Cob: Foundation Strength Relative to Soil Type

[Kim West]

A while back I made a post relating the clay content of our soil here. A nice fella, I do not remember who it was, wrote to me that there is no way I can know the make-up of our soil without lab tests. Yesterday I found a book that I knew we had, yet did not know where it was. It is entitled "Soil Survey, Ouachita County Arkansas". After a bit of time studying the maps and information in the book, I now have a generic lab test result for our soil. Granted, it was not taken directly from this property, but it was a sample of this soil type.

I found that our land is classified as Kirvin fine sandy loam. When this book was published, the classes of soil were standard country-wide. That was in 1973 so things may have changed somewhat, although the description would not change with the class name if indeed the class name has changed.

=======================================================================================
What follows are some of the properties of Kirvin soil:

Depth to seasonal high water table: 30-60 in.
Depth from surface of typical profile:
    0-9 in......fine sandy loam........AASHO A-2 or A-4.........30-40% passing #200 sieve...shrink/swell low
    9-55 in....red clay, plastic.......AASHO A-7...................75-95% passing #200 sieve...shrink/swell moderate
    55-73.....silty clay loam..........AASHO A-4 or A-6..........60-75% passing #200 sieve...shrink/swell low

Kirvin soil limitations relative to foundations of dwellings: Moderate if slopes are less than 15%; moderate bearing strength; moderate shrink/swell potential; severe if slope greater than 15%.

As relates to septic systems: severe-slow percolation.

========================================================================================

Firstly, let me say that the depth to seasonal high water is no problem. We have lived here for nearly 13 years, through near-drought and through high precip years. Where I plan to build is atop a hill, the highest point on the property, and water has never been a problem, even during floods and/or high precip years.

Secondly, The clay content given above pretty much agrees with the jar tests that I did. I had said that 6-12 inches down is pure red clay. The lab tests say that 9 inches down is 75-95% non-sand soil. This hypothetically could contain silt, but there are not three layers in my clay samples, only two. After the gentleman drew my attention to the need for a lab test, I did get a magnifying glass and do another examination using it. I found that under magnification the fine sand content that wasn't visible to my naked eye was indeed there. I owe him, whoever he was, a thank you for influencing me to back up and re-examine my samples. Had I not come across this soil survey I may have continued erroneously thinking the clay was pure. Thank you!

Anyway, the reason I posted this is because of the previous discussions we have had on rubble trench foundations, and the fact that they have been around a while and work very well. I'm wondering if part of the success of those foundations was because they were set on stable, weight bearing soils. Above, it says the foundation limitations here are only moderate, meaning they can be overcome with good planning, so long as the slope is less than 15%. There is no problem here with that. Where I planned to build does indeed have a slope of less than 15%. My concerns, though, is the A-7 AASHO rating of the soil at 9-55 inches deep. A-1 is gravelly soil with a high bearing strength, and the scale goes down to A-7 which is clay with little strength if wet. If I built on top of the ground I would be working with an A-2 rating which is not bad, but once I dig down for the foundation I will be in A-7 soil which is the worst for weight bearing. I know I've been told "tamp, tamp, tamp...and good drainage is a must", and I know that is a very important part of building, but I am not *completely* comfortable that would be sufficient for the near-pure clay soil that is under the shallow topsoil. Somewhere I read that lime can be used to stabilize clay soils, and I am wondering if anyone here knows how I can use lime to stabilize this soil. I really would HATE to put out alot of work to have my home fall down due to a lack of bearing strength under the foundation! LOL! If anyone here has any info on how I can stabilize my soil with lime, please, please let me know.   :D

Kim
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<DIV><FONT face=Arial size=2>A while back I made a post relating the clay 
content of our soil here. A nice fella, I do not remember who it was, wrote to 
me that there is no way I can know the make-up of our soil without lab tests. 
Yesterday I found a book that I knew we had, yet did not know where it was. It 
is entitled "Soil Survey, Ouachita County Arkansas". After a bit of time 
studying the maps and information in the book, I now have a generic lab test 
result for our soil. Granted, it was not taken directly from this property, but 
it was a sample of this soil type.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>I found that our land is classified as Kirvin fine 
sandy loam. When this book was published, the classes of soil were standard 
country-wide. That was in 1973 so things may have changed somewhat, although the 
description would not change with the class name if indeed the class name has 
changed.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial 
size=2>=======================================================================================</FONT></DIV>
<DIV><FONT face=Arial size=2>What follows are some of the properties of Kirvin 
soil:</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Depth to seasonal high water table: 30-60 
in.</FONT></DIV>
<DIV><FONT face=Arial size=2>Depth from surface of typical profile:</FONT></DIV>
<DIV><FONT face=Arial size=2>    0-9 in......fine sandy 
loam........AASHO A-2 or A-4.........30-40% passing #200 sieve...shrink/swell 
low</FONT></DIV>
<DIV><FONT face=Arial size=2>    9-55 in....red clay, 
plastic.......AASHO A-7...................75-95% passing #200 
sieve...shrink/swell moderate</FONT></DIV>
<DIV><FONT face=Arial size=2>    55-73.....silty clay 
loam..........AASHO A-4 or A-6..........60-75% passing #200 sieve...shrink/swell 
low</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Kirvin soil limitations relative to foundations of 
dwellings: Moderate if slopes are less than 15%; moderate bearing strength; 
moderate shrink/swell potential; severe if slope greater than 15%.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>As relates to septic systems: severe-slow 
percolation.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial 
size=2>========================================================================================</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Firstly, let me say that the depth to seasonal high 
water is no problem. We have lived here for nearly 13 years, through 
near-drought and through high precip years. Where I plan to build is atop a 
hill, the highest point on the property, and water has never been a 
problem, even during floods and/or high precip years.</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Secondly, The clay content given above pretty much 
agrees with the jar tests that I did. I had said that 6-12 inches down is 
pure red clay. The lab tests say that 9 inches down is 75-95% non-sand soil. 
This hypothetically could contain silt, but there are not three layers in my 
clay samples, only two. After the gentleman drew my attention to the need for a 
lab test, I did get a magnifying glass and do another examination using it. I 
found that under magnification the fine sand content that wasn't visible to my 
naked eye was indeed there. I owe him, whoever he was, a thank you for 
influencing me to back up and re-examine my samples. Had I not come across this 
soil survey I may have continued erroneously thinking the clay was pure. Thank 
you!</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Anyway, the reason I posted this is because of the 
previous discussions we have had on rubble trench foundations, and the fact that 
they have been around a while and work very well. I'm wondering if part of the 
success of those foundations was because they were set on stable, weight bearing 
soils. Above, it says the foundation limitations here are only moderate, meaning 
they can be overcome with good planning, so long as the slope is less than 15%. 
There is no problem here with that. Where I planned to build does indeed have a 
slope of less than 15%. My concerns, though, is the A-7 AASHO rating of the 
soil at 9-55 inches deep. A-1 is gravelly soil with a high bearing 
strength, and the scale goes down to A-7 which is clay with little strength if 
wet. If I built on top of the ground I would be working with an A-2 rating which 
is not bad, but once I dig down for the foundation I will be in A-7 soil which 
is the worst for weight bearing. I know I've been told "tamp, tamp, tamp...and 
good drainage is a must", and I know that is a very important part of 
building, but I am not *completely* comfortable that would be sufficient 
for the near-pure clay soil that is under the shallow topsoil. 
Somewhere I read that lime can be used to stabilize clay soils, and I am 
wondering if anyone here knows how I can use lime to stabilize this soil. I 
really would HATE to put out alot of work to have my home fall down due to a 
lack of bearing strength under the foundation! LOL! If anyone here has any 
info on how I can stabilize my soil with lime, please, please let me 
know.   :D</FONT></DIV>
<DIV><FONT face=Arial size=2></FONT> </DIV>
<DIV><FONT face=Arial size=2>Kim</FONT></DIV></BODY></HTML>