Building Structurally Sound Foundation layers for Paving
The longevity/success of the paved surface is by large dependent on the foundation layers beneath. A correctly constructed foundation distributes loads and provides consistent support for paved surfaces..
For domestic paving, the foundation usually comprises a base and, where ground conditions or traffic require it, a sub-base. These layers must be formed using suitable materials, installed to the required depth and properly compacted.
Extra bedding material should never be used to disguise an uneven or poorly constructed base. If the foundation is weak, incorrectly levelled or inadequately drained, the finished paving may develop depressions, ruts, rocking units, open joints or standing water.
Check for Buried Services
Before excavating, establish whether pipes, cables or other services run through the area. These may include electricity, gas, water, drainage and communications infrastructure.
Plans can provide useful information, but the position of services should be confirmed where possible using suitable detection equipment. Known or suspected services should be carefully exposed by hand before mechanical excavation begins.
This protects those working on the site and reduces the risk of damage, disruption and costly repairs.
Prepare the Ground
Once excavation is complete, inspect the exposed ground, known as the subgrade. It should provide a firm and reasonably even platform for the pavement.
Before adding foundation material, the subgrade should be compacted and checked for:
- Soft or moving areas
- Loose material
- Ridges, cracks and depressions
- Potholes or ruts
- Waterlogged or frozen ground
- Topsoil, loose fill or unexpected materials
Defective ground should be removed and replaced. If the conditions differ from those identified during the site assessment, review the proposed pavement construction before continuing.
For sites up to 100 m², the site investigation should normally include at least two trial holes. For larger sites, there should be at least one trial hole for every 50 m². Low points should be included because these are where poor ground and standing water are most likely to be found.
Use Suitable Materials
Foundation material should be clean, durable, properly graded and capable of being compacted to form a stable layer. Igneous rock is a preferred, followed Carboniferous limestone.
Suitable unbound sub-base materials include:
- Type 1 unbound granular material
- Type 3 unbound granular material
- Suitable Type 4 asphalt arisings
Recycled or alternative materials may also be suitable if they can provide an equivalent or better result. Appearance alone is not enough to confirm suitability. The material must have the appropriate grading, strength, compaction properties accompanied with the correct certification of specification, typically to SHW Clause 803 for type 1 and 805 for type 3. A geosynthetic mesh (geogrid) installed in layers adds considerably to the foundations structural integrity,helping in preventing creep/lateral movement.
Under no circumstances should asphalt arisings/road planings be used.
A bound base may be formed using asphalt, cement-bound granular material or concrete. For the lighter traffic categories covered by this guidance, a concrete base should be at least strength class C20/25.
Build in the Falls
The levels and drainage falls of the finished paving should be formed through the foundation layers. The laying course is there to provide an even bed for the paving, not to create the main gradient or correct large variations in the base.
Where an impermeable base sits beneath permeable bedding, the base should generally follow the fall of the finished surface. If it does not, water can collect under the paving and issues will almost certainly arise in freeze thaw cycles.
Secondary drainage may therefore be required where a concrete, asphalt or other impermeable base is used. This can include protected drainage holes or weepholes that allow water to leave the bedding layer without carrying bedding material with it.
Lay and Compact in Layers
Foundation material should be installed in manageable layers, with each layer compacted before the next is added.
Where the required compacted thickness exceeds 225 mm, it should be constructed in more than one layer. Each compacted layer should also be at least two-and-a-half times the size of the largest aggregate particle, this is where the correct compaction machinery is important.
The depth that can be compacted effectively in one operation depends on the material and equipment. Check that the compactor is suitable and follow the manufacturer’s instructions.
Unbound material should be moist but not saturated. Material that is dry can be difficult to compact as there’s no lubrication, while saturated or frozen material should not be compacted or used. If the larger and smaller particles have separated during delivery or spreading, remix the material before continuing.
Geotextile separation membranes are commonly used on top of the subgrade as a best practice. In reality with a subgrade CBR strength of 3% or less it is recommended. Geotextile should not be used on steep driveways, as this may cause a slip plane.
Pay Attention to Difficult Areas
Compaction must be consistent across the whole site, including:
- Edges and corners
- Narrow areas
- Locations around drainage fittings
- Areas beside kerbs and restraints
- Previously excavated trenches
- Places larger equipment cannot reach
Trenches crossing the pavement should be reinstated with compatible materials and carefully compacted. Poor trench reinstatement can produce visible lines of settlement through the completed paving.
Where a geosynthetic layer has been specified to improve foundation stability, it should be installed in accordance with its manufacturer’s instructions.
Inspect the Compacted Surface
After compaction, each layer should be firm, closed and stable. It should not move under construction equipment and should be free from cracks, ruts, loose material, segregation or other defects.
Any defective area should be removed through the full depth of the layer and replaced. If construction traffic has travelled over the completed base, inspect it again before installing the paving. Correct damaged levels and remove any contamination.
Do not cover defects with laying course material.
Check Levels as You Work
Levels and layer thicknesses should be checked throughout construction.
The Standard gives the following permitted variations from the intended level:
| Layer | Permitted Variation |
| Subgrade or formation | 20mm above to 20mm below |
| Sub-base | 5mm above to 10mm below |
| Base | 5mm above to 10mm below |
These tolerances do not permit any layer to be thinner than the minimum required construction thickness.
Where an unbound layer needs adjustment, the upper 50 mm should be loosened before material is added or removed and the area recompacted. The laying course should not be used to correct the level of the foundation.
A good foundation will not attract much attention once the project is finished. However, it is one of the main reasons a well-built pavement remains stable, even and properly drained throughout its life.
