Effective Surcharge given Bearing Capacity for Square Footing Calculator

Effective Surcharge Formula:

\[ \sigma_s = q_f - (C \times N_c \times (1 + 0.3 \times \frac{B}{L})) \]

Ultimate Bearing Capacity:

kPa

Cohesion in Soil:

kPa

Bearing Capacity Factor:

Width of Footing:

Length of Footing:

Effective Surcharge (kPa):

1. What is Effective Surcharge in Foundation Design?

Definition: Effective surcharge refers to the additional vertical pressure acting over the ground surface beyond the basic earth pressure.

Purpose: It's crucial for calculating the bearing capacity of square footings in geotechnical engineering.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ \sigma_s = q_f - (C \times N_c \times (1 + 0.3 \times \frac{B}{L})) \]

Where:

\( \sigma_s \) — Effective surcharge (kPa)
\( q_f \) — Ultimate bearing capacity (kPa)
\( C \) — Cohesion in soil (kPa)
\( N_c \) — Bearing capacity factor (dimensionless, typically 9±5%)
\( B \) — Width of footing (m)
\( L \) — Length of footing (m)

Explanation: The formula calculates the surcharge pressure by subtracting the cohesive component from the ultimate bearing capacity.

3. Importance of Effective Surcharge Calculation

Details: Accurate calculation ensures proper foundation design, prevents settlement issues, and maintains structural stability.

4. Using the Calculator

Tips: Enter all required values in kPa for pressure and meters for dimensions. The bearing capacity factor defaults to 9 with ±5% tolerance.

5. Frequently Asked Questions (FAQ)

Q1: Why is the bearing capacity factor 9±5%?
A: The factor 9 is typical for cohesive soils, with ±5% accounting for soil variability and empirical uncertainty.

Q2: What if my footing is perfectly square?
A: For square footings (B = L), the term B/L becomes 1, simplifying the calculation.

Q3: How does cohesion affect the result?
A: Higher cohesion values will result in lower effective surcharge values, as more capacity comes from soil cohesion.

Q4: What units should I use?
A: Use kPa for all pressure values (qf, C, σs) and meters for dimensions (B, L).

Q5: Can this be used for circular footings?
A: No, this formula is specifically for rectangular/square footings. Different shape factors apply to circular footings.