Re: What tests are done to soil to determine weight bearing capacity?

Hello, Alan:
You picked a good one.  Soils are both the simpliest and most complicated 
engineering materials there are!

So many things affect the ability of a soil to support a load -- type of 
soil, particle size, gradation of particles, particle shape, moisture 
content (think mud), and air content are just a few. This Wikipedia 
article is a pretty basic explanation.
  http://en.wikipedia.org/wiki/Soil_mechanics
And other sites:
  http://en.wikipedia.org/wiki/Geotechnical_engineering
  http://www.geoengineer.org/

Also, if you can, locate a copy of Soil Mechanics in Engineering practice 
by Terzaghi and Peck. It's a college-level textbook, but the beginning 
chapters have a simple discussion of soils and soil mechanics. Dr. Karl 
Terzaghi is considered the father of modern soil mechanics.

What you are doing is closer to soil science than soil testing.  You are 
not standing on a construction site in muddy boots trying to determine how 
big the foundation needs to be-- you are trying to get consistent results 
on specific kinds of soils. Since soil is so variable, the only way to 
determine the capacity is to test multiple samples.  These samples should 
be as close as possible to the condition they were in the ground.  Tricky, 
since the act of digging them up 'disturbs' them! 

Soil capacity is described in terms of weight per unit of area -- tons per 
square foot is the most common in the USA, Kg/cm^2 in metric countries. 
Obviously, the size of the bearing will have an effect. The way to test 
soil is to get as undisturbed a sample as possible, and subject it to a 
known weight until it fails. If you graph the weight on a vertical scale 
vs. deflection on a horizontal scale, the line should go more or less 
straight up, then begin to slide off to the side.  Granular soil (sand) 
will have to be confined in some kind of container.  Cohesive soils 
(silts, clay) are subjected to an "unconfined capacity test".
  
Finally, sand should be good for about 1-1/2 tons per square foot, silt 
for 2-3, clay for 3-5 TSF. 

Good luck with your experiments.  Remeber, you can't do it wrong as long 
as you record what you did.

Chas.


P.S.: This outfit makes soil testing equipment.  You should be able to get a 
general idea of how they work from the pictures.  Look especially at 
the "cone penetrometer", the (pocket) "penetrometer", and the "strength 
classifier"
http://www.humboldtmfg.com/index.php