Foundation Engineering / Why piles, and what kinds
Kinds of pile
Displacement or replacement — and why the ground can tell the difference.
Lesson review: expert verification remains open
This lesson has received a full-context editorial review and a check of its technical explanations. Selected arithmetic has been checked separately. This is not independent engineering sign-off.
Checks needed for project use
- A geotechnical reviewer must confirm drainage and stress basis, bearing-factor formulation, correlations and settlement influence factors. Partial factors and project-specific ground conditions are outside these teaching checks.
- Compare allowable/service and ultimate resistances on a consistent basis. Review pile-type constructability, uplift and displacement classification.
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesWhy this matters
Two piles of the same diameter and length, in the same soil, can differ in capacity by a factor of two because of how they were put in. Installation is not a construction detail that follows the design; it is one of the design's main variables.
The primary division is between displacement and replacement piles.
Displacement (driven) piles — precast concrete, steel tube or H-section, timber — are forced into the ground, pushing soil aside. In sand this densifies the surrounding material and raises the horizontal effective stress against the shaft, both of which increase capacity. In clay it remoulds an annulus of soil, which reduces the available adhesion in the short term but allows it to recover as excess porewater pressures dissipate.
Replacement (bored) piles — continuous flight auger, rotary bored, under-reamed — are formed by removing soil and casting concrete into the hole. Nothing is compacted; if anything the ground relaxes towards the void before the concrete goes in. Horizontal stress against the shaft is lower than for a driven pile, so shaft resistance is lower for the same soil.
This shows up directly in the calculation. Module 15's shaft coefficient is β = K·tan δ, where K describes the horizontal stress against the shaft. A driven pile might justify K near 1.0; a bored pile in the same sand is nearer 0.5. With interface friction angles of 30° and 25° respectively, the bored pile's β is about 40% of the driven pile's.
Same soil. Same diameter. Same length. Roughly two-fifths of the shaft resistance, because of how the hole was made.
Check yourself
Why does a driven pile generally develop more shaft resistance than a bored pile of the same size in the same sand?
Practice
A column carries 1000 kN. At an allowable bearing pressure of 150 kPa, what plan area would a pad footing need, in m²?
Try it
Pad or pile?
One column load, two ways to carry it. The pile's advantage is footprint, not economy.
Pile installation
What is fixed
- 0.6 m diameter, 15 m long, Nc = 9 at the base.
- Working load taken as ultimate over a nominal factor of 2.5 — illustrative, not a Eurocode check.
- Pad area required
- 6.7 m²
- Pad side (square)
- 2.6 m
- Pile shaft Qs
- 848 kN
- Pile base Qb
- 153 kN
- Pile ultimate Qult
- 1001 kN
- Piles needed
- 3
- Shaft85%
- Base15%
A 2.6 m pad is entirely buildable. Unless settlement, uplift, lateral load or unreliable near-surface ground says otherwise, piling this column would be a reflex rather than a decision.
Summary
- Six reasons to pile: weak ground, settlement, concentrated load, uplift, lateral load, and ground that changes
- Settlement — not capacity — is the most common of them
- Displacement piles densify and stress the ground; replacement piles relax it
- A bored pile's shaft coefficient was about 40% of a driven pile's in the same sand
- Pile type is usually chosen on constraints, and the capacity calculation follows