Sinking yard -- what could cause it?

I live in a four-family building of townhomes, less than five years old. The yard in front of the building has been sinking for some time, two or three years. The sinking runs a line from one sewer clean-out valve at the side of the building, down that side, turning a corner under the driveway (which has cracked) into the yard to the next sewer cleanout valve, and out from there. About three feet out from there is a small plum tree that his now sitting in a sinkhole, it's probably 8 to 10" lower than the ground around it. Besides this sinking, there is an area between that and the homes, right in front of a decorative rockbed, which seems to be sinking in the middle, forming almost a saucer-look to it. That part is almost exactly behind the tree and about four or five feet back.
This has been noticed with concern by the lawn people, and most recently two sewer companies who came to fix a backup, because they said it can be indicative of a break in the sewer line. However, they dropped a camera down the lines and found no breaks.
Both my neighbor and myself are getting concerned because it continues to get worse. There are no basements, and when my carpet was pulled up recently, I found a crack in the concrete extending from one end of the unit to the other -- the builder tells me this is not a big deal, all concrete buckles, and as long as the crack is going in one direction and not spidering off, and as long as the crack is less than 1/4" wide and both sides are level, it's nothing to be concerned about. The sidewalk has also formed a horizontal crack, and the slabs are becoming quite uneven.
I realize that normal settling occurs after building, but this seems extreme. Other than a sewer line break which has been ruled out, what could cause ground sinking like this?
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Mother Nature likes soil to have about an 88 Protctor density.
The soil under commercial construction is usually required to be a "select" material. This material will have specific properties that allow it to be compacted. Select material will be low in clay content and no loam or biodegradables. Typical compaction required on commercial work will tend toward 95 Proctor density.
These requirements seldom are considered or required for residential work. Consider digging a trench across old, well settled soil. You cannot put all the dirt back in the ditch without using tamping equipment. After a year of rain, freeze/thaw, you can usually see ditch lines where Mother Nature has finally regained her normal compaction. This is also true of dirt outside basement excavations and the loose fill shoved up behind a curb on which the sidewalk is poured. If these areas have not had mechanical compaction as they are placed they will settle.
Perhaps your area was "filled in" land that is just now settling. Another possibility would be a break in a storm or sanitary sewer that would allow the slow erosion of the subsoil. You have eliminated the sanitary, but you may look into municipal or other storm drainage. Other possibilities are buried trash. We had one reoccurring sink hole in which we dumped multiple loads of dirt. When I finally dug it up, we found the last remnants of a Model A at about 15 feet. I suspect an old well as there were many glass bottles also.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Keep the whole world singing. . . . DanG
"me" <no.spam.please> wrote in message

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me wrote:

I'll bet money the sewer cleaners will be back to clear another blockage. (I'm thinking that the sewer has sunk too and now has a "belly".)
It's certainly suspicious that the sinking appears to follow the sewer line. I wouldn't entirely rule out that they missed a break with the camera. Another possibility is water service line; they often lay the water line in a trench near the sewer (but not next to). Even a small leak could erode a lot of earth over several years. The utility might help in determining if there is a leak.
Could be natural causes too, like an underground spring or an abandoned well, septic tank.
Whose responsibility is this in a townhouse development? Jim
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Dan did a good job of explaining it, but I will try as well.
The people who built the building and did the excavation work did not do it right. They did not back fill properly. There are two fixes. Wait until it quits moving and then repair any damage. Fix it now by re-excavating much of it and this time back filling properly.
I chose the second method and took my builder to court. On the other hand the first method is likely to be cheaper, as look as you are sure it has finished compacting. Depending on local conditions this could take years longer or it could be done. In my experience a couple of really wet years usually settles things nicely.
There is also an odd chance that there is a sewer problem. Not likely in my opinion.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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<< I live in a four-family building of townhomes, less than five years old. The yard in front of the building has been sinking for some time, two or three years. >>
Your real concern right now is getting out of the situation with as small a financial loss as possible. If you are a renter, then you haven't as much a stake in the outcome, but if you have any equity in the structure it will soon become a real money pit. This building must have been slapped together with other money saving techniques. The sinking is only the first short cut to become obvious. Predictably, there will be others even nastier. Have a critical look around and then decide whether to stay or go. Good luck.
Joe
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Uncompacted fill, decomposing materials in the fill, underground caverns and sink holes, giant mole people from Neptune, take your pick. You'd need a backhoe to examine it, or pour more fill on top until it stops settling.
Jeff
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Jeff Cochran wrote:

Your last idea is REALLY stupid advice if the problem is a true sinkhole. The next to last idea could be just as bad.
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Sporkman wrote:

While it may be a sinkhole, in most parts of the world Jeff's idea is much more likely.
--
Joseph E. Meehan

26 + 6 = 1 It's Irish Math
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Joseph Meehan wrote:

I grew up in a community which made its living from coal mines. Occasionally somebody would find their basement floor a lot lower than on the previous visit because the timbers holding up the mine roof under the house rotted and collapsed.
I now live near a community named "Mine Hill" because they used to dig for iron ore. Somebody recently had a shaft collapse which damaged their yard or their house and then complained that they had no idea there were mines in the area.
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me wrote:

if you dont own the property dont worry about it.. the owners never worried about it when they had it built.. the ground was probably filled with earth that kept rotting and never settled and its still trying to settle... in my area you can only use fill that will not rot when filling lots..... that is what they should have done in your area and probably never did.. its too late now....
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the
driveway
out
his
in
exactly
two
to
unit
concrete
also
could
Unfortunately, I do own the townhome, and being in an association, do own the yard (common area) along with the rest of the homeowners. I suspect that unless there is reasonable cause to think there could be future structural damage (and who makes that decision) or other issues, digging up the yard and refilling isn't something the association is going to want to spend $$ on (and would probably have to special assess if they did0. If it is settling, I wish I knew when it was going to stop.
Thanks for all the responses.
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"me." <no.spam.please> wrote:

The general rule is that fill dirt sinks for 7 years
--
Free men own guns, slaves don't
www.geocities.com/CapitolHill/5357/
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me wrote:

1. The builder dug a sewer line, as much as five or more feet deep. 2. He put in the sewer pipe. 3. He filled the trench with leaves and dead frogs. 4. He put sod on top.
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WHERE ARE YOU? If you're in an area of Karst (soluble limestone) you could be seeing a literal cave-in . . . the gradual type. Sinkholes abound in certain areas of the country and are quite possible in several widespread regions of the country.
me wrote:

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if a tree has been removed or tree roots removed...as the roots/stump rot and decompose they shrink ...this can cause the sinking of the ground?
"me" <no.spam.please> wrote in message

the
out
in
exactly
two
unit
also
could
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On Sat, 17 Jul 2004 07:58:18 -0500, someone wrote:

Well since the sinking is in a line following the pipe, the 99.9% answer is that the fill on top of the pipe was not compacted vey well and is settling. Not a big deal, just level it.
-v.
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