## Tuesday, February 8, 2011

### How to Calculate Overburden Pressure

OVERVIEW

The pressure due to soil overburden is important to know in order to design stable foundations,  load bearing walls, and footings under the ground surface. This pressure is due to the weight of the soil layers above, as well any materials on top of the soil, such as concrete or ice. The presence of saturated soils at the design elevation can have a significant impact on overburden pressure as well. For the purpose of this discussion, overburden does not include solid rock, and the point of concern must be at the surface of underlying solid bedrock, or above the bedrock.

STEP 1.

Calculate the pressure due to the soil alone acting on the point at the elevation of concern, Eo. This point is typically located beneath more than one layer of soil. Use a table with a row for each separate soil layer, or substrate, with a value for each layer's density and depth in separate columns.

If there is water present, divide the soil layer where the water into two row entries, one for dry condition, and one row for saturated condition below the dry. For the dry layers, the soil pressure at the bottom of the layer is the sum of the dry density multiplied by the layer depth for that soil.

The bottom layer will be the layer containing the elevation Eo where we are calculating the pressure.
The layer depth will then extend from the bottom of the upper layer to the Eo elevation.

For example, for a Dry Gravel with a top elevation of 98.0 feet and a bottom elevation of 92.0 feet, the density in place is 125 lbs per cubic foot (pcf). In the table row for this layer, place a value of 98.0 in the first column, 92.0 in the second column, 6.0 in the third column (98.0 - 92.0 = 6.0), and 125.0 in the fourth column,

The total pressure due to this layer is then (98.0 - 92.0) * 125 = 750 pounds per square foot (psf). Place the value for the pressure due to each layer in the next column (fifth column) of the table.

STEP 2.
For saturated soils, reduce the calculated soil pressure due to the water being displaced by the soil. The density of the soil that is saturated is computed by taking the dry density of the soil minus the density of water. The density of water is 62.4 lbs/cu.ft. at 50 deg F. For the saturated layers, the soil pressure at the bottom of the layer is the dry density, less the density of water, multiplied by the layer depth for that soil. Be sure to use similar units, for example, use feet of soil and pounds per cubic feet for density.

In our example, the Eo elevation is 88.0 feet. The layer depth extends from the bottom of the dry gravel layer at 92.0 feet to 88.0 feet. Place 92.0 in the first column, 88.0 in the second column, and 4.0 in the fourth column.

Saturated gravel has a dry density of 125 pcf, so the wet density is then 62.6 pcf, so place this value in the fourth column. The total pressure from wet gravel layer, which is 4.0 feet thick, would be 250.4 psf. Place the value for the pressure due to each wet layer in the fifth column of the table.

STEP 3.

Now the total pressure at Eo can be computed. Sum the pressure computed due to each of the overlying layers which have been entered into the fifth column of the table. The total pressure is the sum of all these soil layers. For the two layers in our example, the total of the layers is 750.0 + 250.4 = 1000.4 psf.

TIP

A close approximation of soil density may be obtained from Standard Penetration Testing (SPT) and from examination of extracted split barrel samples. These tests are standardized by the American Society for Testing and Materials (ASTM). The ASTM also outlines a thin walled extraction cylinder method so that soils are less disturbed by the sampling method.

WARNINGS

• Estimates of density may be used, but large variations of natural in-place densities do occur.
• The water (or glacial ice, etc) maybe be above the soil surface. Subtract the density of water from all soils in the first case, and add the total weight of ice in the second case.

REFERENCES

ASTM: Standard Penetration Test [http://www.astm.org/Standards/D1586.htm]
Essentials of Soil Mechanics by David MCarthy [http://www.amazon.com/Essentials-Soil-Mechanics-Foundations-Geotechnics/dp/0131145606/ref=dp_ob_title_bk?ie=UTF8&qid=1296526528&sr=8-10]