Sub-base Compaction with a Vibrating Plate

Compaction happens in several stages, and each stage builds on the one before. One pass over the finished gravel is not enough.

• Compacting the formation: once the topsoil, roots and soft earth have been dug out, the bottom of the excavation is compacted first. This bottom layer is called the formation. If it is soft or clayey, the stone above it will keep moving.
• Building the sub-base in layers: crushed stone is spread in layers usually around 10 to 20 cm thick. A lighter plate copes with the thinner end of that range, while a heavier one can handle thicker layers. Each layer is compacted on its own before the next one goes on.
• Overlapping passes: the plate is guided steadily in strips that overlap slightly. Each layer usually needs roughly 3 to 6 passes, ideally alternating lengthwise and crosswise.
• Checking levels and falls: after every layer, levels and falls are checked with a straightedge and a string line. Next to a house the surface should fall away from the building, usually by around 1 to 2 %.

One point that older guides often get wrong: the bedding layer of fine grit or sharp sand directly under the paving is not compacted before laying. It is only screeded level. The vibrating plate comes back once the paving is laid, fitted with a protective rubber mat so the blocks settle without being damaged.

Well-graded crushed stone compacts well. It is a mix of coarser and finer particles: the large stones carry the load and let water drain, and the fines fill the gaps between them. Rounded river gravel or single-size stone tends to shift around under the plate instead of locking together.

The total depth of the sub-base depends on what will use the surface:

• Paths and patios: a thinner sub-base is usually enough, often somewhere around 15 to 25 cm
• Driveways and parking for cars: the sub-base is usually deeper, roughly 30 to 50 cm, laid in several layers with coarser stone at the bottom
• Heavier vehicles: the build-up should be designed by an engineer or an experienced contractor

Why so many layers? A single thick layer only compacts near the top and stays loose further down. Several thinner layers give even strength through the full depth. On impermeable clay it is worth thinking about drainage, and sometimes about a geotextile membrane between the soil and the stone so the two materials do not mix.

How much work the job takes, and therefore what it costs, depends mainly on:

• Type of ground: sandy, free-draining soil behaves far better than wet clay
• Number and thickness of layers: a driveway needs several times more compaction than a path
• Access: narrow spots, steps and areas next to walls and kerbs need a smaller machine or hand tools
• Moisture and weather: a waterlogged base has to dry out first, a very dry one needs light watering
• Slopes: the plate is harder to control on a gradient and progress is slower
• Underground services: shallow pipes and cables have to be located and avoided first

Before you start, check with your local building authority whether the new paved area or driveway needs permission. Rules differ from place to place, and things like a new dropped kerb, drainage onto the road or a large hard surface may need approval. If you live in a block of flats or the land is shared, the building management or owners' association has the final say.

• Too thick a layer at once: it feels firm on top but settles below the surface over time.
• Too few passes or moving too fast: the area looks finished, but some spots stay soft.
• Skipping the edges: the base is weakest next to kerbs and walls, and that is where paving sinks first.
• Wrong moisture content: dry stone will not compact, and soaked material turns to mud.
• Unsuitable material: rounded gravel, dirty building rubble or soil with organic matter in it.
• Ignoring a soft formation: even perfect stone will sink if the bottom of the dig is soft.

For a path or a patio you can do this yourself with a plate from a hire shop. For a driveway or on poor ground, consider a professional. Have underground services located before you dig. If you damage a gas pipe or smell gas, leave the area straight away and call 112 or your local emergency number. Any repair must be done by a qualified, registered gas engineer.

1. Mark out the area, levels and falls with string lines and pegs.
2. Remove topsoil, roots and soft earth down to firm ground.
3. Compact the formation with several passes of the plate and replace any soft spots with stone.
4. Set the kerbs or other edge restraint in line with your chosen method.
5. Spread the first layer of crushed stone roughly 10 to 20 cm thick, depending on the weight of the plate.
6. Check the moisture: a handful of material squeezed in your fist should hold its shape without water running out.
7. Run the plate over the area in overlapping strips, starting at the edges and working towards the middle.
8. Repeat the passes crosswise until the plate stops leaving visible marks.
9. Check levels and flatness with a straightedge, top up any dips and compact again.
10. Lay and compact the remaining layers the same way, up to the level just below the bedding layer.

Wear sturdy boots, gloves and ear defenders while you work. Never run a petrol plate in an enclosed space. On a slope, guide the plate up and down the fall, never across it. Take care near old walls and cellars, because strong vibration can damage weak structures.

Before the bedding layer and paving go down, go through this list with your contractor:

• The formation was compacted before any stone went on
• The stone was laid in layers and each layer was compacted separately
• Walking on it in heavy boots or running the plate over it leaves no marks
• There are no soft or sunken spots along the edges and kerbs
• A straightedge laid on the surface shows no noticeable gaps or humps
• The fall runs away from the house and water has nowhere to collect
• The sub-base depth matches what was agreed for the expected load
• For driveways and larger areas, a bearing test such as a plate load test was carried out if agreed