Deep foundations transfer structural loads through weak or compressible surface soils to stronger strata at greater depth. They are essential for high-rise buildings, bridges, heavy industrial facilities, and sites with poor near-surface conditions.
Driven piles are installed by impact or vibratory hammers. They densify surrounding granular soils and provide good quality control through driving records.
The ultimate axial capacity of a pile is the sum of shaft resistance and base resistance:
\[Q_u = Q_s + Q_b\]where:
For cohesive soils (α-method):
\[Q_s = \alpha \, s_u \, A_s\]For cohesionless soils (β-method):
\[Q_s = \beta \, \sigma'_v \, A_s\]Depends on the strength of the material at the tip and the method of installation.
Single pile settlement is usually small if the pile is founded in competent material. However, pile groups can experience additional settlement due to interaction and the compression of the soil mass within the group.
Group efficiency factors and equivalent raft methods are commonly used to estimate group capacity and settlement.
Piles must often resist lateral loads from wind, earthquakes, or earth pressures. Analysis methods include:
Static load tests remain the most reliable method of verifying capacity. Dynamic testing (e.g., PDA) and rapid load tests offer faster alternatives. Integrity testing (PIT, CSL, thermal integrity profiling) is used to assess the quality of installed piles.
Deep foundation design is both a science and an art. It requires sound theoretical knowledge, careful interpretation of site data, and practical judgment informed by construction experience. When executed well, deep foundations provide reliable, long-term support for the most demanding structures.
This concludes our 10-part series on core topics in geotechnical engineering. We hope these posts provide a solid foundation for further study and professional practice.