House Settling 3+ inches
House Settling 3+ inches
(OP)
Looking for a little help or opinion here. My brother has a slab on grade house that has "settled" over 3 inches during the past year. It began in the interior of the house and has, just over the past couple of months, begun to settle in other areas (ie front porch, sidewalk, etc) Insurance co. is pretty much saying it is not a sinkhole (he has sinkhole insurance). Borings were not done directly over the worst areas as they are in the interior of the house. A boring was done down to around 15 feet to bedrock just outside of the house. Not surprisingly, the low bidder did the work. And we're not talking low bidder by 1k or 2k, but by several thousand (red flag in our minds). My question, being totally ignorant of soil mechanics/dynamics among other things, is this (well several, actually): What is a proper test to determine whether or not the problem is a sinkhole or not? Is 3+" of "settling" realistic" House is 15+ years old, one level, located just south of B'ham, AL. It was a very dry summer last year. We were probably 16-20 inches short normal rainfall. Anyway, I'm probably not providing enough of the puzzle, so I apologize in advance. Any advice is appreciated. Thanks.






RE: House Settling 3+ inches
RE: House Settling 3+ inches
This is a complicated question that is incredibly difficult to answer without seeing the site.
For example, my mother in law had similar problems but as I was in a different country I had to rely on her local expert to give her advice. Even though I had been involved in this type of work previously I was no use to her at all without seeing the site.
Have they had a structural/civil/geotechnical engineer investigate the site? If not then who specified the fix? What did they recommend?
If it started on the inside of the house then I would supect that it could be a pipe leaking under the foundations. Was a pipe survey carried out?
As it is after a dry period, it may not actually be that your house is sinking, but that the surrounding soil is swelling again as it has absorbed moisture(and the soil under the footing is still dry).
I would specifically recommend against supporting only part of the house on the bedrock if this is what is planned. This can cause even worse problems if the remainder is still on soil and continues to settle.
It is a pity that it is so far along, because I would really recommend that your brother doesnt get the work done until someone knowledgable is sure that it will fix the problem.
RE: House Settling 3+ inches
csd72,
I'm going from bad memory on some of this, so I may not have the answers, but here goes:
1) An engineer did go out to the site and did some work and a report was created (nice and vague, huh?)
2) A fix, to the best of my knowledge, has not been discussed in-depth (no pun intended)
3) I don't remember hearing anything about a pipe survey or the possibility of a leaking pipe. Good point.
4) The problem began right in the middle of the drought, not afterward (this is when most of the settling occured)
You're absolutely right about the eventual fix. Whatever the fix is (helical piles, grout)etc, it will be done right. No partial support will be tolerated.
Thanks.
RE: House Settling 3+ inches
there's some geophysical explorations that can be used to look for karst formations but it won't be real cheap and it may not yield any answers. i'm not sure how that kind of insurance works but if it does provide indication of sinkholes, i'm guessing it'd pay for itself. i can pass along some information if interested. as far as drilling to identify such conditions, sometimes it's a coin toss as to whether you obtain relevant information.
RE: House Settling 3+ inches
All you should be concerned with is differential settlement. Roads routinely settle or rise far more than that without a problem because there is relaitvely no differential settlement. Your brother needs a thorough professional analaysis of the site to determine the cause and what the possiblr remediation measures are.
RE: House Settling 3+ inches
RE: House Settling 3+ inches
The lowered water table is highly suspect to me. Has the area seen a draught of this magnitude in the past 15 years?
Mike McCann
McCann Engineering
RE: House Settling 3+ inches
I've carried out a number of building investigations for problems of this kind, but have never seen a settlement of 3" (75mm) in a slab on grade. (in Ireland, we don't experience the same degree of variation in our Climate - yet!) The settlements I've seen, of slabs on grade, have generally been due to poor compaction of the hardcore beneath.
Reading between the lines, it seems that the house foundations have not shown the same signs of movement - is this the case? This also suggest that the slab movement is due to soil shrinkage - more pronounced in upper soil layers. How deep are the founds?
If a sinkhole were to develop, I'd expect to see signs of foundation movement - e.g. cracking in supporting walls.
Check out the attached reports.
RE: House Settling 3+ inches
Mike, I tried to post in geotech forum, but somehow it has been deleted. Not happy about that. Last summer was pretty bad, but I would be surprised if it was that order of magnitude worse than any other year over the past 15. Of course, maybe it was that little extra dryness that did it. Everything's decided at the margins, huh?
renko, I believe it is correct that the foundations are not seeing movement. I need to double-check that, however. Couldn't speak to foundation depth, though I'd guess less than 24 inches around the edges. Fortunately, not much problems with frost line down here.
Thanks again.
RE: House Settling 3+ inches
It sounds to me like something has happened recently to cause this, something along the lines of a leaking pipe.
You generally dont get a stable building start to subside without a root cause.
But without knowing what the report said, it is impossible to make an informed comment.
Can you get a photocopy of the report, delete out all specifics on company, adress e.t.c. and then post it on the forum. We then can hopefully make a more informed comment.
RE: House Settling 3+ inches
RE: House Settling 3+ inches
I'll see what I can do about posting the report.
renko,
Thanks for the link.
By the way, why would my post have been deleted over at the geotech forum? Is that improper or something? It was there for a short while after I reworded the subject line, then it disappeared. Without an explanation, that made me a bit angry.
Thanks again.
RE: House Settling 3+ inches
If it appears that your post is for a personal project rather than a professional problem, the post stands a high likelihood of being deleted in most any forum here. Justr follow the lead of csd72 and it should remain.
Mike McCann
McCann Engineering
RE: House Settling 3+ inches
Everyone here has been very helpful, and I appreciate it. I'll see if I can get that report so I'm "legal"
RE: House Settling 3+ inches
RE: House Settling 3+ inches
Try rewording and changing the layout of the post from the beginning then, and see if that flies, with csd72's recommendations too.
Mike McCann
McCann Engineering
RE: House Settling 3+ inches
I had agreed with some others here that the first thing I would consider was shrinkage of moisture sensitive clay. Are there large trees around which could have stolen the moisture?
But my opinion was based on the house being on a slab foundation, rather than with separate footings and infill slabs. Settlement of the slab relative to the footings would tend more to the idea of loss of ground under the slab. Where it could have gone is not for guessing, but for investigation.
RE: House Settling 3+ inches
Technically this would rule out mechanical engineers asking for advice about their personal structural problems.
Personally I don't have an issue with your question and the moderators for this structural forum seem to feel the same way; our geotech friends may not agree.
RE: House Settling 3+ inches
There are a couple of trees of size around, but assuming a root ball equal in size to the spread of the tree, they don't come within ten+ feet of the house. I'm curious what trees can do in a situation like this. Please explain.
As to the construction, and this is prime example of my limited knowledge and info, I believe it is a slab foundation, and not separate footings with infill slabs. My response to renko probably caused the confusion here. Looking at typical construction drawings for slab-on-grade, I notice that the perimeter is thicker than the main interior slab. This is the region I was referring to as not being more than 24 inches below grade. I guess it's possible that it could be a 4" (or whatever) thick slab everywhere. The sample drawings I've seen had the "integral" footings.
Thanks.
RE: House Settling 3+ inches
I certainly appreciate the tolerance shown in this forum
RE: House Settling 3+ inches
RE: House Settling 3+ inches
RE: House Settling 3+ inches
From what I gather, and hopefully I'm wrong, only a couple or three borings were done. Is this adequate?
Thanks again, everyone.
RE: House Settling 3+ inches
Yea, Geo's are a dirty bunch. I ought to know as my brother-in-law is one, and a geologist to boot.
It was kind of shocking when my other brother-in-law got his PE as an EE.
Answering your question though, I would do one or two borings on each side of the structure where the settlement was observable - enough to do an underground map of any bad material encountered. Don't have a contractor do it - use a licensed Geo(tech).
Another scenario here too - if the water table has remained high for all those years and was recently lowered, then, in addition to the clay scenario, if the house was built on fill with a lot of wood and vegetation intermixed, the high water table could have prevented the wood/plant material from breaking down until the water table lowered. Remember that piles generally only rot above the low water mark where they are subject to a higher concentration of oxygen. If they are always immersed in water, they should not rot.
All the more reason to find out exactly what's down there. There is a logical reason.
Mike McCann
McCann Engineering