Machine excavation of a building pit

A building pit is an open, area-wide excavation. It is usually several metres across and covers the whole footprint of the building or a large part of it. Trenches for strip foundations and trenches for service connections are separate items. They use different buckets and a different method. You will usually need a pit for:

• A house with a cellar or a partly sunken basement
• A house on a sloping plot where part of the ground floor cuts into the hillside
• A ground-bearing slab where unsuitable soil has to be removed and replaced with compacted hardcore
• A swimming pool, cesspit, sewage treatment plant, rainwater or attenuation tank, or an underground garage
• An extension with a basement next to an existing house

Why use a machine even on smaller builds? Digging a few dozen cubic metres by hand would take weeks, and rain would wear down the sides in the meantime. An excavator does the same work in a day or two, and a good operator can level the bottom to within a few centimetres.

How the volume in cubic metres is calculated

Excavation is measured "in the bank", meaning the volume the soil filled in the ground before it was dug. The volume includes the building footprint, the working space around it and the extra volume of any sloped sides. The working space is a strip around the future walls. Bricklayers and waterproofing crews need it to apply waterproofing and insulation to the basement walls. Depending on the design, it is usually roughly half a metre to a metre wide.

Excavated soil loosens and takes up more space than it did in the ground. This is called bulking. The difference is smaller with sandy soil and larger with clay and stony ground, often a fifth to a third more. So you cannot work out the number of lorry loads just by multiplying the pit dimensions. When you compare quotes, check whether the contractor works out the volume from the drawings, from an actual survey of the dug pit, or from the number of lorry loads.

Permission, design and setting out

Digging the building pit is part of the construction work, so it falls under your local building regulations. Do not assume that soil work can start before your permission is in place. Check with your local building authority what approval your project needs and from when you can start digging. If work is stopped for lack of approval, you are left with an open pit, which is awkward for your plot and for your neighbours. In a block of flats or on shared land, also talk to the building management or owners' association before anything is dug.

Before digging, a surveyor should set out the building from the drawings. They place profile boards or reference points outside the future pit, where the machine cannot knock them over. They also fix a datum, a height reference point. Without proper setting out, the house can easily end up tens of centimetres away from where it should be, or too close to the boundary.

Buried services

Gas pipes, electric cables, water mains, drains or telecom lines can run under the lawn. Request records from the network operators in advance and have the routes marked on site. Many contractors also scan the ground with a cable avoidance tool before digging. Near marked services the bucket is not used. The soil is dug by hand or with great care, under the conditions the operator sets.

If the excavator damages a gas pipe or you smell gas, stop the machine at once. Move away, keep everyone else away and call the gas network's emergency number or 112. Do not use naked flames or switches nearby. If an electric cable is damaged, nobody may touch it. Only the network operator and a qualified electrician may deal with it.

Ground investigation

For a house with a basement or on a slope, a geotechnical ground investigation is well worth it. The engineer finds out what soil lies at foundation depth, whether there is groundwater and what slope angles are safe. Without this information, the slope of the sides and the drainage are decided during digging, often only after the first collapse.

Access, space and removal of spoil

• The access route must carry an excavator and lorries; soft ground after rain may not hold them
• You need space on the plot for a temporary spoil heap of soil kept for backfilling
• Trees, overhead lines and fences must not be in the machine's way
• If lorries stand on a public road, you may need a permit to use it; ask your local council
• Agree in advance where surplus soil goes and who arranges its disposal or reuse

If there are neighbouring buildings, it is sensible to photograph their condition before work starts. If a crack appears later, you can show whether it was there before.

The process depends on the size of the pit, but the basic steps are nearly always the same:

1. Check the setting out, the datum and the marked positions of services.
2. Strip the topsoil, the top fertile layer, and store it in a separate heap at the edge of the plot.
3. Mark the outline of the pit, including the working space and the top edge of the slopes.
4. Dig in layers from one edge, loading soil straight onto lorries or onto the temporary heap.
5. Shape the slopes to the agreed angle as you go, or install the shoring.
6. Check the depth against the datum with a level or laser.
7. Trim the last few centimetres above formation level with a smooth ditching bucket without teeth, or by hand.
8. Form a sump and drain the bottom if water appears.
9. Hand the pit over for inspection of the formation by the structural or geotechnical engineer, if the design requires it.

Topsoil is kept separate because it can go back on the garden. Mixed with clay and gravel from deeper down, it is no longer much use, and it also adds to the volume that has to be carted away.

The last digging step matters more than people expect. The teeth of the bucket loosen the soil below the bottom. If the pit is dug too deep and then filled back with loose soil, you get a soft spot that will settle. An experienced operator digs just above the required level and finishes the bottom with long, slanting passes of a smooth bucket.

How long it takes

With good access and easy soil, a pit for a typical family house with a basement usually takes one to three days. Rocky ground, a tight site, slow lorry turnaround or groundwater all add time. The pit should not stay open longer than necessary. Rain and frost weaken the bottom, so follow on quickly with blinding concrete or a gravel bed. That protects the finished formation.

Sloped sides or shoring

An unsupported vertical side is safe only to a small depth, usually around 1.25 metres, and even then only in firm, dry, cohesive soil. Deeper pits must be either battered back (sloped) or shored. A collapsing side can bury a person, even when the wall looks solid.

Battering is the simplest solution if there is enough room on the plot. The sides are dug at an angle that suits the soil. Loose sand needs a gentler slope, stiff clay can stand steeper. The downside is a larger dig and later more backfill.

Shoring is used where a slope is not possible, for example near the boundary, next to a neighbour's house or beside a road. Options include steel sheet piles, king post walls with timber or concrete infill, trench boxes or bored piles. Shoring is designed by a structural engineer and priced as a separate item. When choosing a contractor, ask whether they can provide it or who they work with.

Digging next to a neighbouring building

If the pit goes deeper than the foundations of a nearby house and comes close to them, the soil under those foundations can lose its support. A structural engineer must design the solution. Typical methods are digging in short sections, underpinning the foundations, or shoring. Do not dig without a design, even if it seems like only a short stretch. In some countries you also have to formally notify your neighbour before such work, so check what applies where you live.

Groundwater and rainwater

Water in the pit softens the bottom and makes the slopes less stable. A small inflow can be handled with a sump in the lowest corner and a submersible dirty-water pump. Where the water table is higher, it has to be lowered, for example by pumping from wells or wellpoints. A specialist should design this. Divert rainwater running off the surrounding ground away from the edge of the pit so it does not run down the slopes.

If the ground investigation finds a high water table, that also affects the basement design. You will probably need watertight concrete or pressure-resistant tanking, and it is better to know this before digging than after.

Small pits and tight sites use mini excavators and backhoe loaders, which many people simply call a "JCB". Larger pits use tracked or wheeled excavators with a longer reach that load straight onto lorries. A bigger machine is faster, but it needs firm access and room to slew. When you get a quote, ask which machine the contractor plans to use and how it will get onto your plot.

These factors have the biggest effect on how demanding, and therefore how costly, the dig will be:

• Soil type: sand and loam dig easily, stiff clay and gravel are harder, rock may need a hydraulic breaker
• Depth of the pit and the need for battering or shoring
• Access to the plot, gate width, slope of the ground and distance from the road
• Buried services that need hand digging
• Groundwater and the need for pumping
• Distance to the tip and how the soil is loaded
• Old foundations, concrete, tree roots or rubble in the ground
• Required accuracy of the bottom and road cleaning after lorries leave

On older plots, diggers often find remains of an earlier house, an old well, a cesspit or buried rubbish under the lawn. Such finds slow the work, and sometimes they also have to be removed and disposed of. If you know about anything like this, tell the contractor in advance.

Where the soil goes

Part of the soil usually stays on site for backfilling and landscaping. The rest is taken away. Excavated soil you do not reuse on site can count as waste and must go to a site licensed to accept it. Agree in advance whether the excavation contractor handles removal and disposal or whether you arrange it yourself. Soil contaminated with oil, building rubble or old waste cannot simply be spread over the garden.

Excavation is only the first link in a chain, and each trade needs something from the one before:

• The surveyor sets out the building before digging and, on larger jobs, also surveys the finished pit
• The geotechnical or structural engineer inspects the formation to see whether the soil at the bottom matches the design assumptions
• The concrete crew needs the pit as soon as it is handed over, before the bottom gets wet or frozen
• The plumber needs to know where drains and service pipes will pass through the foundations
• The waterproofing crew needs enough working space around the basement walls
• Backfilling comes only after waterproofing, its protection board and the drainage are in, and it is done in layers with compaction

Why is timing so important? If the concrete crew arrives two weeks late, the pit can look completely different after rain. A muddy bottom has to be cleaned out, sometimes deepened and replaced with gravel. So the dig date is planned around the concrete pour, not the other way round.

• Over-dug bottom filled with loose soil: a soft spot forms under the foundations, the house settles unevenly and cracks appear in the walls
• No working space: the waterproofing crew has no room, the job is rushed and the basement gets damp later
• Slopes too steep: sides slide into the pit, endanger people and bury the finished bottom
• Spoil heaped right at the edge: it loads the slope and raises the risk of collapse; keep it usually at least a metre back, further for deeper pits
• Water not dealt with: a soaked bottom loses bearing capacity and mud stays under the concrete
• Topsoil mixed with subsoil: the garden loses its good top layer and there is more to cart away
• Digging without marked services: a damaged pipe or cable, work stopped and a risk of injury
• Pit left open too long: rain and frost spoil the bottom and the slopes slowly crumble

Many of these mistakes are not visible straight away. A soft spot under the foundations may show only years later, when the house is finished and repairs are very difficult. That is why checking the bottom pays off even on small builds.

Safety on site

An open pit is especially dangerous for children and for anyone who visits the site in the evening. Fence off the edge or mark it clearly. While the machine is working, nobody should stand within reach of the arm or under a lorry being loaded. Nobody goes into the pit until the operator has finished and the slopes are known to be stable. Use a ladder or a sloped ramp to get in and out, never jump down from the side.

When you accept the pit, take the drawings and walk round it calmly with the operator or site manager. Have the depth checked with a level against the datum, not by eye. If the design requires the formation to be inspected by a geotechnical or structural engineer, keep their written record. It will be useful later too.

• The pit dimensions match the drawings, including the working space
• The depth matches the design within a reasonable tolerance and has been checked with an instrument
• The bottom is level, with no muddy patches, loose soil or bucket tooth marks
• The slopes have the agreed angle and show no cracks or slipping
• Any required shoring is installed as the structural engineer designed it
• The sump and drainage work and no water stands on the bottom
• Marked services are undamaged and clearly marked
• Topsoil and backfill soil are stored separately and well back from the edge
• Profile boards and the datum are intact
• The access route and public road have been cleaned
• The edge of the pit is protected against falls
• The start date of the concrete crew is agreed and the bottom is protected until then