Foundation engineering is the application of soil mechanics and structural principles to design systems that safely transfer structural loads into the ground.
Foundations are broadly classified into two categories:
These are typically used when competent soil exists at relatively shallow depths and the structural loads are moderate.
Deep foundations are selected when surface soils are weak, compressible, or when loads are very high.
The ultimate bearing capacity of a shallow foundation is commonly estimated using Terzaghi’s or Meyerhof’s equations. For a strip footing on cohesionless soil:
\[q_u = \frac{1}{2}\gamma B N_\gamma + \gamma D_f N_q\]For cohesive soils under undrained conditions:
\[q_u = 5.14 s_u + \gamma D_f\]where $N_\gamma$, $N_q$, and $N_c$ are bearing capacity factors that depend on the friction angle.
A factor of safety (typically 2.5–3.0) is applied to obtain the allowable bearing pressure.
Even if bearing capacity is adequate, excessive settlement can render a foundation unsuitable. Settlement has two main components:
Differential settlement is often more critical than total settlement, as it can induce distortion and cracking in the superstructure.
The choice between shallow and deep foundations depends on:
| Factor | Preference for Shallow | Preference for Deep |
|---|---|---|
| Soil strength near surface | High | Low |
| Compressible layers | Absent or thin | Present at depth |
| Structural loads | Moderate | Very high |
| Groundwater | Manageable | Problematic |
| Adjacent structures | Minimal impact | Sensitive surroundings |
| Cost & constructability | Usually lower | Higher |
A typical foundation design process includes:
Successful foundation design requires close collaboration between geotechnical and structural engineers. The geotechnical engineer provides soil parameters and performance recommendations; the structural engineer ensures the foundation can resist the applied forces and moments while remaining serviceable.
In the next article, we will explore slope stability analysis — another critical application of soil strength principles.