Depth of Footing given Factor of Safety and Safe Bearing Capacity Calculator

Depth of Footing Formula:

\[ D = \frac{(qsa \times fs) - ((Cs \times Nc) + (0.5 \times \gamma \times B \times N\gamma))}{\gamma \times Nq} \]

Safe Bearing Capacity (qsa):

Factor of Safety (fs):

Cohesion of Soil (Cs):

Bearing Capacity Factor (Nc):

Unit Weight of Soil (γ):

N/m³

Width of Footing (B):

Bearing Capacity Factor (Nγ):

Bearing Capacity Factor (Nq):

Depth of Footing (D): m

1. What is Depth of Footing given Factor of Safety and Safe Bearing Capacity?

Definition: This calculator determines the required depth of a footing based on soil properties, bearing capacity factors, and safety considerations.

Purpose: It helps civil engineers and construction professionals design stable foundations that can safely bear structural loads.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ D = \frac{(qsa \times fs) - ((Cs \times Nc) + (0.5 \times \gamma \times B \times N\gamma))}{\gamma \times Nq} \]

Where:

\( D \) — Depth of Footing (meters)
\( qsa \) — Safe Bearing Capacity (Pascal)
\( fs \) — Factor of Safety (±5%)
\( Cs \) — Cohesion of Soil (Pascal)
\( Nc \) — Bearing Capacity Factor dependent on Cohesion
\( \gamma \) — Unit Weight of Soil (N/m³)
\( B \) — Width of Footing (meters)
\( N\gamma \) — Bearing Capacity Factor dependent on Unit Weight
\( Nq \) — Bearing Capacity Factor dependent on Surcharge

Explanation: The formula calculates the minimum footing depth required to safely distribute structural loads to the soil while considering soil properties and safety factors.

3. Importance of Footing Depth Calculation

Details: Proper footing depth ensures structural stability, prevents settlement, and accounts for soil bearing capacity and environmental factors like frost depth.

4. Using the Calculator

Tips: Enter all required parameters with appropriate units. The calculator provides default values for common scenarios, but these should be verified for your specific project.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical factor of safety for footings?
A: Common values range from 2.5 to 3.5 (±5%), with 2.8 being a typical default for many applications.

Q2: How do I determine soil cohesion?
A: Soil cohesion is determined through geotechnical testing like triaxial or unconfined compression tests.

Q3: What affects bearing capacity factors?
A: They depend on soil type and the angle of internal friction (φ). Tables are available for different soil conditions.

Q4: Why is unit weight of soil important?
A: It affects both the surcharge effect and the soil's resistance to shear failure.

Q5: How does footing width affect depth?
A: Wider footings typically require less depth as they distribute loads over a larger area, but this depends on soil conditions.