal with as you only have suspected soft ground in localized areas.

 

[tarmy]

Civil/geotechnical engineer and soils report is what you need. They will invasive test and recommend a compaction/grading solution for the specific foundation you will be using.
Here in NorCal, they are required for most new home foundations.

 

[zak77]

Benchmark Geotechnical Labs

AASHTO Accredited

benchmarkgeolabs.com

You want a company such as this. These guys are in Mass but i believe they can test anything from anywhere but i do not know the logistics of how things are done. I've spoken to the owner and he is very knowledgeable in this field.

 

[jack stand]

Are you saying that you're confident in the soils under the footings but worried about the interior slab subgrade?

 

[subarctic_moose]

Are you saying that you're confident in the soils under the footings but worried about the interior slab subgrade?

Correct, at least with my original plan using the frost wall. The footing would actually be 12” by 18”, plus the 18” frost line would I believe would put everything down at 30”. I am “pretty confident” that we didn’t go below this line and also didn’t disturb any significant amount of soil in the area of the footings, just the interior slab area as you were inquiring. But I wouldn’t want to stake a building or possible foundation issue on that.

The primary concrete contractor in the area was out yesterday and didn’t seem concerned about the work that had been done so far. He also pointed out that we could simplify things and go the mono slab route. I did not realize this was an option in snow country with frost but after researching this forum I can understand the idea a bit more given it’s a metal frame shop only with no second floor/residence. He also gave me the number of his own earth works guy that I’m waiting to hear back from and get more info about what he would typically do in this situation for site prep.

Plumas Country strangely does not have mention of soils report in the garage plans submission checklist. Just architectural drawings, setbacks, and fire safe requirements.

 

[ConCretin]

I'm not going to argue with the advice provided by previous posters that you employ some professional help. It's the safest route forward and will help protect your investment. With that said, if there is no disturbed soil under your foundation, there is obviously no concern there. As for the sub base under your slab, I'd probably just proof roll it and install a good base layer. As long as the native soil is compactable, there is no need to dig it out and replace it. I wouldn't worry too much about a small amount of organic material remaining after the root removal. There is the perfect world and there is good enough. You'll have decide what your comfortable with.

Assuming local codes permit, a monolithic slab or frost protected shallow foundation might be viable options for your structure but there are a number of considerations for each you'll have to take into account. Frost walls with a floating slab is a little more expensive but it probably more fool proof.

 

[7th Kahuna]

I hesitate to contribute to this conversation as I am not an engineer and it sounds like you are asking the right questions. But I do want to put your mind at least somewhat at ease. As long as the foundations are on virgin soil, they are generally where the load is concentrated. Talk to the engineer about the loads within the garage. Those are usually relatively minor. My guess is extensive excavation would not be required. Maybe some localized compaction, maybe a soil binder, maybe some additional steel or concrete. Again, it depends upon your loads inside the structure and the composition of your soil. I think LLWillysfan is on the right track.

 

[Rusted Nut]

I agree with LLWillysfan. An 8x16 footing will support two floors and a roof load, so you should OK with just a roof load. Wouldn’t hurt to have a geo tech come out probe soil for a professional opinion.

 

[bb29510]

just go to home depot and rent a jumping jack for the day

 

[BombShelter]

Interesting discussion, I don't think it comes up much up here. I'm definately not an expert but if there's concerns, couldn't you run a sprinkler to find out if the backfill wants to settle more, then just load up on the class 5, we used to do 4"-5," sometimes more, with soft or wet ground and compact the **** out of it.

I've watched the new subdivision going up a few blocks over for the past two years, they just plow everything, terra-form so all the homes can have a pretty basement walk-out then build. There doesn't seem to be much concern for anything.

 

[tarmy]

I'm not going to argue with the advice provided by previous posters that you employ some professional help. It's the safest route forward and will help protect your investment. With that said, if there is no disturbed soil under your foundation, there is obviously no concern there. As for the sub base under your slab, I'd probably just proof roll it and install a good base layer. As long as the native soil is compactable, there is no need to dig it out and replace it. I wouldn't worry too much about a small amount of organic material remaining after the root removal. There is the perfect world and there is good enough. You'll have decide what your comfortable with.

Assuming local codes permit, a monolithic slab or frost protected shallow foundation might be viable options for your structure but there are a number of considerations for each you'll have to take into account. Frost walls with a floating slab is a little more expensive but it probably more fool proof.

While I agree with this generally, there is differential settlement potentiAl. We have no data on how the pad was created even if by cut. The point of a geotechnical is to get that sorted out and to determine if the soil is truly adequate for the intended loads or any point loads like a lift.

 

[rancherbill]

-talk to an Engineer.
-compaction testing is done all the time.
-If you have question still, put in piers on undisturbed dirt. All new subdivision garages are done this way here, because massive amounts of dirt is moved creating new districts.

 

[Chaznsc]

How much fill was placed to level the pad? Any photos you can share? An example: You may can scrape off 2 feet of material, roll out the subsoil, then proofroll with a loaded dump truck. Any movement indicates areas that need attention.

 

[bb29510]

call a geotech, it be two hours minimum $200 then they will requires a proctor $250 so that almost $500, three day wait and they going tell you its uncompacted, or rent a jumping jack for $200 and be done with it in half a day

 

[CraigStu]

I am curious in this situation where it is a relatively light weight building vs multiple stories. How would a jumping jack result compare w/ simply driving the back hoe or excavator (that will be there to dig the footers) back and forth a couple hours? Does the intermittent pounding do a better job vs simple weight?

 

[Chaznsc]

A geotechnical engineer will be unable to test more than a foot into the subsoil. The best way to check this is to roll back to the OG surface and test or roll it.

 

[rancherbill]

How much fill was placed to level the pad? Any photos you can share? An example: You may can scrape off 2 feet of material, roll out the subsoil, then proofroll with a loaded dump truck. Any movement indicates areas that need attention.

I don't know what proof rolling involves but anything that involves tire packing is WRONG. There is a specific procedure and equipment required to compact soil. Part of it is doing small lifts not 2 feet at a time.

 

[tarmy]

A geotechnical engineer will be unable to test more than a foot into the subsoil. The best way to check this is to roll back to the OG surface and test or roll it.

Around here they can test to over 50’ deep. They also test the finished pad so ensure that it is within spec.

 

[Scotto]

If you do determine you want to try compacting the soil more, I'd suggest renting a trench compactor with sheep pad rollers. I used this when filling stump holes for my garage and it did an unbelievable job compacting my clay soil in 4"-6" lifts - it would compact until the clay soil would actually come up around the rollers if I let it sit in one spot too long. MUCH better than I could get with running over with my tractor and it's fun being remote controlled, lol.
If you're a DIYer and just want to get it done, over a weekend rent one of these compactors (mine was $200/weekend) and a backhoe/loader (maybe $300-$400 if you can find a local business) and go to town excavating down maybe 18 inches, then compact that, then add 4-6" lifts and compact those. Put off to the side any organic topsoil you may find. I would also dig down and try and get any remaining roots
For the footers, I'd pay a little more to go down deeper, maybe 36", to make sure you're getting to virgin soil.




Here's some photos of what I dealt with when I dug out the stumps for my garage. It sounds like you need some bigger equipment to deal with the large existing stump and the taproot that was left - you should really get anything organic out of there. I was able to move around my stumps with my 40 horse loader but it was tough, lol.

 

[subarctic_moose]

How much fill was placed to level the pad? Any photos you can share? An example: You may can scrape off 2 feet of material, roll out the subsoil, then proofroll with a loaded dump truck. Any movement indicates areas that need attention.

I don’t think a lot of fill has been placed to level, but hopefully the photos give some idea. Mostly
just back-fill in stump holes is what I was worried about. But good point that the dirt being push to the low side might have the same issue.

The stump by where my tractor is parked is the slightly high side and where the front of the building would be. Where you can see some dirt has been pushed to is a little lower and where the back of the building has been placed. The left side would be within 3-5 feet of the large tree/tree line still standing and the right side 24’ from the house (marked with stakes) and leaving room for a 12’ lean-to coming to 12’ from the house (also marked with stakes). The property line is about 25-30 feet past the tree-line. The house faces a view in the opposite direction so not worried about losing any view out the back, and it would actually be a bonus to create a visual barrier to the neighbors.

I do have some leeway forward/backward on placement. Right now the front of the building would be just behind the big stump still standing, moving forward would place it over it but would receive the amount of dirt having to come to the low side at the back to level.

 

[subarctic_moose]

I agree with LLWillysfan. An 8x16 footing will support two floors and a roof load, so you should OK with just a roof load. Wouldn’t hurt to have a geo tech come out probe soil for a professional opinion.

Our code says “all footings must extend 12” is to undisturbed soil”. Would this not call for a 12” x 16” footing in this case as opposed to 8” or am I reading that wrong?

 

[Chaznsc]

I see a lot of organics in that soil.

 

[tarmy]

I see a lot of organics in that soil.

Yep…

I am not trying (although I may be) a ****, but….

OP…you are going to spend tens of thousands on a great garage…why take a chance.

Lots of good advice and it still seems like you are set on winging it. A few hundred/grand to be sure. Guessing on foundation design/soils may not go so well. Good luck and post up picks as it goes…

 

[subarctic_moose]

Yep…

I am not trying (although I may be) a ****, but….

OP…you are going to spend tens of thousands on a great garage…why take a chance.

Lots of good advice and it still seems like you are set on winging it. A few hundred/grand to be sure. Guessing on foundation design/soils may not go so well. Good luck and post up picks as it goes…

Oh definitely not wanting to wing it here (and I’m not trying to convey that impression). I’m putting too much money into this to take any sort of chance and the photos you’re looking at are reflective of the first pass at removing trees and obstructions. As stated above I’m going to be pulling a permit and doing whatever is necessary to ensure site prep passes inspection and compaction tests. I’m still waiting to hear back from a guy that does most of the earthwork for the concrete company but no luck yet.

My original question was how much work and expense I’m likely looking at to get this pad in order. After working with the original contractor I thought I had a decent idea of what was required but that was not the case. This is all helping me get a much clearer picture.

 

[subarctic_moose]

If you do determine you want to try compacting the soil more, I'd suggest renting a trench compactor with sheep pad rollers. I used this when filling stump holes for my garage and it did an unbelievable job compacting my clay soil in 4"-6" lifts - it would compact until the clay soil would actually come up around the rollers if I let it sit in one spot too long. MUCH better than I could get with running over with my tractor and it's fun being remote controlled, lol.
If you're a DIYer and just want to get it done, over a weekend rent one of these compactors (mine was $200/weekend) and a backhoe/loader (maybe $300-$400 if you can find a local business) and go to town excavating down maybe 18 inches, then compact that, then add 4-6" lifts and compact those. Put off to the side any organic topsoil you may find. I would also dig down and try and get any remaining roots
For the footers, I'd pay a little more to go down deeper, maybe 36", to make sure you're getting to virgin soil.




Here's some photos of what I dealt with when I dug out the stumps for my garage. It sounds like you need some bigger equipment to deal with the large existing stump and the taproot that was left - you should really get anything organic out of there. I was able to move around my stumps with my 40 horse loader but it was tough, lol.

Great info. And that is a heck of a root ball!

 

[subarctic_moose]

Thanks for the advice and knowledge everyone! I was actually able to track down our one geotech in town. Very friendly guy, just fired off an email at his request with some photos for a consultation. Hopefully he can do a site visit soon, I’ll share whatever info he comes back with.

 

[Zeke]

I can't imagine building on natural forest ground w/o some preparation. In nature's way it has to be very rich in organic material. I live in a city so what do I know? Still, you have questions and so would I. I think I would have started with moving a lot of soil away from the site to nearby or hauling it out if need be. Then I'd bring in base. For drainage and compaction and certainly free of roots and whatnot.

 

[tarmy]

Thanks for the advice and knowledge everyone! I was actually able to track down our one geotech in town. Very friendly guy, just fired off an email at his request with some photos for a consultation. Hopefully he can do a site visit soon, I’ll share whatever info he comes back with.

Post his scope of work (nothing proprietary) so we can help with that. Make sure your architect and him have a coordination section so that foundation design/recommendations are reviewed as well. Usually some discussion will occur so that the most efficient/cost effective foundation structure can be implemented. Also, report prep may be extra fees if they are reviewed (out here they have to be submitted and approved with the structural plans). Not sure what your inspectors actually inspect…out here the steel and form boards get checked and setbacks as well prior to pour.

Ask your guy for costs for him to review the pad condition prior to trenching…

 

[f121]

Regarding not knowing where the good soil is vs the made ground - it will be really obvious when you drag an excavator bucket through the ground what is original and what is fill.

 

[bb29510]

if you can grow a tomato in it, its bad soil

 

[subarctic_moose]

if you can grow a tomato in it, its bad soil

Oof. Shots fired. I can barely grow tomatoes in my planters. We have pretty wild temp swings here even in summer, 90+ during the day and high 30s/low 40s at dawn in July. Between that and the damn deer I haven’t harvested a whole lot. As the saying goes “$1000 in supplies and 100 hours in labor to grow a pepper, e-mail me for more financial advice”.

 

[subarctic_moose]

Post his scope of work (nothing proprietary) so we can help with that. Make sure your architect and him have a coordination section so that foundation design/recommendations are reviewed as well. Usually some discussion will occur so that the most efficient/cost effective foundation structure can be implemented. Also, report prep may be extra fees if they are reviewed (out here they have to be submitted and approved with the structural plans). Not sure what your inspectors actually inspect…out here the steel and form boards get checked and setbacks as well prior to pour.

Ask your guy for costs for him to review the pad condition prior to trenching…

Will do! Might be a bit before he can make it out here. I rush at this point I’m committed to next year and not this one.

General question, regardless of what we have to excavate still to hit compaction, is there any harm (I.e cause more work later) by trying to continue leveling and scraping material off the high sides with the box blade? I get that there will be more work and material replacement in the future, but wondering if I can make some form of progress that can keep things moving.

 

[tarmy]

Will do! Might be a bit before he can make it out here. I rush at this point I’m committed to next year and not this one.

General question, regardless of what we have to excavate still to hit compaction, is there any harm (I.e cause more work later) by trying to continue leveling and scraping material off the high sides with the box blade? I get that there will be more work and material replacement in the future, but wondering if I can make some form of progress that can keep things moving.

My advice (what I would do ) is leave it until you get better informed as to what you really have. Any efforts you make now may result in more earthwork later or more handling of material if the recommendation is remove and replace in lifts and a certain compaction/moisture level as you go. They usually have max lift depths, rock size/materials types. All that will be based upon the buildings’ needs.

 

[soreneck racing]

Another consideration that might be asked of your engineering/architectural/contractor firms is the use of screw piles as part of your footer. Not sure what their availability is in your area or the costs associated with them. Something to think about!

 

[Scotto]

I see a lot of organics in that soil.

Yea, it looks like that to me as well. It's tough though, your soil is totally different than my area (as you can see from my pics above). You go down 6-8" inches and it's clay/gravel which you can build on, so not much organic to worry about. I actually scrapped all the topsoil off the site beforehand because there's so little and then started digging out stumps. Definitely best to talk to the geotech and not do anymore work that could cause even more issues, unless you're removing material and putting it away from the build site.

Also, for 30x40 garage there are way too many stumps still in that area. Like I said earlier, you need a bigger machine to get those stumps outta there.
If/when you need more fill, look around for a local septic installer. I was able to get triaxle loads of virgin base material for $50 a load. I got like 8 of them - couldn't beat it for the price. Just had to be a little flexible on when I got the material.

 

[subarctic_moose]

Thanks for the advice here everybody. Took a step back and talked with my wife some more and came up with some slightly different plans. It’s going to make more sense to go with a stick frame and add in some living space above the shop, so footers/stem will be the way to go. The other good news is that we found out now and not later that we need to move the placement over about 30-40 feet. The bad news is that it means I could have just ground out the stumps as they are no longer in the foundation area instead of paying the guy 2k to spend all that time digging them out with a too-small backhoe. Now I have a hole in my pocket with not much to show for it. Again… my fault. Another 500 bucks and I would have had my own grinder bought for the tractor. Oh well. Hopefully a good lesson that saved me from making a bigger mistake later.

 

[subarctic_moose]

My advice (what I would do ) is leave it until you get better informed as to what you really have. Any efforts you make now may result in more earthwork later or more handling of material if the recommendation is remove and replace in lifts and a certain compaction/moisture level as you go. They usually have max lift depths, rock size/materials types. All that will be based upon the buildings’ needs.

Another consideration that might be asked of your engineering/architectural/contractor firms is the use of screw piles as part of your footer. Not sure what their availability is in your area or the costs associated with them. Something to think about!

I will make sure to ask once I actually hear back from someone. Trying to follow up with the geotech but no response after I sent over my info a week ago. The go-to earth works contractor that was recommended to me is booked up, has a full voicemail box and does not return calls or texts. Luckily my wife knows some other folks in town through her job at the school who do foundation specific earth work. They’ll be coming out next month to take a look and see if they can help out and bring in some heavy equipment from a job nearby. I think those kinds of relationships are the only way you can get things done out here.