Backfilling with soil and compaction

Backfill means filling a hollow: a trench, a pit, a building excavation or the gap between a foundation wall and the surrounding ground. Fill, sometimes called embankment, raises the ground level instead, for example when levelling a plot, under a ground-bearing floor or under a driveway. Both follow the same rule: lay the material in layers and compact each layer before the next one goes on.

Why not just push the soil back in and let it settle on its own? Loose material has a lot of air between the grains. Excavated soil also bulks up once it is dug out, so it takes up more space than it did in the ground. Without compaction, rain, frost and loads keep pressing it down for years. The result is dips around the house, water running towards the foundations and cracks in the paving. Compaction does the same job in a single day, under control.

The price-list unit is the cubic metre. In practice the volume can be measured in different ways: by the volume of the excavation, by the compacted volume in the drawings or by the loose volume of material delivered. These figures can differ noticeably. Before you place the order, agree with the contractor how the volume will be measured.

Three related items share this price-list page. They differ in material, so they suit different places and are handled differently.

Backfilling with soil and compaction

This uses suitable soil, usually the soil dug out on site. It suits garden trenches, backfill around foundations where no paved area will be built and anywhere you want to restore the original ground. The soil must not contain topsoil, roots, timber, building rubble or large stones. Organic matter rots over time and leaves voids.

The type of soil matters a lot. Sandy and gravelly soils compact easily and drain well. Clays and loams react to moisture. Dry, they form hard lumps that will not compact properly. Waterlogged, they wobble under the plate compactor like a jelly. Clay can be used, but only at the right moisture, in thinner layers and with a suitable machine.

Backfilling with gravel or crushed stone

Gravel or crushed stone is the choice where the backfill has to carry load and must not settle: under a path, terrace or garage driveway, at service connections under a road, or next to foundations that paving will later butt up against. Compacted layers use a well-graded material, such as crushed stone 0/32 or 0/63, where the fine particles fill the gaps between the coarse ones. In the UK this kind of material is commonly sold as sub-base (Type 1). It compacts into a firm layer that can carry load.

Clean washed stone, for example 16/32, is a completely different product. Gaps stay open between the stones and water flows through them, so it works as a drainage surround around a land drain. It compacts only slightly and is not suitable as a load-bearing layer under paving without further treatment. Mixing up these two materials happens often on site, and then someone wonders why the gravel will not compact.

Sand and gravel fill with compaction

Sand and gravel, for example 0/32, is a dug aggregate with rounder grains and a share of sand. It is used for larger fills: raising the ground under a ground-bearing floor, under a foundation slab as specified in the drawings, under paved areas or to level a plot. It compacts well and evenly and copes with water better than loamy soil. Under the floor of a house, the drawings often specify sand and gravel or crushed stone of a given thickness and required bearing capacity.

How to decide

• A garden or lawn will stay there: good excavated soil is usually enough, with topsoil on top.
• Paving, a path or a driveway will go there: crushed stone or sand and gravel, at least in the upper layers.
• The ground under a building needs raising: sand and gravel or crushed stone as specified, with the bearing capacity checked.
• Water needs to drain away: washed stone wrapped in geotextile, not clay.

Backfilling cannot be undone without digging again. So it pays to stop beforehand and check that everything underground really is finished.

• Pipes and cables are laid and checked. Drains need the correct fall, and water pipes should have passed a pressure test. Gas and electricity connections usually need an inspection or sign-off by the network operator before they are covered.
• The services are surveyed. For connections and utility lines, the route should be measured and recorded before backfilling. Afterwards it is too late.
• The foundation waterproofing is protected. A dimpled membrane, XPS insulation boards or other protection should cover it, so stones in the backfill cannot damage it.
• The concrete is strong enough. Basement and foundation walls are backfilled only once they have hardened sufficiently. The structural engineer often requires the floor above to be built first, so it braces the wall from the top.
• The bottom of the excavation is free of water, snow and mud. Never backfill into water or onto a frozen base.
• It is clear where material will be stockpiled and where machines can go. An excavator, loader and plate compactor all need access. A narrow gap along the neighbour's fence or a flower bed that must not be damaged changes the whole method.

If the backfill is part of a building project or of landscaping work, it must follow the drawings and any permit conditions. Whether your particular change to ground levels needs approval depends on local building regulations, so check with your local building authority. For a block of flats or shared land, the first contact is the building management or the owners' association.

Pipe surround and the first layers

Pipes and cables first get a surround: fine material around the line, usually sand or fine crushed stone without sharp stones. The surround usually reaches roughly 10 to 30 cm above the top of the pipe. The drawings or the pipe manufacturer give the exact depth. Along the sides of the pipe, the material is carefully packed by hand or with a light tamper, so the pipe sits firmly and cannot move sideways. Nobody runs a heavy plate directly over the pipe until there is enough cover above it. A coloured warning tape goes in the surround or just above it, so anyone digging later knows there is a service underneath.

Laying in layers

Above the surround, the material goes in evenly spread layers. With a standard plate compactor, one loose layer is usually about 20 to 30 cm thick. With a light plate it is less, with a heavy roller more. The machine manufacturer gives the exact figures. If a layer is too thick, only the top gets compacted and loose material stays underneath. This is the usual cause of later settlement.

The machine passes over each layer several times in overlapping strips, until the surface stops visibly sinking and the plate starts to bounce. Around a building, the crew works evenly round the perimeter, so one side of a wall is not pushed harder than the other.

Choosing the machine

• Plate compactor: the all-round machine for sand, gravel and crushed stone. A light plate for narrow trenches, a heavier reversible plate for larger areas and thicker layers.
• Trench rammer (jumping jack): narrow with a harder impact, suited to trenches, wall edges, manholes and cohesive clay soils that a plate compacts poorly.
• Roller: for large areas and fills under roads and car parks.
• Excavator with a compaction plate on the arm: for deep and wide excavations where a person with a plate cannot safely go.

Moisture content

Material compacts well only within a certain moisture range, called the optimum moisture content. In practice, squeeze the soil in your hand: it should form a lump that holds together but does not smear and no water runs out. Very dry gravel and sand get a light spray of water. Waterlogged clay is left to dry, spread out or replaced with a more suitable material. Compacting saturated clay is usually wasted time, because the material only moves from side to side.

Next to foundations and basement walls

Right against a wall, compaction is gentle, with a lighter machine and thinner layers. A heavy plate or roller next to a fresh wall can push it over or damage it. The top layer of backfill next to the house slopes away from the building, so water does not run to the foundations. If paving, a gravel strip or a terrace will be built there, the upper layers are made of crushed stone.

How long it takes

Backfilling a trench for a service connection usually takes a few hours to a day. Backfilling around a house or a larger fill under a floor takes one to several days, depending on access and quantity. Unlike concrete, there is no curing time. If the bearing capacity has to be tested, though, the next work such as the concrete blinding starts only once the test results are in.

How demanding the backfill is depends on more than volume. The same number of cubic metres can mean easy work with a loader one time and a full day of wheelbarrow runs the next.

The job is easier with:

• Good access for an excavator or loader and the material stockpiled right next to the excavation
• Sandy or gravelly soil at a reasonable moisture
• A wide excavation where a plate can work without obstacles
• Dry weather and no groundwater in the excavation

The job is slower and harder with:

• Carrying material by hand through a garden, a house or a narrow passage
• Many services in one trench, manholes and crossings that need hand compaction around them
• Loamy or clay soil, wet after rain, that has to dry out or be replaced
• Unsuitable excavated soil that has to be taken away and replaced with imported material
• A required bearing capacity test and repeated compaction if the test fails
• A deep excavation with trench supports that are removed step by step as the backfill rises

• Backfilling in one go, without layers. The excavator fills the whole trench and a plate runs over the top. The surface looks firm, but after one or two winters a dip forms along the trench.
• Soil with topsoil, roots or rubble. Organic matter rots and rubble leaves voids. This shows up as local sinkholes, often years later.
• Stones against pipes or waterproofing. Under load, a sharp stone punctures the pipe or the membrane. The result is a damp basement or a leaking drain that has to be dug up again.
• A heavy machine directly over the pipe. The pipe deforms or a joint fails. A CCTV drain survey later finds a sagging pipe or water standing in a dip.
• Waterlogged clay or frozen material. Wet clay will not compact, and frozen soil settles once it thaws. The same goes for snow pushed into the excavation.
• Backfilling against an immature wall or from one side only. The wall can tilt or crack. That is an expensive repair which patience would have avoided.
• Drainage stone instead of a load-bearing layer. Washed stone shifts under paving, and the paving turns wavy and loose.
• Ground sloping towards the house. Rainwater runs into the backfill, down to the foundations, and looks for a way into the basement.
• No warning tape or survey. Nobody knows exactly where the service runs, and someone hits it during future digging.

You can reasonably handle a shallow trench yourself, for example for irrigation, a garden cable laid by an electrician or a land drain, or compacting the base for a small path. Plate compactors can be hired from tool hire shops. Stay out of deep trenches, excavations with unstable sides and any gas, electricity or mains water connections from the network operator. Those belong to a professional contractor. If you smell gas while working, move to a safe place immediately, do not use flames or electrical switches, and call the gas emergency number or 112.

1. Find out where underground services run and never dig or compact blind.
2. Clear water, mud, roots and stones from the bottom of the trench.
3. Prepare the material: sand for the surround, suitable soil without topsoil or rubble, and crushed stone if needed.
4. Pour sand around the pipe or cable and pack it down by hand along the sides.
5. Add a protective layer of sand above the line and lay the warning tape.
6. Place the remaining material in layers of roughly 15 to 20 cm.
7. Check the moisture: the soil should hold its shape in your hand without smearing.
8. Run the plate over each layer several times in overlapping strips.
9. Finish compaction along walls, around manholes and in corners with a hand tamper.
10. Slope the final layer away from the house and spread topsoil or the paving base.
11. After a few rainfalls, check whether the surface has settled and top up any dips.

When using a plate, wear sturdy boots, ear defenders and gloves. Do not leave the plate running in one spot, and lower it into the trench with two people rather than dropping it.

For ordinary backfill in a garden, a simple check is enough. A compacted surface does not give way under your boot, a steel rod is hard to push in, and the plate leaves no deep marks. Uneven spots where the rod suddenly goes in easily reveal a layer that was not compacted properly.

Under floors, foundation slabs, roads and car parks, the drawings usually specify a bearing capacity or degree of compaction. This is tested with a static plate load test, which measures the deformation modulus, or with a quicker dynamic plate test. For important fills, the density of the compacted layer is sometimes compared with a laboratory test of the material. The designer sets the values to achieve, based on EN standards and national rules, not the backfill contractor. If a test fails, the layer is compacted again or replaced and tested once more.

For a drain connection, it is worth having a CCTV survey done after backfilling. It shows whether the pipe has bent during compaction and whether the joints are holding.

At handover, walk over the area, compare it with the drawings and ask for the documents you will need later, for completion sign-off or for further building work.

• Services were checked, tested and surveyed before backfilling
• The pipe surround is sand or fine material without sharp stones
• Warning tape is laid above all services
• The material used matches the agreement or the drawings and contains no topsoil, roots or rubble
• Backfilling was done in layers and each layer was compacted
• The foundation waterproofing and its protection are undamaged
• The surface is even, with no dips or soft spots
• The ground around the house slopes away from the building
• A test report is available for areas with a required bearing capacity
• It is agreed how the volume was measured and what happens if the backfill settles in the first months