Structural and base materials · Aggregates & Groundworks
Crushed Rock and Unbound Granular Fill
This entry is about the material itself — what is delivered by the load and compacted in layers — and deliberately not about the positions it fills, which the library publishes as construction components.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What crushed rock and unbound fill is
This is the crushed rock, gravel, sand and recycled aggregate that is delivered by the load, spread in layers and compacted, with nothing added to bind it. Nothing cures. The material gets its behaviour from the interlock between its particles and from how tightly it has been compacted, which is why two loads of identical stone can behave quite differently depending only on what was done to them after they were tipped.
Grading is the first thing that separates one material from another. A well-graded material contains a spread of particle sizes, so the small particles fill the spaces between the large ones and the whole mass locks together densely when it is compacted. A single-sized material does the opposite: it stays open, drains freely and does not lock at all. Neither is better than the other in the abstract — they are chosen for different purposes — but confusing them is one of the more consequential mistakes available in groundworks.
Compaction is the second. The material is placed in layers and compacted layer by layer, because a thick lift compacted from the top is dense at the top and loose at the bottom. Moisture content matters while this is happening: too dry and the particles will not move past one another into a dense arrangement, too wet and the water holds them apart. What the right condition is, for a given material, is a ground-engineering determination rather than a judgement made on site by eye.
The material's source is the third variable. Quarried crushed rock, natural gravels and sands, and recycled aggregate produced from crushed concrete and demolition material are all used, and recycled material carries its own questions of consistency, contamination and what it was before. Whatever the source, this entry is about the substance being delivered and compacted. Where it sits in a build-up — whether it is acting as a load-spreading layer under a pavement, as placed ground beneath a structure, as a separating layer, or as the fill of a trench — is a matter of position, and the library publishes those positions as construction components rather than as materials.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Hardcore (UK trade usage)
- Crushed rock fill
- Unbound granular material
- Scalpings (UK trade usage)
- Crusher run (North American trade usage)
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Compacted layers beneath ground-bearing slabs, drives, paths, patios and hardstanding.
- Raising, replacing or making up ground where levels have to be built up in controlled layers.
- Working platforms formed so that plant and materials can move across a site.
- Bedding and surround to buried pipes and ducts, and backfill to trenches.
- Free-draining material placed behind retaining structures and around land drains.
- Loose surfacing to tracks, paths and parking areas where a bound surface is not wanted.
Consideration
Where it is commonly considered
- Where load has to be spread over ground that cannot accept it concentrated, and a granular construction has been designed.
- Where levels are being made up and the ground being built has to have known rather than discovered properties.
- Where drainage through the construction is part of what the layer is for.
- Where recycled aggregate is available and its source and consistency can be established.
- Where site investigation has been carried out and the design of the construction follows from it.
- Where compaction can actually be carried out — access for plant, workable material, and the space to do it in layers.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where no site investigation has been carried out and what lies beneath is unknown.
- Where material is being placed and compacted without anyone having specified what material or what compaction.
- Where the water in the ground, or the water the construction will carry, has not been considered.
- Where a well-graded and a single-sized material are being treated as interchangeable.
- Where recycled material of unknown origin is being used and no one has established what it contains.
- Where fine material beneath could migrate up into the layer, or the layer could work down into it, and no separation has been considered.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Substrate dependency
- The layer is only as good as what it sits on. A compacted granular construction spreads load; it does not create bearing capacity that the ground beneath does not have, and what the ground can accept is established by site investigation.
- Installation dependency
- Placing in layers and compacting each one is the whole method. Moisture condition during compaction, the plant used, and the number of passes are determinations made by the designer rather than adjusted by eye on site.
- Moisture behaviour
- Water moves through this material rather than being stopped by it, and grading decides how freely. Where water goes once it is in the construction, and where it leaves, is a design question that the material alone does not answer.
- Documentation
- The material arrives against a description of its source, its type and its grading, and for recycled aggregate a record of its origin. That description is the only way of knowing what has actually been delivered.
- Maintenance
- Where the material is left as a loose surface, it migrates under traffic, ruts, and needs periodic raking and topping up. Where it is buried, the surface above is what shows any settlement in it.
- Repairability
- A buried granular layer can only be reached by taking up whatever is over it, so any inadequacy in it is expensive to correct later and cheap to avoid now.
- Weathering and exposure
- Frost action, saturation and freeze-thaw affect a granular construction differently depending on how much fine material it holds and how well it drains, and that is a matter for the design rather than for the delivery ticket.
- Surface wear
- Where the material forms the trafficked surface itself, the hardness and shape of the particles decide how it stands up to turning wheels and how quickly it migrates.
Exposure
Exposure and environmental conditions
- Where does water in this construction come from, and where does it go?
- Is there groundwater at or near the level being worked at, and does it move seasonally?
- Is the material going to be exposed to frost before it is covered?
- What will cross the finished construction, and how often?
- Are there trees nearby whose roots or whose effect on the ground matter here?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The formation beneath is prepared and its condition established before any material is placed on it.
- Where fine soil lies beneath, a separation arrangement between it and the granular layer is a design decision made for those particular soils.
- The material is placed in layers, so the sequence and the thickness of lifts belong to the designer rather than to the plant operator.
- What is laid over the layer — a slab, a membrane, a bedding course, paving — determines how the top of it has to be finished.
- Where a membrane or a damp-proof layer is laid over the material, a blinding or a finer surface is commonly needed so it is not punctured.
- Drainage within and at the edges of the construction is designed with it, because a granular layer that fills with water behaves differently from one that drains.
Appearance
Appearance and finish considerations
- In most constructions the material is buried and its appearance is irrelevant; what shows is any settlement in the surface above it.
- Where it is the finished surface, colour and particle size are the whole of the appearance, and both come from the source rock.
- Recycled material varies visibly from load to load, which matters where it is left exposed.
- A single-sized loose surface reads as open and rounded; an angular well-graded material reads as compacted and tight.
- Edges are what keep a loose granular surface where it was put, so how the construction is contained is both an appearance and a performance matter.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What did the site investigation find, and what has been designed on the strength of it?
- What material has actually been specified, and does what arrived match it?
- In what layers is it being placed, and how is compaction being carried out and checked?
- Is separation from the soil beneath needed, and has it been designed?
- Where does water in this construction go, and what happens in a wet season?
Upkeep
Maintenance and repair considerations
- If the surface over this layer settles, what would be involved in finding out why?
- Where the material is a loose surface, who will rake and top it up, and how often?
- How are the edges being held, and what happens where vehicles turn?
- Has what was placed, and the compaction that was carried out, been recorded anywhere?
- Would the same material be obtainable if part of the construction had to be made good?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Compaction is carried out layer by layer, and the layer thickness is part of the specification rather than of the day's progress.
- The moisture condition of the material at the time of compaction affects the result and is managed rather than ignored.
- The plant used has to suit the material and the confined or open space it is working in.
- Trafficking a partly compacted layer with delivery vehicles changes it, so the sequence of the works matters.
- Material stockpiled on site segregates and picks up contamination from the ground beneath it.
- Working in wet weather on fine-containing material can spoil a formation that was sound the day before.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The specification for the material, stating its type, source and grading.
- For recycled aggregate, the record of its origin and what it was produced from.
- Delivery documentation for what actually arrived, kept alongside the specification.
- The designer's requirements for layer thickness, compaction and any testing.
- Records of the compaction or testing that was carried out, and by whom.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What did the site investigation find, and how has it shaped this construction?
- What material is specified here, and why that grading rather than another?
- In what layer thickness is it being placed, and how is compaction being verified?
- Is a separation arrangement needed between this layer and the soil beneath it?
- Where does water in this construction go, and how is that arranged?
- Is recycled material being used, and what is known about where it came from?
- What is being laid directly on top of this, and how does that affect how the surface is finished?
- What records will exist of what was placed and what compaction was achieved?
- Who is responsible for the design of this part of the works?
Blind spots
Commonly overlooked points
- Nothing here cures: the material's behaviour comes from grading and compaction, and a load of the right stone badly compacted is not the right construction.
- Well-graded and single-sized materials do opposite things, and the words for them are used loosely in ordinary conversation.
- Spreading load is not the same as creating bearing capacity; what the ground beneath can accept is a separate question with a separate answer.
- Fine soil below can work up into a granular layer and the layer can work down into it, which is what separation arrangements exist to prevent.
- Compaction in thick lifts leaves the bottom of the lift loose, and nothing about the finished surface reveals it.
- Once it is buried under a slab or paving, this layer is among the hardest parts of the construction to put right and among the simplest to get right while it is still open.
What this page does not do
- This page states no grading, no layer thickness, no compaction requirement, no bearing value and no material designation: every one of those is a ground-engineering determination made from site investigation by qualified professionals.
- Nothing here describes where such a material should be used or how deep any construction should be. Those are design matters for the parties responsible for the works.
- A compacted granular layer does not make ground able to carry a load, and no statement on this page should be read as indicating that it does.
- Local requirements govern groundworks, drainage and the use of recycled material in a given place, and they are confirmed with the relevant authority.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
Related entries
Often used alongside
Entries commonly encountered together with this one. Co-occurrence only — never a claim that any combination is compatible or suitable.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Aggregates & Groundworks Materials
Reference entries for the unbound and geosynthetic materials that make up the ground beneath a building — crushed rock, fills, bedding, drainage media, geotextiles and the cellular products laid with them.
Browse all aggregates & groundworks entries →