Structure and support · Foundations & Ground
Pile Cap and Ground Beam System
This entry treats caps and ground beams as an interface assembly, explaining how a spread line of load is gathered into discrete support points, why the ground beneath the beams is deliberately excluded from the load path, and what to confirm with the designer.
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 pile caps and ground beams is
Piles arrive at discrete points; walls, floors and columns arrive as lines, areas and concentrations. Caps and ground beams are the layer that reconciles the difference. A cap sits over a pile or a group of piles and collects what reaches it, and the beams span between caps to give continuous support to whatever is built off them.
The defining behaviour is that these elements span. A ground beam is not a strip foundation lying on the ground; it is a beam whose supports are the pile heads, and the soil beneath it is deliberately kept out of the load path. That is the testable distinction from the nearest sibling, the piled foundation itself: piles are trying to engage the ground along and beneath them, while caps and beams are trying not to touch it.
What sits directly under the beams is therefore a designed absence. Where the ground can swell - clay recovering after trees have been removed, or material heaving against a buried element - a void former or compressible layer is placed beneath the beam so the ground has somewhere to move without pushing on it. That layer belongs to the system even though its purpose is to collapse or to disappear.
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.
- capping beam system
- pile cap and tie beam
- suspended ground beam foundation
- ground beam grillage
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.
- Collect loads arriving as lines and points and redirect them into the discrete positions where piles have been installed.
- Give walls, columns and ground floors a continuous and accurately positioned surface to bear on.
- Keep the ground directly beneath the beams from bearing on them or swelling against them, so that movement in that ground is not delivered into an element designed to span.
- Absorb the difference between where a pile was meant to be and where it ended up.
- Tie the pile positions together so that the foundation acts as a connected arrangement rather than as separate supports.
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.
- AttachmentPile head connection zone
The region where reinforcement projecting from a trimmed pile is embedded into the cap. It is where pile and cap become a single element, and its geometry is fixed by where the pile actually finished rather than by where it was drawn.
- Primary supportPile caps
Blocks over single piles or pile groups that gather load and redirect it downward. Where a cap sits over a group, it also has to share load between piles that will not all be in exactly the position or the condition assumed.
- Primary supportGround beams
Beams spanning between caps that carry walls, floors and any load applied between pile positions. Because they span, their depth follows the distance between supports and the load they gather rather than the wall thickness above them.
- Cavity or voidVoid or compressible zone beneath the beams
The deliberate separation between the underside of the beam and the ground, formed by a void former or a compressible layer. It exists so that ground able to swell or heave has somewhere to go without loading an element designed to span.
- SubstrateWall and column bearing surface
The top of the beams and caps, which becomes the datum for everything above. Its level and position govern how the superstructure is set out, and its width has to suit whatever bears on it including any external leaf.
- Control layerDamp-proof course seating
The line on the beam where the damp control of the wall begins and where it has to be joined to the floor arrangement. A structural element is carrying a control-layer duty here, as it does on a strip, but at a level set by the beam rather than by the ground.
- Service zoneService and drainage penetration provision
Openings formed through beams for drainage and incoming services. Because the element is spanning, an opening is a hole through a beam rather than through a lump of concrete, so its size and position belong to the beam design.
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.
Everything in this assembly exists to change the shape of a load path. Load arrives spread along a wall and has to leave at discrete points, so the beam gathers it and the cap redirects it into the piles below. Where a wall crosses a cap the beam is barely working; between caps it is doing everything. Pile positions, not wall positions, therefore decide where the beams have to be deepest.
The void beneath is a component and not an absence. Because the beams span, the ground under them is excluded from the load path by intention; where that ground can swell, the void former or compressible layer is there to give it somewhere to move into rather than bear on the beam soffit. If that layer is omitted, crushed before the beam is cast, or filled with spoil, the beam is loaded from below by something the design assumed would not reach it. Swelling ground can also act against the sides of buried beams and caps, which is a separate design matter.
Cap and pile act together only through the steel crossing between them. Reinforcement projecting from each trimmed head has to be embedded within the cap, and the cap reinforcement has to be arranged around it. A pile that finished away from its intended position moves that crossing point and can leave the group off-centre beneath the load it was meant to receive, which is resolved by redesigning the cap.
The beams then become the datum for everything above. The wall damp-proof course sits on them, the ground floor bears on or spans between them, and every drain and incoming service has to cross them somewhere. A hole cut through a beam after casting is a hole through a spanning element, so penetration positions belong with the beam design, and where the floor bears decides whether the beam carries the floor as well as the wall.
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
Caps and beams are commonly encountered in these families, cast in place or delivered as precast units. Which arrangement suits a given grillage is a structural design matter informed by the pile layout and by the supplier documentation.
Cavity or void
Void-forming and compressible products beneath beams are commonly encountered in this family, in forms intended to compress or collapse under the pressure they receive. Void formers made to break down after casting belong to a different product family. What a particular ground condition calls for is set by the geotechnical assessment and the product documentation.
Control layer
The damp control that begins on the beam is commonly encountered in these families. How it is joined to the floor arrangement decides whether the transition works, since no single component resists ground moisture on its own.
Substrate
Masonry built off the beams is commonly encountered in these families. Whether any is appropriate at this level, given exposure and the ground chemistry identified on the site, is for the designer to establish from manufacturer information.
Jointing and sealing
Sealing around penetrations through beams is commonly detailed with components from these families. What a given penetration requires depends on what is passing through it and on the continuity the barrier arrangement has to maintain.
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 piles below
This layer exists because piles finish at points and the building does not. The tolerance achieved during installation is absorbed here, so pile position records are the first thing cap design needs and the last thing an owner should let go unrecorded.
Read about Piled Foundation →Ground floor spanning between beams
A suspended ground floor commonly bears on the ground beams, which means the beams carry the floor as well as the wall. Bearing positions, the space left beneath the floor and any ventilation of that space are decided with the beam layout.
Read about Beam and Block Floor →Timber floor bearing and underfloor space
Where a timber floor is used, the beams define both the bearing line and the underfloor space beneath it. How that space is ventilated and how timber is kept clear of the concrete are junction questions rather than floor questions.
Read about Suspended Timber Ground Floor →Wall bearing on the beam
The beam has to be wide enough for everything the wall brings, including any outer leaf and any insulation between them. A wall set out after the beams are cast can find itself short of bearing at exactly the point the beam is weakest.
Read about Load-Bearing Masonry →Damp continuity at beam level
The damp-proof course starts on the beam rather than on the ground, so its relationship to the external ground level is set by the beam and by the finished landscape around it. Both have to be considered at the same time.
Read about Damp-Proof Continuity →Drainage crossing spanning elements
Every drain has to pass through or beneath a beam, and beneath is rarely available because the void is doing something else. Openings therefore have to be designed into the beam, which means drainage layout has to be settled early.
Read about Buried Drainage Network →
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.
- Movement
- The arrangement is intended to take the ground immediately beneath the beams out of the load path, so that swelling or shrinkage there is not bearing on a spanning element. Whether the ground on a particular site will swell, and what provision that calls for beneath and beside buried elements, is a geotechnical assessment rather than a standard detail.
- Moisture
- The beams sit in wet ground and carry the start of the damp control for the wall above. How the buried faces are treated, and how the damp-proof course relates to the finished external level, are decided by the designer for that exposure.
- Durability
- Ground chemistry and groundwater act on elements that will be inaccessible for the life of the building. What the ground investigation reported about aggressiveness is what informs the concrete and any protection specified for these elements.
- Buildability
- Excavation for caps, trimming pile heads, placing void formers and casting deep beams all happen in a confined and often wet zone. The void formers in particular have to survive everything that follows until the beam is cast over them.
- Interfaces
- This is an interface system by nature: it exists between piles and structure and touches drainage, floors, walls and damp control at once. Almost every question about it is a question about something it meets rather than about the element itself.
- Documentation
- As-installed pile positions, the cap and beam drawings and the record of what was placed beneath the beams are what allow later work to proceed safely. Cutting into a ground beam without them risks a spanning element and a barrier at once.
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.
- Are the beams designed to span between pile positions, and does anything rely on the ground beneath them?
- What provision is made under the beams where the ground could swell, and how is it protected until the beam is cast?
- Are the recorded pile positions available to the engineer designing the caps, and have deviations been assessed?
- Where do drains and services cross the beams, and are those openings shown on the beam drawings?
- Does the ground floor bear on the beams, and has that load been included in their design?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A ground beam is not a strip foundation cast at a lower level, because it is designed to span between supports rather than to bear on the soil.
- The void beneath a beam is not wasted space or poor workmanship; it is a designed separation giving ground that can swell somewhere to move without bearing on the beam soffit.
- Provision beneath a beam does not deal with heave by itself, because swelling ground also acts on the sides of buried elements and as uplift on pile shafts, so it forms one part of a strategy set by the geotechnical designer.
- Correcting a misplaced pile is not a matter of adjusting the wall above, since the redesign happens in the cap that connects them.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What ground conditions led to a void or compressible layer being specified, or to it being left out?
- How much pile position deviation can the cap design absorb, and what happens beyond that?
- Is the ground floor carried by the beams, and does that change their depth or reinforcement?
- How are the buried faces of the beams treated, given what the ground investigation found?
- Which drawing shows the damp-proof course seating and its junction with the floor arrangement?
- What record will exist of pile positions, void former placement and beam penetrations after completion?
What this page does not do
- Cap and ground beam design is structural work for a qualified engineer, coordinated with the specialist responsible for the piles.
- No depth, reinforcement, void provision or penetration size is stated here, because each follows from the pile layout and the ground on that site.
- Forming an opening through an existing ground beam affects a spanning element and should not be attempted without structural advice.
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.
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 →