Sub-base compaction with a plate compactor

Soil and imported stone both contain air between the grains. If you lay paving or concrete on such a layer, the grains slowly settle under load, water and frost. The results are dips, puddles, cracked slabs and leaning kerbs. Compaction makes this settlement happen in advance. Vibration and pressure rearrange the grains so they lock together and the layer stops compressing.

You need compaction almost every time something load-bearing is built on the ground:

• sub-base for block paving, paths and patios,
• driveways and parking bays,
• sub-base layers under a ground-bearing slab or ground floor,
• backfill of service trenches and around foundations,
• bases for garden sheds, garages or swimming pools.

Two things are compacted separately. The formation is the bottom of the dig, the natural ground you build on. The sub-base is the imported material on top, usually crushed aggregate in different grades. Both are compacted, but in different ways and with different results.

Preparing and compacting the formation

After digging, all topsoil, roots and soft spots are removed first. Topsoil is never compacted, because it contains organic matter that rots and settles over time. The formation is trimmed to the required fall and then run over with a plate or roller. Sandy and gravelly soils compact well. Clay and loam respond much less to vibration, and wet clay starts to "pump" under the plate: the surface ripples and water squeezes out. When that happens, the crew stops. Wet patches are dug out and replaced with stone, or the formation is left to dry.

On cohesive soils a separating geotextile is often laid. It stops fine clay from working its way up into the stone over the years and weakening the sub-base.

Placing and compacting in layers

Crushed aggregate is never placed at full depth in one go. It is spread in a layer the machine can compact right through, and only then is the next layer added. With light plates this usually means layers of roughly 15 to 20 cm. Heavier reversible plates and rollers can handle thicker layers, according to the manufacturer's data. A layer loses height as it compacts, so the material is spread slightly higher than the finished level.

Each layer is covered in overlapping passes, usually four to six times. The operator starts at the edges and works towards the middle, and on a slope works from the bottom up. An experienced operator can tell when a layer is done: the plate starts to bounce, the sound changes and the passes leave no marks. Heavier machines have a built-in compaction indicator that shows this directly.

Edges, corners and around services

A plate cannot get right up to a wall, into corners or tight against manholes and inspection chambers. These spots are finished with a vibrating trench rammer, sometimes called a "jumping jack", which has a small foot and is easy to handle. Over newly laid pipes, the bedding and surround are first compacted by hand or with a light machine. The heavier machine only goes on once there is enough cover. The sub-base should also extend beyond the edge of the future paving, so the kerbs do not end up on loose ground.

How long it takes

The passes themselves are quick. Most of the time goes into delivering, spreading and wetting the material. A typical driveway of a few dozen m² with two or three layers usually takes one to two days. Compaction has no drying or curing time, so the next trade can start on the finished sub-base straight away. If heavy rain or frost comes in between, the surface is checked before work continues and re-compacted if needed.

This page covers two price-list items: compaction with a plate compactor and compaction with a vibrating roller. Both are priced per m². They differ in the machine, how deep it compacts and the type of job it suits.

Light forward plate

This machine travels in one direction and is turned by hand. It suits paths, smaller patios and compacting the sand bed. It is also used to vibrate finished block paving through a rubber or polyurethane mat. For thick layers of stone under a driveway it is too weak.

Reversible plate

This heavier machine moves forwards and backwards without turning. It is the standard choice for sub-bases under driveways, car parks and the ground slabs of family houses. It handles thicker layers and works well in enclosed yards and along walls.

Vibrating roller

A walk-behind twin-drum roller or a ride-on roller pays off on larger, open areas such as long driveways, bigger car parks and access roads on commercial sites. It compacts deeper and covers more m² per hour. It does need room to turn and access for the transport vehicle, and on small areas its advantage disappears. Near older buildings, weak neighbouring foundations or fresh masonry, vibration needs thinking about first. Both rollers and heavy plates send shocks into the surroundings.

Trench rammer

The rammer is for narrow trenches, corners, the area around chambers and for cohesive soils. A plate only vibrates the surface, while a rammer drives the material down with repeated blows. In practice the machines are often combined: the plate or roller does the main area and the rammer does the edges.

When you compare quotes, ask three things. Which machine will the firm use, how thick will each layer be, and does the rate per m² cover one layer or the full build-up? Differences here are the most common reason why quotes that look the same cannot really be compared.

Sub-base layers use crushed stone with a continuous grading, typically 0/32 or 0/63. In the UK this is often sold as Type 1 sub-base. For levelling under paving, a finer crushed grade such as 4/8 or a similar product is used. Crushed grains have sharp edges and interlock. Rounded river gravel or pea shingle just rolls about under vibration and never forms a firm layer, so it has no place under areas that will carry vehicles.

Why does moisture matter? Dry material compacts badly, because the grains create a lot of friction and the fines blow away as dust. Waterlogged material pushes water around and will not compress. The ideal is slightly damp aggregate. Squeeze a handful: it should hold its shape without water running out. In summer the layers are sprayed before compaction. After rain, the crew waits until the water has drained away.

Frozen material and a frozen formation are never compacted. When they thaw, the layer settles and all the work is wasted.

Compaction is a quick job, but it is easily held up by things that should have been sorted out beforehand. Before the crew arrives, check that:

• the dig is finished at the required depth and fall, and the topsoil has been removed from site,
• all services in the area are laid, surrounded and recorded (drainage, water, electric cables, land drains, ducts for future lighting),
• the aggregate is ordered in the correct grade and quantity, with extra to allow for compaction,
• the finished levels and falls are agreed, ideally marked with pegs or a string line,
• there is access for a lorry and the machine, and space to tip the material,
• water is available for wetting the stone.

If the area belongs to a block of flats, for example a shared yard or car park, agree the work in advance with the building management or owners' association. For new hard surfaces, driveways and parking areas, check the local building regulations with your local building authority. Rules on permitted development, drainage of surface water and dropped kerbs onto the public road vary, so do not simply assume no approval is needed.

A simple job is a flat, open area with gravelly ground, good access and material tipped right where it is needed. The job gets harder when:

• material has to be carried or barrowed through a narrow gate or across a garden,
• the area has many corners, curves, steps and service penetrations that need the rammer,
• the ground is clay or waterlogged, so soft spots must be replaced and geotextile or drainage laid,
• compaction takes place on a slope or in narrow trenches,
• the sub-base is thick and built up in several layers,
• the design calls for a bearing capacity test,
• the work is right next to older buildings and vibration has to be limited,
• the area is small, so bringing the machine takes a lot of time compared with the work itself.

Compaction sits between the excavation and everything that goes on top, so several trades depend on it. Excavation and all service connections must be finished first. Digging through a finished, compacted sub-base later disturbs the whole area, and a backfilled trench never behaves quite like the ground around it. The plumber, the electrician and whoever installs the drainage should therefore have their pipes and ducts in before any stone goes down.

After compaction come the kerbs, the bedding layer and the paving, or the formwork and concrete for a slab, or the asphalt. Kerbs are set on a concrete bed on the compacted sub-base, never in loosened soil. Under ground slabs there is usually a blinding layer of concrete or a damp-proof membrane. Laying these needs a flat, firm surface without sharp stones sticking up.

If there is a design for the project, the designer or structural engineer sets the layer thicknesses and the required bearing capacity. For paving that carries cars, people often follow the paving manufacturer's recommendations, which are usually given in the technical data sheets.

• Too thick a layer at once. The surface looks firm, but the bottom stays loose. After a year or two the paving sinks in ruts under the wheels.
• Topsoil or roots left in. The organic matter rots and local dips appear across the area.
• Compacting dry or waterlogged material. The layer never reaches the density it needs and settles unevenly.
• Rounded gravel instead of crushed stone. The sub-base shifts under load and the paving starts to undulate.
• Poorly compacted edges. Kerbs lean outwards over time and the outer rows of paving drop.
• No geotextile on clay. Clay works into the stone, the sub-base loses strength and the surface heaves in frost.
• A heavy machine directly over new pipes. Pipes can crack or deform, and you only find out when the drain blocks.
• Wrong fall built into the sub-base. Water then sits under and on the paving and damages the whole construction in winter.

You can compact a smaller area such as a garden path or patio yourself with a hired plate. Do not take on compaction in deep trenches, next to the foundations of older houses, or over services whose exact position you are unsure of. A petrol or diesel plate gives off exhaust fumes, so never use it indoors or in deep excavations. Expect vibration and noise. Ear defenders, sturdy boots and gloves are essential.

1. Remove the topsoil and roots, and trim the dig to a fall of roughly 1 to 2 % away from the house.
2. Run the plate over the formation, and replace soft or wet patches with stone.
3. On clay or loam, lay a separating geotextile with generous overlaps.
4. Spread 0/32 crushed aggregate in layers as recommended by the plate manufacturer, usually no more than 15 to 20 cm with light machines.
5. Spray dry material until it is damp but not running with water.
6. Pass over each layer four to six times in overlapping strips, working from the edges to the middle.
7. Finish corners and the strip along walls with a hand rammer or tamper.
8. Check flatness and fall with a straightedge and spirit level, then fill any low spots and compact again.
9. Once the paving is laid and sand is brushed into the joints, vibrate it with the plate fitted with a rubber mat.

The simplest check is to look and to walk on it. A well-compacted sub-base shows no footprints, does not spring underfoot and does not rut when a car drives over it. Check flatness with a 2-metre straightedge. Deviations should stay within what the paving manufacturer or the design allows.

For driveways, car parks and bases under ground slabs, bearing capacity is checked by measurement. This is done with a static plate load test or the quicker light weight deflectometer, also known as a dynamic plate test. The result is a deformation modulus, which you compare with the value the design requires. A geotechnical or testing company carries out the test and gives you the result in a report. If the design requires this test, arrange it in advance, before anything is built on the sub-base.

• Topsoil and organic material removed from the whole area
• Material matches the agreed grade and is crushed, not rounded
• Number and thickness of layers as agreed or designed
• Surface flat, with no footprints and no springiness
• Falls run towards the drainage and match the setting out
• Edges, corners and areas around chambers fully compacted
• Sub-base extends beyond the edge of the future paving or slab
• Services in the area undamaged and their positions recorded
• Bearing capacity test report available where the design requires it