How to Determine Soil Bearing Pressure

The ability of soil to support heavy items is found through tables and calculations.

Soil bearing pressure is a crucial measurement when building and paving. The measurement assesses the ability of soil to bear pressure upon the ground in a specific area. This is important to determine whether it is safe to erect a structure in a particular location. The soil bearing pressure varies depending on the drainage of an area and the type of soil. Soils under concrete slabs experience the most pressure and must have a safety factor built in to maximize the effectiveness of the measurement and help prevent failure. Measuring soil bearing pressure requires a thorough education in equations and formulas to make an accurate calculation, but an educated guess is possible with a professional table and handheld penetrometer, or pounds-per-square-inch meter.

Things You'll Need

• Penetrometer
• Compactor
• Mallet
• Shovel

Instructions

1. Measure surface soil bearing pressure with a handheld penetrometer. This instrument tells you if the soil in the upper region of the earth is stable. Compact the soil with a manual, plate or jack compactor before taking the measurement.

2. Sink the penetrometer into the soil to the upper part of the gauge with a mallet. Take a measurement after one to two blows for light construction projects and after five to 10 blows for large, heavy construction such as concrete slabs.

3. Take a deep soil bearing pressure measurement for sunk foundations and similar construction. The soil composition is different at various levels of depth. Deeper soil reflects a different pressure-carrying ability.

4. Dig into the soil to the depth at which you will be building. Compact the soil and use the penetrometer to measure the strength of the soil at that level. Take several measurements with different numbers of blows. Use zero to four for loose soils and up to 50 for very hard, dense soils.

5. Assess the results of your penetrometer test. Heavy, rocky and gravely soils result in pounds-per-square inch ratings of 5,000 to 6,000. Silt to sandy gravel soils range from 3,000 to 5,000, and clay composite soils may be as low as 2,000.

References

• APB Pole Barns: Soil Bearing Capacity
• Shows, Dearman and Waits, Inc.: Bearing Capacity
• ELE International: Soil Testing, Bearing Capacity