Machine excavation for strip foundations and pad footings

The job is digging trenches or pits for foundations with a machine. On a house this is usually a mini digger, a backhoe loader or a tracked excavator. Hand digging is limited to trimming the bottom and to spots the machine cannot reach or where services run.

Strip foundations (width from 50 cm)

Strip foundations are continuous trenches under load-bearing walls. On a detached house they form a closed outline, plus a few internal runs under load-bearing partitions. The structural design sets the width according to the load and the bearing capacity of the soil. For ordinary houses it is usually around 50 to 60 cm, and more for heavy masonry or weak ground. "From 50 cm" in the price list means the trench is dug with a bucket of matching width. Narrower trenches are not dug this way.

Pad footings (width from 80 cm)

Pad footings are separate pits under columns or posts, for example under a carport, a covered patio, a timber building on pads, a steel shed or heavy fence posts. They are usually square or rectangular in plan. They are often deeper and bulkier than strips because they carry a concentrated load from a single point. With pads the machine digs many small holes and has to move and set up precisely for each one. So per cubic metre, pads are usually more labour-intensive than a continuous strip.

How the volume is measured

The price-list item is per cubic metre of excavated soil. The volume comes from the design dimensions: length times width times depth. For strips you add up all the sections. For pads you multiply the volume of one pit by the number of pits. Agree beforehand whether you pay by design dimensions or by the volume actually dug. In practice trenches often end up a little wider, especially in crumbly soil, and the difference can add up.

Also keep in mind that soil takes up more space once it is dug out. This is called bulking. With clays and loams the volume usually grows by a noticeable share. When you order spoil removal, expect more lorry loads than the excavation volume on the drawings suggests.

Digging should only start once your project has the approvals it needs. Foundations are part of the building, so the same rules apply to them as to the whole structure. Check with your local building authority what your project requires and whether building control wants to inspect the trenches before concreting. If you live in a terrace or are digging close to a neighbour's house, talk to the neighbours early. Their foundations may need to be considered, and in some places formal notice is required.

Before the machine turns up, the following should be in place:

• Design drawings with a foundation plan, dimensions, depths and the reference level
• Setting-out by a surveyor, ideally with profile boards placed outside the dig area so the machine does not knock them over
• Located underground services from the utility companies or a locating service (gas, electricity, water, sewer, telecoms), marked directly on the ground
• Topsoil strip, meaning the fertile top layer is scraped off, stored to one side and reused later for landscaping
• Access for the digger and lorry, wide and firm enough, with no low cables or branches
• Space for spoil, or arranged removal and a tip
• A cleared plot with no scrub, rubble or old structures, unless clearing them is part of the order

People sometimes skip locating services because "there has never been anything there". On older plots and where a house has been demolished, though, old supply pipes and cables nobody knew about often turn up. If a gas pipe is damaged or you smell gas, stop work at once, switch off the machine, move well away and call the gas emergency number or 112. Any repair is for a qualified, registered gas engineer. Leave a damaged electrical cable to an electrician and the network operator, and never touch it.

The method depends on the machine and the ground, but the main steps are similar:

1. Take over the setting-out from the surveyor and check the profile boards, grid lines and the datum from which depth is measured.
2. Mark the outline on the ground, usually with spray paint or lime along stretched string lines.
3. Confirm where services run and dig by hand or very carefully near them.
4. Set up the machine and fit a bucket of matching width. A toothless ditching bucket suits the final trim of the bottom.
5. Dig in layers and keep checking depth with a level or a laser.
6. Place spoil on the side where it will not get in the way of later work, and far enough from the edge that it does not load the trench walls. Or load it straight onto a lorry.
7. Take out the last few centimetres carefully and clean the bottom by hand so no loose soil is left.
8. For pads, check the position of each pit against the grid lines and the spacing between pits.
9. Check dimensions, depths and the formation, then hand the excavation over for inspection by the structural engineer, building control or the site manager.

The formation is the bottom of the trench, where the foundation meets undisturbed ground. It must be firm, level and intact. If the bottom shows different ground from what the design assumed, such as made ground, soft clay or a spring, the structural engineer needs to decide what to do before concreting.

Type of soil

Cohesive soils such as clays and loams dig well, and the walls of a narrow trench usually stand longer. They are sensitive to water, though: after rain the bottom softens and loses bearing capacity. Sands and gravels dig easily, but the walls run, the trench widens and the volume grows. Rock or large boulders may need a hydraulic breaker, which slows the job considerably. Made ground, old cellars and remains of earlier foundations bring surprises nobody can see at the start.

If you have a site investigation or soil report, give it to the excavation contractor in advance. It saves arguments on site.

Choice of machine and bucket

For a house, a mini digger or a backhoe loader is the usual choice. A mini digger fits onto tight plots and does less damage to the surroundings, but it is slower with larger volumes or hard ground. A bigger excavator digs faster and deeper, but needs more room and firm access. The bucket width should match the strip width. Digging a strip with a wide bucket means excess soil now and excess concrete later.

Foundation depth

The bottom of the foundation must be below frost depth, so that freezing water in the soil cannot lift the foundations. Frost depth depends on climate, altitude and soil. In mild regions it may be only around half a metre, in colder ones well over a metre. On clay soils, nearby trees can also call for deeper foundations because roots dry out and shrink the clay. The design and local regulations always set the actual depth. If the bearing layer lies deeper, the dig goes down to it. On sloping plots strips are often stepped, so the trench does not have to be deep along its whole length.

For the same number of cubic metres, the dig can be a straightforward one-day job or a demanding contract lasting several days. It is easier when:

• The plot is level and dry, and the machine can reach it without obstacles
• The soil is cohesive, with no stones or roots
• There is enough room around the dig to store spoil
• The work is continuous strips without frequent moves
• Services are accurately located and outside the dig

It becomes more labour-intensive and costly when:

• Access is narrow, through a neighbour's land or across a garden that has to be protected
• The machine hits rock, concrete, old foundations or tree stumps
• Groundwater collects in the trench and has to be pumped out
• Services cross or run alongside the dig and part of it has to be done by hand
• The trench is deep and the walls need support or battering back
• The volume is small, for example a few pads, but the machine still has to be brought in
• There is nowhere to put the spoil and everything has to be taken away immediately

Spoil removal, tipping, hand trimming, pumping and rock breaking are usually separate items in price lists. Ask the contractor exactly what the excavation item includes and what will be charged on top.

Foundation excavation is the first link in a chain, not a standalone job. Setting-out and service locating come before it, reinforcement and concreting follow. The better these steps are lined up, the shorter the time the trench stays open.

• The surveyor should be available during the dig in case profile boards are damaged or positions are in doubt.
• The electrician may lay a foundation earth electrode in the trench where the design calls for one. This has to happen before concreting, so it can be inspected and tested later.
• Service entries for water, drainage, electricity and possibly gas pass through or under the foundation in ducts. Know their positions in advance, because drilling through foundations afterwards is wasted effort.
• The structural engineer or building control should approve the formation before concreting.
• The concrete supplier and pump should be booked so that concreting follows the dig as closely as possible.

On a typical house in good ground, machine excavation for strips is often done in one or two days. Pads for a carport or patio are usually finished in a few hours. Stones, water, services, awkward access and spoil removal all add time, particularly if the lorry cannot keep pace with the digger.

It often happens that the trenches are finished but the concrete is a week away. Meanwhile it rains, the walls collapse and the bottom turns to mud. Then the trench has to be cleaned out and sometimes dug deeper. Ideally concrete goes in the same day or the day after the trench is approved. If a longer gap is likely, the last layer can be taken out just before concreting, or the bottom can be protected with a blinding layer of concrete if the design allows it. In winter, keep the bottom from freezing. Concrete should not be placed on frozen ground.

• Inaccurate setting-out or moved profile boards. The house then sits wrongly on the plot, walls are out of square or boundary distances are not kept. After concreting this is almost impossible to correct.
• Over-dug bottom backfilled with soil. Loose soil compresses under load and the foundation settles. Any over-dig should be filled with concrete, not soil.
• Digging to full depth with a toothed bucket. The bottom is left churned up and uneven. Take the last layer out with a toothless bucket and trim it by hand.
• Too shallow. A foundation within the frost zone can be lifted in winter, and cracks appear in the walls, sometimes only years later.
• Spoil heaped right at the edge. Its weight pushes on the walls, they collapse into the trench and anyone inside is at risk.
• Trench left open too long. Rain, frost or drying out damage the formation. A softened bottom has to be removed before concreting.
• Ignoring different ground at the bottom. If made ground or a soft layer appears and concrete goes in without the engineer's opinion, the foundation can settle unevenly.
• Undermining a neighbour's foundations. Digging close to an existing building, especially deeper than its foundations, can damage it. A structural engineer must assess this beforehand.

Think about safety with deeper trenches too. Nobody should work beneath unsupported trench walls, and a trench left overnight should be fenced off so that no one falls in, especially children and animals.

Digging foundation trenches is structural work in trenches, so it belongs to an experienced operator. You can still help the job run smoothly and catch problems early, without entering the trench or operating the machine:

1. Clear the plot of loose material, garden furniture, plant pots and anything stored where the digger will work.
2. Trim low branches over the access route and mark soft spots, manhole covers and garden features that must not be driven over.
3. Ask the utility companies or a locating service for service plans and keep them on site for the operator.
4. Agree with the contractor where spoil and topsoil will be stored and mark the areas with pegs.
5. Photograph the profile boards and markings before the dig, so any later movement can be spotted.
6. Check the depth from outside the trench with a tape and a straight timber laid across it, measured from the agreed datum.
7. Watch the weather forecast and remind the concrete supplier of the planned date as soon as the dig is finished.
8. Keep children and pets away and check the fencing around the open trench every evening.

Before concreting, walk the excavation with the contractor, ideally with building control or the structural engineer as well. A tape measure and a level reveal most problems. Focus on the following:

• Positions of strips and pads match the setting-out and the design
• Trench widths and pad dimensions are not smaller than designed
• Depth is achieved along the full length, measured from the agreed datum
• The bottom is level, firm and free of loose soil, mud and water
• There are no roots, stones or remains of old structures on the bottom
• The walls are stable, with no slips or overhangs
• Spoil is stored at a safe distance from the edge or has been removed
• Services are undamaged and their crossings are marked
• Steps on a sloping site match the design
• The formation has been approved by the engineer or building control and the approval is recorded
• The volume for invoicing is agreed according to the chosen measuring method