Hardcore Sub-Base and Compaction: Material Types, Layer Thicknesses and Compaction Requirements for Slabs and Driveways
Quick Answer: Sub-base for domestic slabs and driveways is typically Type 1 MOT crushed limestone, granite, or recycled concrete, compacted in maximum 150mm layers using a vibrating plate compactor (for thin layers) or 75kg vibrating roller (for thicker beds). Minimum sub-base depth is typically 100mm for paths, 150mm for domestic driveways, and 250mm+ for HGV access. Each layer must be compacted to refusal before the next is placed.
Summary
The sub-base is the structural foundation layer below paving, slabs, and driveways. Its purpose is to spread the imposed loads from the surface down to the natural ground, to provide a stable construction platform, to aid drainage, and to prevent frost heave in susceptible soils.
Getting sub-base right is often the difference between paving that stays level for 20 years and paving that cracks, sinks, and becomes uneven within 5. Sub-base failures are almost always due to: insufficient thickness for the applied load, incorrect material specification, inadequate compaction, or failure to address underlying poor ground conditions.
Material selection and compaction technique are the two critical variables. Type 1 MOT (also called Type 1 DOT Specification) is the industry standard granular sub-base material for UK construction and is suitable for almost all domestic and light commercial applications.
Key Facts
- Type 1 MOT — graded crushed stone to the Specification for Highway Works (Series 800); the standard UK sub-base material
- Type 2 MOT — lower specification; includes some fines; not suitable where drainage is important; less common now
- Recycled concrete aggregate (RCA) — acceptable for many sub-base applications; must be free from contamination; check local authority acceptance for driveways
- Compaction in layers — maximum 150mm compacted layers for plate compactor; maximum 200mm for roller; thicker loose layers will not compact through
- Maximum uncompacted layer — roughly 200mm for Type 1 (becomes approximately 150mm compacted); never place in single lifts >200mm for plate compaction
- Compaction to refusal — compact each layer until no further settlement occurs under the compaction plant
- Geotextile membrane — geotextile (woven or non-woven) placed on natural ground before sub-base prevents subgrade contamination of sub-base and improves load spread
- Minimum depths — 100mm for lightly loaded paths, 150mm for domestic driveways, 200–250mm for commercial driveways, 300mm+ for areas accepting HGVs
- Subgrade preparation — remove all topsoil, organic material, soft spots before placing sub-base; do not compact onto soft/wet ground
- Test by proof rolling — fully loaded lorry or loaded bowser rolled over compacted sub-base to check for deflection; any soft spots require further work
- Frost protection — sub-base below frost penetration depth (450mm) is recommended for driveways; where sub-base is shallower, use frost-resistant materials or accept some seasonal movement
Quick Reference Table: Sub-Base Design Depths
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Try squote free →| Application | Minimum Sub-Base Depth | Notes |
|---|---|---|
| Pedestrian path (low use) | 100mm Type 1 | No vehicle access |
| Domestic garden path | 100mm Type 1 | Light foot traffic |
| Domestic driveway (cars only) | 150mm Type 1 | Standard residential |
| Domestic driveway (vans/light delivery) | 200mm Type 1 | Frequent van access |
| Commercial car park | 200–250mm Type 1 | Low-speed vehicles |
| Commercial driveway (HGV occasional) | 300mm Type 1 + capping if needed | Occasional lorry |
| HGV circulation/loading area | 350–450mm (structural design required) | Regular lorry use |
Detailed Guidance
Type 1 MOT Material Specification
Type 1 MOT is specified in the Specification for Highway Works (SHW), Clause 803. It is a well-graded crushed rock or recycled concrete aggregate with:
- Maximum particle size: 63mm
- Grading passing 63mm to passing 75µm sieve specified within bands
- Less than 10% by mass finer than 75µm (to limit fines that reduce drainage and frost resistance)
- Material: crushed limestone, granite, dolerite, or recycled concrete aggregate
Type 1 is self-draining when compacted — the interlocking angular aggregate provides a rigid, load-spreading structure. This is critical: poorly graded or rounded materials (e.g. gravel) do not achieve the same stability under repeated load.
Type 1 is available from quarries and builders' merchants as "Type 1 MOT", "75mm road base", or "crusher run". Always check the specification sheet; "crusher run" from some suppliers may not meet the grading requirements.
Subgrade Preparation
The quality of the sub-base depends on the quality of the subgrade (the natural ground or fill) below it. If the subgrade is poor, even a perfect sub-base will fail. Key subgrade requirements:
- Remove all topsoil — topsoil contains organic matter, is compressible, and is not suitable for load-bearing; minimum 150mm removal in most cases
- Remove soft spots — if excavating reveals soft, wet, or otherwise poor areas, over-excavate and replace with compacted Type 1 or capping layer material
- Compact the subgrade — run a vibrating plate or roller over the exposed subgrade surface before placing geotextile or sub-base; this eliminates loose, disturbed material
- Drainage — if the subgrade is wet or waterlogged, consider French drains or land drainage around the perimeter before placing sub-base; building a sub-base on waterlogged ground is problematic
Geotextile separator: Place a non-woven geotextile (minimum 200 g/m² for driveways) on the prepared subgrade before placing sub-base. This:
- Prevents clay and fine particles from migrating up into the sub-base (reducing permeability and load-spreading ability)
- Improves load distribution over weak subgrade
- Allows long-term water drainage through the sub-base into the ground
For soft or silty soils, a woven geogrid (stabilising geogrid) can be placed on the subgrade to further improve load distribution.
Compaction Technique
Equipment:
| Equipment | Application | Max Layer Thickness |
|---|---|---|
| Forward plate compactor (300–400kg) | Small driveways, paths, confined areas | 100mm |
| Reversible plate compactor (400–800kg) | Standard driveways and slabs | 150mm |
| Vibrating roller (1–3 tonne) | Large areas, commercial driveways | 150–200mm |
| Rammer (trench compactor) | Trenches, confined spaces | 75–100mm |
Method:
- Spread sub-base in loose layers not exceeding the compaction equipment's maximum layer depth (typically 150mm for a reversible plate compactor)
- Compact in parallel overlapping passes; overlap each pass by at least 150mm
- Check that the material is compacting — if the plate bounces, material may be too wet (too wet to compact effectively); if too dry, lightly water to improve compaction
- Continue until no further settlement is visible under the compactor
- Check level — rake out high spots, fill low spots, and re-compact
- Place next layer; repeat
Moisture content: Type 1 MOT compacts best at near-optimum moisture content. In practice, material delivered in normal UK weather is usually at a suitable moisture. If material is very dry (from extended dry summer storage), lightly spray with water before compaction. If very wet, allow to drain for a day before compacting.
Compaction testing: For domestic work, visual assessment and depth measurement are usually sufficient. For commercial work, a plate bearing test or dynamic cone penetrometer test (DCP) confirms that the required bearing capacity has been achieved.
Levels and Falls
Sub-base should be laid to finished levels that provide the designed falls for the surface:
- Driveways: minimum 1:60 fall (approximately 16mm per metre) away from buildings
- Patios: minimum 1:60 fall away from the house
- Car parks: minimum 1:100 fall to drainage channels or soakaways
Do not use sub-base to achieve level correction for large discrepancies — address these at subgrade level. Sub-base should be approximately uniform depth across the area.
Special Considerations: Clay Subgrades
On clay soils, sub-base behaviour depends on the clay's moisture content and behaviour under load:
- In dry summer conditions, clay shrinks — sub-base may settle slightly into desiccation cracks
- In wet winter conditions, clay swells — sub-base may be pushed up
- Frost can heave clay and the sub-base above it
To reduce frost heave:
- Ensure total construction depth (sub-base + base + surfacing) exceeds 450mm
- Use frost-resistant aggregate in the upper layers
- Ensure drainage is good so clay stays at consistent moisture content
On highly shrinkable clays (near trees), the sub-base and surfacing above may experience significant seasonal movement regardless of sub-base quality. This is managed through flexible paving systems rather than rigid concrete slabs.
Frequently Asked Questions
Can I use recycled crushed concrete as sub-base?
Yes, recycled concrete aggregate (RCA) is acceptable for sub-base under most domestic applications, provided it meets the grading requirements of Type 1 specification, is free from contamination (some old demolition concrete contains asbestos or other hazardous materials — check with supplier), and is accepted by your local planning authority or building control for the specific application. RCA is widely used and environmentally beneficial.
How do I know if my compaction is adequate?
The simplest field test: after compaction, run the compaction plant over the layer again. If no further settlement is visible, compaction is adequate. Another test: stand on the compacted surface — it should feel firm, not "springy". For critical commercial work, a dynamic cone penetrometer (DCP) test or plate bearing test provides a measured assessment.
Do I need a licence to import and place Type 1 MOT on my driveway?
For domestic driveway use, no specific licence is needed. Planning permission may be required for driveways depending on the surface material and size (see planning permission driveways if it exists). No licence is needed to purchase or place Type 1 MOT on private property.
How thick should the blinding layer be above the sub-base?
If you're laying a concrete slab above the sub-base, a 50mm blinding layer (weak concrete or lean mix) between the sub-base and the structural slab is standard. The blinding provides a clean, level surface for reinforcement placement and prevents the structural concrete from losing water into the sub-base. If block paving is going directly on the sub-base, a 30–50mm sharp sand bed is used instead.
Regulations & Standards
Specification for Highway Works (SHW), Clause 803 — Type 1 MOT specification
Building Regulations Approved Document A — structural slab design on sub-base
BS 8500-1:2015 — Concrete specification; applies to concrete slabs placed on sub-base
DMRB (Design Manual for Roads and Bridges) HD 26/06 — pavement design for highway applications
BS 1924-2:2018 — Hydraulically bound mixtures; relevant for more sophisticated sub-base types
Specification for Highway Works — National Highways — Type 1 MOT specification (SHW Clause 803)
Marshalls Landscape Specification — Sub-Base Guidance — practical paving sub-base guidance
Natural Stone Specialist — Sub-Base Guide — practical installation guidance
strip foundation design — sub-base under strip foundations
oversite concrete slab — concrete slab design over sub-base
excavation safety trench support — safe excavation when installing sub-base
soil investigation trial pits — ground conditions affecting sub-base specification