How shallow can foundation be?

Hi
I started to build a dormer recently, its 235 sq m. I observed the foundations being poured and about 36-40 metres of concrete went into it. The trenches are very wide in places because the foundations were dug more than once (don't ask!).
I meaured the depth in a few places and which varied from over 3000 mm in places to 2500 mm in others but worringly as low as 1750 mm on an external wall corner. Steel mesh has gone in also. The ground is without doubt solid and excellent and has been inspected by an engineer. The foundations contain a number of steps.
Should I be concerned? It is obvious that this is well below the official spec so I was a bit worried on seeing it. Have people seen houses built to this spec which survived long term? Can mesh and width compensate for shallowness? BTW there will be concrete slab floors upstairs.
Thanks.
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Chad wrote: ...

What was the "official spec"? This is almost 6-ft depth at the minimum which would be well below frost-line at most any place. If the ground was undisturbed and solid and seen and the excavation was inspected and approved by an engineer it would seem adequate.
But, that's just a guess based only on the depth and your description and a generality about frost depths; it would require far more to make anything sort of realistic assessment.
Why not ask the architect/engineer/inspector?
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Official spec 300-400 mm concrete, no steel. bottom of concreete 600 mm below finished ground level.
Engineer was happy with trenches but does not inspect concrete during or after pour.
dpb wrote:

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Chad wrote:

Well, one would assume he would have observed the depth and compared to the spec and that the concrete would then be at the bottom of the trench. :) Consequently, if he approved the trench for pouring, it would seem as though the depth was considered adequate.
You quoted a "minimum" of 1750 in your original post which is nearly three times the depth of the minimum "below grade" spec and over 4X the actual required concrete. W/O seeing it, surely sounds as though it well exceeds the design.
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My mistake. I should have said 175 mm, not 1750 mm. Trench was deep enough alright but in this trouble spot the concrete is 7 inches short of the top.
dpb wrote:

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Chad wrote:

You lost me now...if the trench was deep enough relative to the finished grade level, then unless the grade of the finished pour is wrong, there should be adequate material in the pour. But if the top of the foundation isn't the right height, nothing will work in constructing the structure on top correctly. I can't see a competent engineer doing an inspection before the pour letting that go by, nor should have the jurisdiction inspector after the pour.
Can you describe more precisely the geometry of the situation? If there is really only 6" of foundation at one area, that would seem quite shallow and I'm having a hard time figuring out how it could meet the requirement. Seems like it would have to have been dug into a hillside and the depth measured from the original ground level not the finished grade. If so, then that would seem to indicate the inspection wasn't up to par.
I think you need to talk to them about the situation directly and ask the question straight out and get an explanation of why it is adequate. Here, if you have an actual spec and can point out a deficiency to the City Engineer, he has the authority to halt construction until the defect is remedied or otherwise cleared by reanalysis/review as not a serious deficiency. If you have a formal contract, it should also provide you recourse that substandard work be corrected. I don't know what rules apply in your area, obviously.

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Hi,
The house is built into a hill. The trenches at the front are about 17 inches deep whereas at the back they are much deeper. Like I said the front trench looks about 7 /8 inches short of the top. There is no jurisdiction inspector. The only inspection that has been done was of the trench before the pour. The responsibility for anything going wrong after that is with the builder.
dpb wrote:

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Chad wrote:

...
Makes no sense to me...who/what was the "engineer" that did the previous inspection and how come he has no responsibility to see that the resulting pour would be adequate? As noted before, what was he inspecting if it did not include the height of the finished foundation in relationship to the bottom of the trench and the finished grade?
Who wrote the "specification" and in what document is it contained? What are the provisions for ensuring it? If the builder has the responsibility for "anything going wrong", then it would seem that all you need to do is point out there is an apparent problem. (But, I doubt a contract reads that way in actual wording.)
With whom do you have a contract and what does the contract say about resolving any dispute or question? (Which, of course, raises the question of did you have any legal counsel for review of the terms of the contract(s) before you signed them?)
Is there a lending institution involved? Undoubtedly they would be interested in knowing of any substandard construction and possibly could be of help in ensuring any defect were corrected before doing so became any _more_ expensive. Also, you'll need to work out any arrangements w/ them if you have any intent of using payment to require resolution before going on.
End the end, I ask again as you haven't responded to the question of -- Have you asked the builder directly the question of your concern? What does he say? And the engineer/architect/whoever you got the plans from/whoever else has been involved...
Do you have building permit, if required? Are you trying to do this avoiding getting one where it may have been optional? Did you not find out whether one was required and "just go ahead" as someone suggested in a totally separate thread recently? So many questions, so few answers....
I don't think anybody here (me included) can help any further. You've got to get busy and take the bit in your teeth and pursue this or just sit back and let things take their course and hope for the best...
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dpb wrote: ...

But, that aside, will be most interested to hear the resolution of the issue so keep us posted on what transpires.
Good luck with it...
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Thanks for your patience. You have some good suggestions. To respond to some of the questions.
The builder when approached about this denied there was any problem with shallowness. He also said that if we didn't trust him there was no point in he being there. The engineer was also unapproachable on the matter and reiterated his confidence in the foundations despite not having inspected the pour. The same engineer wrote the spec and yes we DO have a building permit.
We simply want to get a feeling for how shallow foundations can be as we really would prefer to take our chances and let things take their course. The prospect of a redig is truly offputting. Problem is I dont know if we're being neglifient in ignoring foundations which on some walls are only 7-8 inches deep. Got any gut feeling on this? Remember the ground was very good and steel mesh was used
Thanks.
dpb wrote:

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Chad wrote:

Impossible to say w/o much more information than is available. Would need basically enough information to do an independent analysis/evaluation of the design and site. Specifically, amongst a myriad of other details, is no indication of the loadings expected to be borne by the foundation and no real data on the ground or climate. Also you have indicated the depth varies drastically from one location to another so part of what might an alleviating factor would be if this particular section is very short in relation to all the rest. Still is a puzzle to my why, given the amount of excavation that must have gone on otherwise this is so shallow. I also wonder if the structure is built into the hillside and thus apparently the foundation is below grade, is that factored into the design for the rest of the structure?
All in all, my "gut feel" would be that 8" of concrete would be adequate to support a single-story residence, but would still have some trepidation on possible frost heave and so on if there were a significant section above a frost line of the finished elevation. There's another of those key elements that is unknown here -- if it is indeed going to be 5-6 ft or so below finished grade, it undoubtedly will be stable from frost, etc., and the only question is whether it is strong enough. But again, w/o more information than possible to know here, can't really even guess for your particular situation.
As for the builder, the "don't trust me?" ploy is a favorite of many to cow a client. It isn't a question of "trusting" or not, it's whether the job is up to snuff. Question is, how does he answer the question to demonstrate the work as performed meets the engineer's specifications to which he is supposed to be conforming?
As for the engineer, was this a one-time purchase of a set of plans or is he part of the project on a continuing basis? If the latter, I'd provide a written letter and photographs w/ measurements of the questionable areas and ask for written confirmation/approval of adequacy. Actually, I'd do the same even if not, and see what happened. I would also strongly consider finding another opinion before proceding w/ construction if I were concerned. Despite the unpleasantness of the action and possible ramifications of needing corrective action, it will certainly be far less of a pita to deal w/ now rather than down the road if a problem does occur because you let it slide.
There is no City Engineer or other agency who issued the building permit that can be consulted? That seems very strange, indeed.
Again, have you discussed this w/ the lender/legal counsel? (That's rhetorical ??, not necessarily prying).
Bob Morrison, if you're lurking, you got any other suggestions on how to follow up? I'm just a (non-structural/civil engineer here, spouting more or less platitudes). :) Obviously, can't tell OP whether his got an actual problem or not, and that's not my question.
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10 in stepdown (3 inches)-> ------------------------------------- | | 6 in (DUG) | | 8 in (Dipped) 12 in | | 9 in (Dipped)
Hi There has been further work done on this problem of the shallow foundation. The engineer requested that I dig along the trench in the trouble spots so he could later inspect. His position will be that if there is less that 9 inches of concrete then a remedy is required. In the course of digging two holes I was impressed by one of the depths I saw (15 inches). Above is the diagram of the front projecting wall of the study along with rough depths. Each vertical tick is 2 feet or less in length roughly. Where you see the six inches is where I dipped the cement on the night but also where I have dug a one foot wide square hole alongside the concrete. Through this hole I could observe that there was about 6 inches of concrete on top of a solid square rock about six inches deep itself. Combined depth of concrete and rock is 12 inches. This was the last bit of trench filling done on a very slow job (10 hours) and is just below a 3 inch step. It is quite possible the builder ran out of time.
Now I would like to know the best remedy or if one is even required. There is talk of building a lean mix concrete bridge over this shallow corner instead of dead building. I am not familiar with this. Will it bind well and will it improve the foundations or is it possible it could make them worse. Is a more drastic remedy required. There was also talk of cutting and dowelling concrete or drilling in steel. I go to the internet for help as my engineer still has not come out to inspect this despite many phone calls.
Thanks.
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Chad wrote:

Was wondering what transpired... :)
Not sure I fully understand again, particularly I don't know what you mean by "Where you see the six inches is where I dipped the cement on the night....". Don't know what to say in response since I don't know what you mean.]
It would seem there's at least a short section around that corner that is well under the engineer's specification for depth. If the rock is only a single rock of 6" square and 6" deep, that would not in my mind count as anything other than soil. If, otoh, it's part of an actual ledge and that's why the trench was shallow, that's something else again and might justify there being no concern.
At this point, given that this is only the end of the first week in January after the holidays, I'd probably give the engineer a little slack on getting out this past week but request resolution in a timely fashion. If you have done all the site preparation for his inspection, I'd use the formal letter to the office notifying them of that and requesting their response. Usually that will work.
I think unless I couldn't get a response from that after a reasonable time I would expect them to come up w/ the solution or approval. Again, you need to be formal in this so he's required put his name on the line where it's traceable.
I go back again to the question of where's the city or building permit folks in all this?
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Thanks dbp

To answer your last question, in our jurisdiction their is no concept of a building inspector of that sort...

On the night of pouring I was able to dip the corner in question while the cement was still wet and I came up with the readings you see of 6, 8 and 9 inches.

I wouldn't think there is a huge ledge there but certainly the rock is longer and wider than 12 inches. Put this way it was someting which didnt come up with the digger which the engineer asked them to dig out but which it seems they decided to leave in.
I know I'm jumping the gun but I'm worried about it, any thoughts on possible remedies?
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I measured the ledge a bit further. It runs for at least 3 ft along the wall and goes in to at least the middle of the trench based on having dipped over it. It is 5 inches deep I was able to measure it at its outside because its edge coincides with the edge of the trench. ie coincidentally it ends where the side of the concrete ends. If it wasnt so smooth it could easily be mistaken for concrete. Seems substantial.
Chad wrote:

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Chad wrote:

...
That may make it ok, my first guess would be the engineer will probably decide so, particularly if he can determine that it is actually tied to more and not just a single rock, albeit a large one.
Ideally, the builder/excavator should have gotten approval from the engineer to either pour as he did or for a on-site fix/workaround before completing the pour, but that, of course, is concrete already in the dam. :( But, imo, that does put the onus back on him to make any necessary corrections.
My recommendation still is to notify the engineer's office officially of you're having the site ready for the followup inspection and requesting approval or instructions/plans for whatever rework/additonal work is required. If you were to want to be passive-agressive about it, you could always cc: the builder, lending institution and your counsel on the letter. :)
I'm still a little unsure about the overal depth of this with respect to frost line and the finished grade, but that would/should be a part of the engineer's inspection/approval, not just the depth of the foundation pour itself.
One last point I may have over-belabored...are you absolutely sure there's no equivalent of the building inspector or are you relying on word-of-mouth of friends or "they say"? It's very unusual in my experience for a locality to require permits but to have absolutely no enforcement whatsoever, so I'm only suggesting you make sure you know actual ordinances, etc., not just rely on conventional wisdom...
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You've been great about this. Yes I'm sure about building inspections. In fact somebody from a nearby jurisdiction which does have strict building control inspections commented on how lax our system here is. I suppose thats why I went running around trying to do it.
Don't worry about frost line as there will be a lot of dead building at this spot. It is the lowest part of excavation. Im sure they would have let the builder pour over it. But he definitely should have moved his step down to a later point and poured 10 inches over it instead of 6 or 7.
The engineer has been informed and I am disappointed that three days later he has still not examined this hole. Now the builder may arrive before he does. I will follow up with an e-mail but am still anxious until I get a response.
More later hopefully.
dpb wrote:

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Chad wrote:

...
While I'm sure you're antsy, three days isn't that long especially when you take in to account the time of year. I'm sure this must be a small job in their business and while it's a big job for you, to expect him to rush out is, while understandable, not surprising he hasn't.
I would try very diligently to communicate to the builder and the engineer both here so that you don't have the builder show up until you have approval. If you haven't told the builder you want/have asked for an approval before continuing, best do so.
I'm still amazed any locality would have requirements for permits assuming that those permits require design details, etc., and have no provision for enforcement. If the permit is only a tax with no other requirement, that I could see. May I ask where this is?
I'm outside nearest city 3-mile jurisdicitional limit where they city can require compliance and there isn't but a health ordinance in the county so don't have to file anything other than the septic/sewer plans. They would like permits to be filed, but can't require it. Something like that I can see, but the requirement to file the permit and then have nothing seems bizarre! :)
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