How to Calculate the Bearing Capacity of Soils

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How to Calculate the Bearing Capacity of Soils

Soil bearing capacity determines the load that a soil can support per area. Calculating the bearing capacity of soils vary greatly due to many factors such as the type, cohesiveness and compactness of the soil as well as the presence of groundwater. There are numerous variations of the formula used for these calculations depending on the different factors involved and background knowledge of soil mechanic terminology is required to understand all the variables. The formula we will use is Terzaghi's Bearing-Capacity Equation (bearing capacity = c*Nc*s-c + q-bar*Nq + 0.5*gamma*B*Ngam*s-gam) which is a fundamental equation used in soil mechanics.

Things You'll Need

  • Calculator
  • Soil data from tests of soil samples
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    • 1

      Collect data about the soil's cohesion in kPa (c), shape factor for cohesion (s-c), unit weight in kN/m3 (gamma), least lateral base dimension in meters (B) and shape factor for soil wedge (s-gam).

    • 2

      Find the overburden pressure in kPa (q-bar) by multiplying the soil's unit weight (gamma) by the depth to the base of the soil in meters.

    • 3

      Calculate the overburden multiplier (Nq) using the equation a^2/(2*cos^2(45+PHI/2)). Use a=2^((0.75*PI-PHI/2)*tan(PHI)) and PHI=angle of internal friction in radians. Use the overburden multiplier (Nq) to find the cohesion multiplier (Nc) using (Nq-1)*cot(PHI).

    • 4

      Determine the wedge weight multiplier (Ngam) through the equation (tan(PHI)/2)*(Kpy/cos^2(PHI)-1). The Kpy value is based on the PHI value in Step 3. PHI values of 0, 5, 10, 15, 20, 25, 30, 35 and 40 will have Kpy values of 10.8, 12.2, 14.7, 18.6, 25.0, 35.0, 52.0, 82.0, 141.0 and 298.0 respectively.

    • 5

      Use all of the values from the previous steps and insert them into the equation c*Nc*s-c + q-bar*Nq + 0.5*gamma*B*Ngam*s-gam to determine the bearing capacity of the soil. Use kPa as the final unit for your answer.

Tips & Warnings

  • You can use a hand penetrometer to determine soil bearing capacity by computer rather than doing all the calculations manually.

  • Tables are available with estimated bearing capacity of soils by soil type. You can use these as a rough guideline for a quick estimate.

  • The above equation does not take all factors into account since it is the very basic, fundamental one used for calculating soil bearing capacity. Complex, modified versions of this equation will yield more accurate results.

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  • Photo Credit Hemera Technologies/ Images

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