How to Calculate Ground Bearing Pressure

Written by pat smith
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How to Calculate Ground Bearing Pressure
Soil (Arid soil image by Igor Baryshev from Fotolia.com)

Ground bearing pressure lets you know how much pressure soil can handle, which is helpful for gardening, farming, and building foundations and buildings. There are many factors going into bearing pressure, but thankfully there's an equation to help you calculate it. You'll need to gather some data about the soil, and then calculate it using Terzaghi's Equation (q-ult = cNcs-c + q-barNq + 0.5gammaBNgam*s-gam, where q-ult=bearing capacity).

Skill level:
Moderate

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Things you need

  • Calculator
  • Soil sample data

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Instructions

  1. 1

    Gather soil sample data, including the soil's cohesion, measured in units kPa and represented by c in the equation. Shape factor is s-c in the equation. Unit weight is measured in units kN/m3 and is gamma in the equation. Base dimension is measured in meters and represented by B. Soil wedge shape factor is s-gam in the equation, and overburden pressure is calculated (gamma * soil base).

  2. 2

    Calculate Nq, the overburden multiplier, using the equation a=2^((0.75PI-PHI/2)tan(PHI)). PHI is the angle of internal friction in radians, or the angle of the soil wedge sample (entered in degrees). Now plug Nq into the equation Nc=(Nq-1)*cot(PHI).

  3. 3

    Calculate Ngam with Ngam=(tan(PHI)/2)*(Kpy/cos^2(PHI)-1). Kpy is based on PHI, so if the angle of your soil sample in degrees is 0, Kpy is 10.8. If PHI=5, Kpy=12.2; if PHI=10, Kpy=14.7; if PHI=15, Kpy=18.6; if PHI=20, Kpy=25.0; if PHI=15, Kpy=35.0; if PHI=30, Kpy=52.0; if PHI=35, Kpy=82.0; if PHI=40, Kpy=141.0; if PHI=45, Kpy=298.0. Calculating soil samples larger than 45 degrees will not yield the most accurate results.

  4. 4

    Plug in all the values you've found into the original equation: bearing capacity=cNcs-c + q-barNq + 0.5gammaBNgam*s-gam.

Tips and warnings

  • Make a list of all the variables you'll need in the equations and make sure they're consistent across the equations.

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