Foundation Engineering and Terzaghi's Theory
Explore the science of what lies beneath. Learn why footing depth matters as much as soil strength and how the 'Angle of Internal Friction' determines soil stability.
The Interface of Structure and Earth
The entire weight of a building must eventually be absorbed by the soil. If the pressure from the building exceeds the **Bearing Capacity** of the soil, the building will sink (Settlement) or tilt (Shear Failure). Developing an accurate estimate of soil capacity is the most critical task in Geotechnical Engineering.
Terzaghi's Bearing Equation
The Three Components of Capacity
- Cohesion ($c N_c$): The "stickiness" of the soil. Pure clays rely almost entirely on cohesion. Sand has zero cohesion.
- Surcharge ($\gamma D_f N_q$): The weight of the soil above the foundation level. Deep foundations are stronger because the surrounding soil "holds" the footing in place.
- Wedging ($0.5 \gamma B N_\gamma$): The resistance created by the soil wedge directly under the footing. Wider footings are inherently more stable in sandy soils.
Angle of Internal Friction ($\phi$)
This is the angle at which soil remains stable in a pile without sliding.
- Loose Sand: $\phi \approx 25$-$30$°.
- Dense Sand: $\phi \approx 40$-$45$°.
As $\phi$ increases, the Bearing Capacity Factors ($N_c, N_q, N_\gamma$) increase **exponentially**. A small change in soil density can double or triple the foundation's capacity.
Frequently Asked Questions (FAQ)
What is 'Differential Settlement'?
If one side of a building sinks $20$mm and the other side sinks $20$mm, the building stays level. But if one side sinks $20$mm and the other sinks $5$mm, the building tilts. This "Differential Settlement" is what causes the most dangerous cracks in structural walls and frames. Even if the bearing capacity is safe, engineers must check settlement limits.