House Settling 3+ inches 1

[caravoy]

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.

 

[caravoy]

snubbed by the geos, huh? Man, that hurts. Tough crowd. Hard as rock (rim shot, then come the boos)

I certainly appreciate the tolerance shown in this forum

 

[cvg]

tree roots can be up to 50% larger than the spread of the tree. Cottonwood roots can easily extend 50 feet or more and intrude under foundations, into swimming pools, sewerlines etc. The can absorb and evapotranspirate an enormous amount of water from the ground, from a leaking pipe etc.

 

[hokie66]

It depends on the species, but some trees can be very thirsty and spread their roots much farther than the crown. I am on the other side of the world from you, but it is unfortunately a common occurrence here in drought for houses of rigid construction to suffer distress due to volume change of the underlying clay material. Testing of the soil should determine if the material is moisture sensitive or not, and to what degree.

 

[caravoy]

Well, I've certainly received a lot of great information from you all. I sincerely appreciate it. Could you give me some idea of what would be a proper testing methodology for this situation. That may be a loaded question, but I'm looking for info like # of test borings, depth of borings, layout of borings in relation to the house, etc. as well as maybe an expected order of magnitude $ amount that something like this would run.
From what I gather, and hopefully I'm wrong, only a couple or three borings were done. Is this adequate?

Thanks again, everyone.

 

[msquared48]

Caravoy:

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