Structure and support · Foundations & Ground
Piled Foundation System
A reference account of piling as a pile-and-ground pair rather than a product, covering how installation method, group behaviour and the head connection govern what the foundation actually does, and what to establish with the geotechnical and structural designers.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What piled foundation is
A piled foundation is used where the ground near the surface will not carry the building, or will not carry it without movement the structure cannot tolerate. Slender elements are taken down through that ground so load is delivered lower, either onto a stronger stratum at the base or into friction along the shaft, and usually with both acting to some degree.
The element and the ground are inseparable. A pile has no capacity that can be quoted independently of where it stands; an identical element installed in different strata is a different foundation. That is why piling follows ground investigation, why installed piles are verified rather than assumed, and why the design is a geotechnical exercise instead of a selection from a range.
Installation method - driven, bored, augered, helical and others - is a variable discussed inside this system rather than a separate system, because the load path is the same in each case. What changes is how the surrounding ground is affected, what plant and access are needed, what noise and vibration reach neighbours, and how much can be observed while the element is being formed.
The distinction from the nearest sibling, ground improvement, lies in where the load ends up. Improvement leaves the near-surface ground carrying the building, only stiffer and less compressible. Piling deliberately bypasses that ground. If the finished building still bears on the layer that was originally inadequate, the answer was improvement; if the load is taken past it, the answer was piling.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- pile foundation
- deep foundation
- piling system
- bored and driven piles
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Take building loads through ground that cannot carry them and deliver those loads to strata that can.
- Limit settlement to what the structure above can accept, on ground where a shallow foundation could not be relied on to stay within that.
- Provide support where the ground is variable or has been filled, so the structure does not follow that variability.
- Resist uplift and lateral force where the structure applies them, which a shallow footing may not be able to do.
- Allow building on sites that would otherwise need extensive excavation or replacement of the ground.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportPile shafts
The slender vertical elements carrying load downward. Their length follows the ground profile together with the load each element has to carry, so lengths often differ across one footprint. Where the arrangement is end bearing the pile is taken down until a stronger stratum is reached; where it works in shaft friction there is no such stratum to reach.
- SubstrateBearing stratum and shaft ground
The ground at the base of the pile and along its length. It is the component that decides what the pile can carry, and the only one that cannot be inspected directly, which is why investigation and post-installation verification take its place.
- Primary supportReinforcement cage or structural section
The steel within a cast element, or the section itself where the pile is a driven steel or precast member. It carries what the shaft alone cannot, particularly bending and tension near the head where the structure applies horizontal force.
- AttachmentPile head connection
The trimmed head and the cut-off level, which is where the pile stops being a ground element and where installation tolerance is least forgiving. What happens above it - the embedment itself and the reinforcement arranged around it - is owned by the pile cap and ground beam system.
- Edge and terminationCut-off level and trimming zone
The point the finished pile is taken down to, above which surplus material is removed. Concrete at the very top of a cast pile is the least reliable part of it, so trimming back to sound material is part of forming the connection.
- ProtectionSleeving and coating in aggressive or filled ground
Provision made where ground chemistry or settling fill would otherwise act on the shaft. What is appropriate follows from the ground investigation and the specialist designer rather than from the structural loading.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
A pile has no capacity of its own. What it carries is a property of the pile and the ground acting together, so the ground investigation is part of the design of the element rather than preparation for it. It is also why installed piles are tested: the component sharing the work is the one nobody can see, and confidence in it has to be built after installation instead of specified beforehand.
Installation is not neutral. Displacement methods push ground aside and change it; bored methods remove ground and leave a face that has to stay stable until it is filled. The method therefore feeds back into what the finished element can do, and into what happens beyond the site boundary, because ground pushed out of the way by one pile has to arrive somewhere else.
The pile becomes part of the building only at its head. Reinforcement projecting from a trimmed head is what ties shaft to cap, so a projection that is damaged during trimming, or a head that has finished out of position, makes the connection rather than the pile the weakest part of the arrangement. Correcting it is a redesign of what sits above, not an adjustment below.
Piles in a group do not behave like the same piles standing alone. Each influences the ground its neighbours are relying on, so the group settles differently from any single element within it. Where fill or soft ground above the bearing stratum is still consolidating, that ground hangs on the shaft rather than supporting it, which is why what lies above the bearing layer matters as much as the layer itself.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Pile shafts and their reinforcement are commonly encountered in these families, whether cast in place or driven as manufactured sections. What is appropriate for a given ground and installation method is a specialist geotechnical decision supported by product documentation.
Attachment
Plates, couplers and connection components at the head are commonly encountered in these metal families. Which is relevant depends on the connection the structural designer has drawn and on the exposure the ground investigation identified.
Protection
Where steel elements pass through ground identified as aggressive, coatings from these families are commonly encountered. Whether any coating is appropriate, and whether coating is the right response at all, is a matter for the specialist designer.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
The capping layer above
Piles almost never receive a wall or a column directly. The capping layer collects load and redirects it into pile positions, so pile setting out and cap design are inseparable and an out-of-position pile is resolved in the cap rather than in the ground.
Read about Pile Caps and Ground Beams →Piles passing through a basement slab
Where piles rise into a basement, every head becomes a penetration through the water-resisting arrangement of a below-ground box. The detail at each head has to satisfy the structural connection and the water-resisting strategy at the same time.
Read about Basement Structure →Piled walls and adjacent retention
The same techniques form retaining walls, and piles installed close to a retaining structure change the ground it is holding. Sequencing and proximity are questions for the temporary works designer as well as the permanent works designer.
Read about Embedded Retaining Wall →Drainage clashing with pile positions
Pile positions are set by the structure and by the ground, and drainage routes are set by falls and connections, so clashes are common. Resolving them after piling is far harder than resolving them on paper beforehand.
Read about Buried Drainage Network →Neighbouring structures, trees and buried obstructions
Existing foundations, services, old basements and tree roots all sit in the ground the piles have to pass through, and vibration or displacement can reach beyond the site. Surveys, condition records and monitoring are specialist matters that belong before work starts.
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Buildability
- Access, headroom, working platform quality and the space needed to handle long elements often decide which installation method is even possible on a site. A method that suits the ground can be ruled out by the gate the plant has to come through.
- Movement
- Settlement of a piled foundation is dominated by group behaviour and by ground still consolidating above the bearing stratum. Predicting it, and deciding what the structure can accept, are geotechnical and structural judgements made together.
- Acoustic
- Installation generates airborne noise and ground-borne vibration that reach neighbouring buildings, people and sensitive equipment. What is acceptable, how it is monitored and what mitigation applies are matters for the specialist contractor and the relevant authority.
- Durability
- Ground chemistry, groundwater and made ground can all act on a buried element that will never be seen again. What the ground investigation found about aggressiveness is the starting point for whatever protection the designer specifies.
- Interfaces
- A piled foundation is only useful through what sits on it, so cap design, tolerance and the position of every penetration through the capping layer are where the system meets the building. Errors here are more common than failures in the piles.
- Documentation
- Installation records, as-installed positions and test results are the only evidence of what is in the ground. They are also what any future alteration, extension or party wall discussion will be judged against.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- What ground investigation supports the piling design, and how far across the site does it actually extend?
- What is the pile intended to rely on: bearing at the base, friction along the shaft, or a combination?
- How will installed piles be verified, and what happens if one does not perform as expected?
- What installation tolerance has the capping design allowed for, and who checks positions before capping?
- What effect will installation have on neighbouring buildings, services and trees?
- Is any ground above the bearing stratum still settling, and has its effect on the shaft been considered?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Piles are not simply long columns driven into the ground, because what they carry depends on the ground around them as much as on the element itself.
- Reaching rock is not the definition of a successful pile, since many piles are designed to work through friction and never reach a hard stratum.
- A piled foundation does not remove ground movement from the discussion, because ground above the bearing stratum can still load the shaft as it settles.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What did the ground investigation find, and how confident is the design in the ground between the investigation points?
- Which installation method is proposed, and what led to that choice on this site?
- What testing regime is planned, and what proportion of the piles does it cover?
- How will vibration and noise be managed, and what condition surveys of neighbouring property are proposed?
- What tolerance is allowed on position and verticality, and how is a pile outside it dealt with?
- What protection, if any, is proposed against aggressive ground or made ground identified on the site?
- What records will be handed over showing what was installed, where, and how it performed?
What this page does not do
- Piling design is specialist geotechnical work and is not something an owner or a general contractor should determine.
- This entry gives no capacity, length, diameter, spacing or test criterion, all of which are properties of a specific design in specific ground.
- Installation can affect neighbouring land and structures, and the legal and monitoring obligations that follow have to be resolved before work begins.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Foundation and Ground Interface Systems
Assemblies where a building first meets undisturbed ground — strip, pad, raft and piled foundations, pile caps, ground improvement and ground gas protection.
Browse all foundations & ground entries →