Effective Surcharge given Bearing Capacity for Purely Cohesive Soil Calculator

Effective Surcharge Formula:

\[ \sigma_s = \frac{q_f - (C_s \times N_c)}{N_q} \]

Ultimate Bearing Capacity (qf):

kPa

Cohesion of Soil (Cs):

kPa

Bearing Capacity Factor (Nc):

Bearing Capacity Factor (Nq):

Effective Surcharge (σs):

1. What is Effective Surcharge in Soil Mechanics?

Definition: Effective surcharge (σs) refers to the vertical pressure acting over the ground surface in addition to basic earth pressure.

Purpose: It's crucial for calculating bearing capacity of purely cohesive soils in foundation design.

2. How Does the Calculator Work?

The calculator uses the formula:

\[ \sigma_s = \frac{q_f - (C_s \times N_c)}{N_q} \]

Where:

\( \sigma_s \) — Effective surcharge (kPa)
\( q_f \) — Ultimate bearing capacity (kPa)
\( C_s \) — Cohesion of soil (kPa)
\( N_c \) — Bearing capacity factor for cohesion (typically 5.14 for clay)
\( N_q \) — Bearing capacity factor for surcharge (typically 1.0 for clay)

Explanation: The formula calculates the effective surcharge needed to achieve a certain bearing capacity considering soil cohesion.

3. Importance of Effective Surcharge Calculation

Details: Proper calculation ensures foundation stability and prevents excessive settlement in cohesive soils.

4. Using the Calculator

Tips: Enter ultimate bearing capacity, soil cohesion, and bearing capacity factors (default values provided). The ±5% indicates typical variation in these factors.

5. Frequently Asked Questions (FAQ)

Q1: What's a typical Nc value for clay?
A: For purely cohesive soils (φ=0), Nc is typically 5.14 (π+2).

Q2: When would Nq be different from 1.0?
A: Nq=1.0 is for φ=0 soils. For soils with friction angle, Nq increases significantly.

Q3: How is ultimate bearing capacity determined?
A: Through field tests (plate load test) or laboratory testing of soil samples.

Q4: What if my calculation gives negative σs?
A: This indicates the soil's cohesion alone provides sufficient bearing capacity without needing surcharge.

Q5: How accurate are the bearing capacity factors?
A: They're theoretical values with ±5% typical variation in real-world conditions.