Effective Surcharge for Circular Footing given Value of Bearing Capacity Factor Calculator

Effective Surcharge Formula:

\[ \sigma_s = q_f - (7.4 \times C) \]

Effective Surcharge (kPa):

Acceptable Range (5% tolerance):

1. What is Effective Surcharge for Circular Footing?

Definition: Effective surcharge (σₛ) refers to the vertical pressure or load acting over the ground surface in addition to basic earth pressure, calculated for circular footings.

Purpose: This calculation helps geotechnical engineers determine the additional load that can be safely applied to soil through circular foundations.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ \sigma_s = q_f - (7.4 \times C) \]

Where:

\( \sigma_s \) — Effective surcharge (kPa)
\( q_f \) — Ultimate bearing capacity (kPa)
\( C \) — Cohesion in soil (kPa)
7.4 — Bearing capacity factor for circular footings

Explanation: The formula subtracts the cohesive component of soil strength from the ultimate bearing capacity to determine the effective surcharge pressure.

3. Importance of Effective Surcharge Calculation

Details: Proper calculation ensures foundation stability, prevents excessive settlement, and helps optimize foundation design for circular structures.

4. Using the Calculator

Tips: Enter the ultimate bearing capacity (kPa), soil cohesion (kPa), and tolerance percentage (default ±5%). All values must be ≥ 0.

5. Frequently Asked Questions (FAQ)

Q1: What is the bearing capacity factor 7.4?
A: This is a theoretical value derived from soil mechanics for circular footings, accounting for shape and depth factors.

Q2: When would the effective surcharge be negative?
A: If cohesion is very high relative to bearing capacity, though this usually indicates an input error as surcharge can't be negative in reality.

Q3: Why include a tolerance percentage?
A: Soil properties can vary, so the tolerance provides an acceptable range accounting for measurement uncertainties.

Q4: How do I determine soil cohesion?
A: Cohesion is typically measured through laboratory tests like unconfined compression or triaxial tests on soil samples.

Q5: Does this apply to rectangular footings?
A: No, rectangular footings use a different bearing capacity factor (typically 5.14 for continuous footings).