Sinkhole activity and organic laden sand/muck 1

[Bill1962]

I live in Palm Harbor, Florida and have recently had a ground subsidence study performed prior to beginning an extension project.
Thet did soil samples with hand auger borings, 2 deep Stendard Penetration Tests and a GPR ( ground penetrating radar).
Findings were: sinkhole activity and burried oraganic laden sand/muck materials with organic contents varying from 15% to 52% by dry weight.
Recommendations: a grouting program estimated by the contractor at between $67,000 and $83,000.

The report notes that "subsurface grouting will not mitigate the sand/ muck problem and that subsequent distress could occur ( and likely will) even after a properly implemented grouting program has been completed."

My question: how serious is this? Do I continue to live here? How significantly does this affect my property value?
Will a lender finance this house? What shold I do? Do I need to speak with an attorney for possible claim against the builder? HELP!

Bill Lichter
Palm Harbor, Florida
727-787-1031

 

[Ron]

Bill,
I'm not sure how large your residence is, but the muck problem is likely worse than the sinkhole subsidence issue. It certainly has a higher probability of being an issue with your house, though you are in a known, active sinkhole area.

It sounds as though you have had a geotechnical investigation done that was carried a bit further than typical, since you had the GPR done. Did they estimate the anticipated settlements? Has there been any fill placed on the site? If so, how much? Is this a one-story structure? Was the investigation done for you or your contractor?

I would suggest you go back to your geotechnical engineer and ask him to get a bit more specific in his evaluation/recommendations. If that is not successful, then perhaps you should approach your builder and offer a buy-back before you pursue litigation.

Start doing your homework. If you pursue litigation, it will be expensive, so try to do as much of the grunt work yourself as possible. Look at your contract with the builder. They often exclude things such as this from their liability.

Good luck.
Ron

 

[dirtsqueezer]

Bill,

I didn't see this dealt with above, but if you're thinking about spending that kind of money, you ought to price how much driven piles would cost. It may cost more than your estimation for grout, but it would definately last longer, and help to secure your property value.
I know a family that had this done, and I could find out what it cost them, if you are interested in that route.

-Jeff

 

[Ron]

Jeff...good idea and I would normally agree, except that his post implies they found a karst structure prone to sinkhole activity. The sinkhole usually develops from one of two scenarios....(1) there is ravelling of overlying sands or other materials through a hole in the top of the limestone into a tenuously domed cavity, or (2) the top of the dome gives way and the overburden drops in.

Driving piles into this type of structure can create problems with the poor quality limestone in his area. It tends to have many solution cavities and the top surface of the limestone is at various subsurface depths and has an irregular profile.

Another issue Bill has to deal with is disclosure. He is required to disclose items such as this for real estate transactions, so he is in a position that he must take some action to mitigate or seek remedy from his builder. Neither is a great choice.

 

[Bill1962]

Someone today told me that grouting " does not work," and that I should do "Underpinning" which is guaranteed for 5 years and will protect the property value.
Van anyone comment on this?
Bill

 

[Ron]

Bill,
Underpinning will likely be required but it will not address slab displacement anywhere in the middle of the house. You will likely have to have a remediation scheme that addresses both. Yes, underpinning is feasible and common in your area.

 

[TJWATKINSPG]

RUNNING A GPR IS THE USUAL IN PINELLAS COUNTY. MANY GEOTECH'S AROUND HERE USE IT AND HAND AUGERS TO DISPROVE SINKHOLE CLAIMS FROM HOME OWNERS. THEY IGNORE THAT CLAY DISPERSES THE SIGNAL. THEY IGNORE THAT THE WATER TABLE CONTRIBUTES TO SIGNAL LOSS. THE STRATIGRAPHY OF SLOW SUBSIDECE SINKS INCLUDE LENSES OF ORGANICS AS PART OF THE SINKHOLE STRUCTURE. (SEE THE BOREHOLE LOG FOR THE REPLACEMENT OF ROMP WELL WHERE PEAT WAS FOUND AT 200 FEET BLS IN WEST TAMPA. UNDER SAND. NO LIMESTONE. THIS IS IN THE FILES AT SWFWMUD.) WHAT BILL DESCRIBED WAS A MODERATELY GOOD ENGINEERING REPORT. THEY DID NOT STOP AT FIST CLAYEY SAND AND LABEL IT SHINK SWELL, THEY DID NOT EVEN STOP AT THE FIRST ORGANIC LAYER. GOOD JOB GUYS IN FINDING THE WHOLE ANSWER.

 

[dynomike12]

The problem with underpinning is that most of the underpinning contractors make their own system in their "backyard". The will only guarantee the material and make it sound scientific but it is only SCH 40 or 80 steel pipe you can buy at Lowe’s. Most systems use the structure for a reaction to hydraulically push the pin pile into the ground. If you advance the piles into the ground all at once you would achieve a FS approx 1. If you advance each one separately before lifting the structure the footing may be over stressed and crack at that point. Even this does not achieve high FS. There are systems out there that are actually engineered (i.e. chance helical piles utilized an empirical relationship relating torque to bearing capacity) but they are a little more expensive. No contractor that I know of will guarantee the effectiveness of their system. They will guarantee only that it was installed properly and the pile material will not fail. They will say that a guarantee cannot be made due to latent or concealed conditions which may exist. Underpinning is usually only effective if it is advanced to hard strata (rock) when sinkhole activity exists. Given the muck/sand that exists there is nothing very stable to transfer the load to (with in a reasonable installation depth).

Grouting is probably you best bet in muck/sand. Be sure they don’t do too high pressures. Sometime the contractor will do this because they get paid by the yard. This can lift the house and cause a lot of collateral damage. Hopefully it will displace the muck and density the sand making your home more stable. However, like the engineer said, “there is a chance it may not work”.

The only other remedial action I may suggest in addition to grouting is chemical injection to upper surface soils. Grouting is typically terminated 10 ft below ground surface if most bearing of the foundation is 3B below the footing (B-footing width). Many times these soils can density. Chemical injection in the 10 ft would help this as wall as alleviate further raveling activity in this critical zone.

 

[Mccoy]

Sounds like a joke!!!
After "a properly implemented grouting program" of the moderate cost of US$ 67000 to 83000, some "distress" will nonetheless be likely to occur!!!
Which is the depth to the buried, karstified limestone formation?
Is there any way to ascertain with reasonable expense the distribution and location of major cavities, if they exist at all underneath the structure?? and tranfer the structure load to the bedrock without the problems previously mentioned by Ron?

 

[Ron]

Mccoy....I doubt this is a joke. Happens a lot in that area of Florida in about a 10-county area. Underlying limestone strata are so erratic it is difficult to plot cavities and even more difficult to determine their volume. It if fairly common for original grouting volume estimates to be off by 100 percent!

 

[Mccoy]

Mmmmmmm,
then we have a real "bad karma" situation, I fully appreciate now Bill's concern!