Calculate the ultimate and allowable bearing capacity of shallow foundations using Terzaghi's bearing capacity theory. This tool is essential for geotechnical engineers and students to estimate the load-carrying capacity of soil.
Understanding Soil Bearing Capacity
Bearing capacity is the maximum pressure that a foundation can apply to the soil without causing shear failure or excessive settlement. In geotechnical engineering, determining the ultimate bearing capacity is the first step in designing safe and stable structures.
Terzaghi's Bearing Capacity Equation
Karl Terzaghi, the father of modern soil mechanics, proposed a general equation for shallow foundations. For a long strip footing, the formula is:
qu = c'Nc + σ'vNq + 0.5 γ B Nγ
- c': Cohesion of the soil.
- σ'v: Effective overburden pressure at the base of the foundation (γ × Df).
- γ: Unit weight of the soil.
- B: Width of the foundation.
- Nc, Nq, Nγ: Dimensionless bearing capacity factors that depend solely on the soil friction angle (φ).
Key Factors Influencing Bearing Capacity
Several parameters play a critical role in the stability of a foundation:
- Soil Type: Cohesive soils (clays) rely heavily on the cohesion term, while cohesionless soils (sands) rely on friction and overburden pressure.
- Foundation Depth: Increasing the depth (Df) generally increases the bearing capacity due to the confining pressure of the surrounding soil.
- Water Table: The presence of groundwater can significantly reduce the effective unit weight of the soil, potentially cutting the bearing capacity in half.
- Foundation Geometry: Square, circular, and rectangular footings require shape factors to adjust the general strip footing equation.
Why Use a Factor of Safety?
The "Ultimate Bearing Capacity" is the point of theoretical failure. In practice, engineers apply a Factor of Safety (FS)—typically between 2.5 and 3.0—to determine the "Allowable Bearing Capacity." This ensures that the actual load applied by the building remains well below the failure threshold and limits settlement to acceptable levels.
How to Use This Calculator
To use the bearing capacity calculator above, follow these steps:
- Enter the Cohesion value obtained from laboratory testing (like a direct shear test or triaxial test).
- Input the Unit Weight of the soil in kN/m³.
- Specify the Depth (Df) and Width (B) of your planned foundation.
- Provide the Internal Friction Angle (φ) in degrees.
- Set your desired Factor of Safety (3.0 is standard for most residential and commercial projects).
Click "Calculate Capacity" to see the instantaneous results based on Terzaghi's simplified factors.