Structure and support · Foundations & Ground
Ground Improvement System
A reference account of ground improvement as an assembly in which the ground is a designed component, explaining why the treatment and the foundation above it form a single decision and what to establish with a geotechnical specialist.
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 ground improvement is
Ground improvement changes the ground rather than the foundation. This entry is organised around the case where the treatment leaves a composite behind: stiffer columns of granular or bound material, the soil between them, and the layer that spreads load onto their heads. Densification in place and dig-and-replace share the same intention and are named here as preparatory treatments, but they install nothing and leave no assembly, so the component and interaction discussion below does not describe them. The intention in each case is the same: to make the near-surface ground able to carry a shallower and simpler foundation than it otherwise could.
This is the entry where the ground stops being a condition to be accommodated and becomes a component to be designed. The practical consequence is that treatment and foundation are decided together. The treatment changes what the foundation has to do, and the foundation chosen decides how much treatment is worth carrying out, so specifying either alone leaves the other arbitrary.
The boundary against the nearest sibling, the piled foundation, is read from the finished load path. After improvement the building is still supported by the near-surface ground, which is why a shallow foundation remains possible. Piling exists precisely because that ground is not going to carry the building. Treatment that ends up transferring load into a deeper stratum through rigid columns sits close to the boundary, and where it falls is a geotechnical judgement.
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.
- ground treatment
- soil stabilisation system
- vibro stone column treatment
- engineered fill platform
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.
- Make near-surface ground stiffer and less compressible so that a shallow foundation becomes a realistic option.
- Reduce the difference in behaviour between one part of a site and another, so the structure above is not following that variation.
- Deal with fill, made ground or disturbed ground in place rather than removing and replacing all of it.
- Provide a working platform that construction plant can operate from as well as ground the finished building can bear on.
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.
- SubstrateTreated ground zone
The volume of ground that has been densified, replaced or bound. It is the component the finished building bears on, and unlike the other components in this category it is not delivered to site but produced there.
- Primary supportStiffening inclusions or columns
The granular or bound elements installed through the weaker material at a set pattern. They do not carry load alone; they work by being stiffer than the soil around them and drawing load towards themselves while that soil carries the rest.
- SubstrateLoad transfer layer and working platform
The layer between the treated zone and whatever is built on it. It carries construction plant while the works proceed. Where stiff or bound inclusions are used it is commonly given a permanent role as well, spreading foundation load across inclusion heads and the soil between them; where a footing bears on the composite treated ground directly, the platform can be a construction expedient only.
- Drainage planeDrainage path through granular inclusions
Where inclusions are granular they are more permeable than the soil around them, which gives water in that soil a route out. This is part of why treated ground settles sooner rather than later, and it also links strata that were previously separate.
- Edge and terminationTreatment boundary and untreated margin
The edge of the treated area, where behaviour changes over a short distance. Foundations near that edge, and any part of the structure that runs off the treated zone, are where differential movement is most likely to appear.
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.
Treatment and foundation form a single design rather than separate ones. What the improvement achieves changes what the foundation has to do, and the foundation intended decides how much improvement is worth carrying out. Specified apart, they are easily mismatched: ground improved for a raft is not necessarily ground improved for isolated pads, since each applies load in a quite different pattern.
Stiffening inclusions work by sharing load with the soil between them, not by replacing it. Load arriving at the surface divides between column and soil according to their relative stiffness, so the pattern of inclusions and the condition of the soil between them are equally part of the design. Where that soil cannot hold its shape around a granular column, the column has nothing to push against and cannot stay stiff.
Which element does the sharing depends on the arrangement. With stiff or bound inclusions a load transfer layer is commonly what spreads foundation load across inclusion heads and the soil between them, and a layer contaminated with soft spoil, reduced once the plant has gone, or left out because nobody realised it had a permanent role changes how load arrives at those heads. Where a footing bears on composite treated ground directly, the footing itself does the sharing and the platform is a construction expedient.
Granular inclusions also change how water moves through the ground. Being more permeable than the soil around them, they give trapped water a path, which is part of why treated ground settles sooner rather than later. That same permeability means the treated zone can become a route between strata, so treatment and the site water-control strategy have to be discussed together.
The boundary of the treated zone is where behaviour changes fastest. A foundation that runs off the treated area, or a treated area that stops short of the most heavily loaded corner, leaves a single structural element bearing on grounds that behave differently along its length. That is the classic arrangement for differential movement, and it is set by the extent of treatment rather than by the foundation drawing.
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
Where inclusions are bound rather than granular, or ground is mixed with a binder, these families are commonly encountered. What is appropriate for a given soil chemistry and treatment method is a specialist geotechnical decision supported by product documentation.
Substrate
Where treatment is finished with a concrete working surface, or the overlying foundation is cast directly onto the treated formation, these families are commonly encountered. Whether that arrangement is appropriate is for the geotechnical and structural designers.
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.
Raft bearing on treated ground
A stiff plate and stiffened ground are alternative responses to the same problem, and using them together is a design choice rather than a belt-and-braces default. The pair has to be assessed as one, because each changes what the other has to do.
Read about Raft Foundation →Shallow footings on improved ground
Continuous footings on treated ground bear across inclusions and the soil between them, so the relationship between footing width and inclusion pattern matters. A footing narrower than the pattern assumed can sit almost entirely on untreated soil.
Read about Strip Foundation →Floor slab over the treated zone
Slabs are usually more sensitive to residual settlement than the frame above them, because a small differential shows as a visible defect. Whether the treatment extends under the whole slab, or only under the loaded lines, is worth establishing early.
Read about Ground-Bearing Slab →External hardstanding beyond the building
Treatment usually stops at or near the building line, so external paving frequently sits on untreated ground next to a building that does not. The step in behaviour appears at the threshold, which is exactly where it is most noticeable.
Read about Pavement Foundation →Neighbouring ground, services and structures
Densification and displacement methods move ground beyond the area being treated, and buried services or shallow foundations nearby can respond. Condition surveys and monitoring around the treated zone are specialist matters settled before work begins.
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
- Improvement reduces settlement but does not remove it, and what remains is judged against what the structure can accept. Residual and differential movement are the questions the geotechnical designer answers, not properties of a treatment method.
- Moisture
- Treated ground can drain differently from the ground around it, which changes how water moves under and beside the building. Whether that helps or creates a new path is site-specific and belongs with the drainage and geotechnical designers together.
- Buildability
- Most methods need heavy plant, working room and a platform to operate from, and some are ruled out by access or by proximity to a boundary. What is technically suitable for the soil may be impossible on the site it is proposed for.
- Interfaces
- The important interfaces are with the foundation above and with the untreated ground beyond the treated edge. Both are places where one element ends up bearing on materials that behave differently from one another.
- Documentation
- Records of what was treated, where, to what extent, and the verification testing that followed are the evidence a shallow foundation was justified. Without them the treated zone is invisible, and a later purchaser or engineer has nothing to rely on.
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 foundation is the treatment being designed for, and would a different foundation change what treatment is needed?
- How far does the treated area extend, and does every loaded part of the structure sit within it?
- What residual settlement is expected after treatment, and can the intended structure accept it?
- How will the improvement be verified once it is complete, and who reviews the results?
- What effect will the works have on neighbouring ground, buried services and adjacent structures?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Ground improvement does not turn poor ground into good ground; it makes ground behave more predictably for a specific foundation and a specific structure.
- Stone or granular columns are not short piles, because they rely on the soil around them for support rather than bypassing it.
- A working platform is not temporary site provision alone, since the same layer often has a permanent role in spreading load onto the treated ground.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What ground investigation supports the treatment design, and how variable is the site across its area?
- Which method is proposed, and what alternatives were considered against the access and the neighbours?
- How is load intended to be shared between the inclusions and the soil between them?
- What verification testing is planned, and what happens if a treated area does not meet expectations?
- How does the treated area relate to the drainage strategy and to any water moving through the site?
What this page does not do
- Ground improvement is specialist geotechnical work, designed and verified by qualified professionals rather than selected from a description.
- No treatment depth, inclusion pattern, platform thickness or settlement figure is given here, because all of them are properties of a specific site.
- Works of this kind can affect ground beyond the site boundary, so neighbouring structures and services have to be considered before starting.
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 →