Soil Characteristics

The chart below summarizes the effects of various soil characteristics on building.

Measurement
Tools
Preferred Value for Building
Slope
level, ruler
< 6%
Permeability
shovel, ruler, water
>100
Density
shovel, scales, 1000 mL beaker of sand
Depends on building plan, but greater is better
Percent Moisture
scales, oven
6-8%
Percent Organics
scales, muffle furnace
< 2%
Support Capacity
rebar, sledgehammer, ruler
Depends on building plan, but greater is better

The procedures used to measure slope and permeability are explained in different topic pages.  Below, we will summarize the method used to measure density, percent moisture, percent organics, and support capacity.


 

Soil Density

Soil density is a factor which will affect the ability of soil to compact.  Density is defined as volume divided by area.  A soil with a low density has a lot of air pockets between the soil particles while a soil with high density has few air pockets.    

To measure the soil's density, first you will need to take a sample of soil.  Simply dig a small hole in the ground, removing the soil as your sample.  

 

Second, weigh the sample to determine its weight.  

 

To determine the soil's volume, you will need a 1000 mL graduated beaker filled with sand.  Fill the hole in the ground with sand from the beaker.  The volume of the hole is:

Volume = 1,000 mL - (Volume of sand left in beaker)


So, if you filled the hole with sand from the beaker and still had 400 mL of sand left in your beaker, then the volume of your hole would be 600 mL.

Finally, you can find the soil's density by dividing the weight of the soil by the volume of the hole.
 

Density formula.


If the volume was 25 mL and the weight was 119 g, then the density would be:

Calculations



Percent Moisture

To find the soil's percent moisture, you first weigh the soil, as you did above.  Then you dry the soil so that all of the water has evaporated out.  Finally, you weigh the soil again and use the following formula:



So, if the soil's original weight was 119 grams, and its weight after drying was 97 grams, then the percent moisture would be:

 

Moisture has a large influence on soil's ability to compact. The optimal soil moisture for compaction is usually around 6-8%.  Soils with less moisture than this are like dry dust which floats up into the air rather than compacting.

Wet soil also doesn't compact well.  Consider what happens when you pick up a handful of mud and try to close your fist around it.  Rather than compacting into a ball of soil, the mud oozes out between your fingers.  


Percent Organics

Soil is made up of a variety of substances.  Some of the soil is bits of sand or rock and some of the soil is organics such as decomposed leaves and other materials.  

You can determine the percent organics in the soil by grinding the dried soil and then heating it in a muffle furnace at 550
°C.  This is like leaving your food in the oven until it burns up into ashes.   All of the organic matter in the soil will burn up and disappear after about 15 to 30 minutes.

Now you weigh the cooked soil.  The percent organics is determined by the following formula:

 

If you used the soil sample which we determined the percent moisture of above, and the weight after cooking was 70 grams, then the percent organics would be:




 

The organics in soil serve a variety of purposes.  They help plants since they are fertilizer which the plants use to grow. They also make soil more permeable.  But organics in soil hurt compaction and cause slippage and stability problems. Soil high in organics may be good for growing crops or installing a septic system but may be bad for installing a large building.  For stability, organics should be less than 2% at the building site.  


Support Capacity

The final characteristic of soil which influences compaction is support capacity.  Support capacity is the ability of the soil to support weight.  

To determine the support capacity of your soil, you will need a 1/2" rebar and a sledge hammer of known weight.  (We will use a 16 pound sledge hammer in this example.)

Place the rebar in the ground and measure the distance between the top of the rebar and the ground.  We will call this distance D1.  

Calculating support capacity.

Now, hold the sledgehammer some distance above the rebar, as shown in the picture below.  The distance between the sledgehammer and the top of the rebar will be called D2.  

Drop the sledgehammer onto the rebar.  The rebar will be driven into the ground some distance.  Measure the new distance between the top of the rebar and the ground.  This distance is D3

The support capacity of the soil is then determined using the following formula:



We dropped our 16 pound sledge from a height of 24 inches.  The bar's initial height about the ground was 12 inches, but after the sledge was dropped on it, the bar's height above the ground was 5 inches.  The area of a 1/2" rebar is 0.2 square inches.  So the support capacity of the soil was:



Support capacity should be reported in pounds per square foot.  To translate from pounds per square inch to pounds per square foot, multiply by 144.  The support capacity of this soil is 39,499 pounds per square feet.

The required support capacity at your site will depend on the weight of your building and on how the building's weight is dispersed through the foundation.