house settling 3= inches

huangyhg

house settling 3+ inches
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.

a geothech engineer would be able to answer this better, but i'm wiling to bet that it's primarily due to the drought.
caravoy,
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.
frv, thanks i've cross-posted on the geotech forum.  the more the merrier
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)
i pretty sure that's a karstic area (geologic conditions solutionize leaving voids--sinkholes) but i've never personally worked there. in my opinion, one boring to 15' isn't going to tell you much about karst formations that might (or might not) exist. i don't deal with karst geologies much so i'll leave detailed discussion about it to those that work a fair amount in such conditions. you might check with neighbors to see if they are experiencing similar conditions. it might be sinkholes trying to form or it could be shotty construction or poor foundation conditions.
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.
you do not always assume that a structure has settled when there is a problem.
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.
yea, obviously you're going to need a professional out there, but from a vague internet point of view and from living in a karst area my initial thoughts point to a lack of soil compaction.  could have been a large area that should have been undercut.  second possibility could be fat clays that are drying out and shrinking.  third and least likely would be a sinkhole since sinkhole formation accelerates as more water is put into it.  since you're in a drought, i'd doubt its that.
might get some more ideas by posting this thread in the geotechnical area under foundation engineering.
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
if the house is 15 years old and the problem has only appeared in the last year, it does suggest that it has been caused by some occurence during or just prior to the last year.  as has been suggested this is most likely due to the drought and subsequent 'shrinking' on the subsoil.
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.
thanks for all your replies.  
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.
caravoy,
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.