Foundation Engineering / Why piles, and what kinds
When piles
Six reasons, only one of which is about strength.
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
Pile selection should respond to a defined problem: unsuitable shallow ground, excessive movement, concentrated loads, uplift, lateral actions or changing ground conditions. Compare shallow and deep alternatives against the same actions, movement limits and construction constraints.
By the end of this lesson you should be able to
- Recognise the six situations that indicate piles
- Say which of them is about capacity and which about movement
- Compare a piled and a shallow solution for the same load
Weak ground near the surface. The classic case: soft alluvium or made ground over a competent stratum. The piles are simply a way of reaching the good material.
Settlement, not capacity. Far more common than the first, and less often stated. The shallow foundation would not fail — it would move too much, or too unevenly. Modules 9 and 10 are what drive this.
Concentrated load. A single column carrying several thousand kilonewtons on a tight plan can need a pad larger than the space available.
Uplift. Wind on a light structure, buoyancy on a basement, overturning on a tall narrow tower. A shallow foundation may mobilise its own weight, overlying ground and other verified resistance mechanisms; a pile may mobilise shaft resistance over its effective length. Uplift and group mechanisms require separate checks.
Lateral load and moment. Bridge piers, retaining structures, quay walls. A pile is a cantilever into the ground — Module 18's subject.
Scour, or ground that changes. River and coastal structures where the surface may not be there in fifty years; expansive clays where the near-surface moves seasonally; collapsing soils that lose strength when wetted. Here piles are about founding below the zone that is not to be trusted.
A 0.6 m diameter bored pile 15 m into the example clay has an estimated ultimate resistance of about 1000 kN. That is not an allowable working load. Separately, a 1000 kN service load at an allowable pad pressure of 150 kPa needs about 6.7 m², or a 2.6 m square pad.
A valid comparison requires both options to satisfy the same factored resistance and serviceability checks. These figures alone do not show that one pile replaces that pad.
Check yourself
A light warehouse is proposed on 3 m of loose fill over dense sand. The pad footings check out for bearing capacity but the predicted settlement is 60 mm, unevenly distributed. What is the case for piling?