Pouring strip foundations directly into the trench

Pouring into the trench makes sense where the trench walls stand up on their own. Typical examples are clay and loamy soils, cohesive gravels with some clay content, and weathered rock. In ground like this, an excavator with a bucket the width of the future foundation digs the trench, and the concrete goes in without any formwork.

Why not always use formwork? Formwork in a trench means digging wider, building and stripping the boards, and then backfilling and compacting the gap around the foundation. In firm ground this adds work without making the foundation any better. Concrete cast against the soil also sits tightly against the surrounding ground.

Formwork is needed when the trench walls keep collapsing, for example in dry sand, or when the top of the foundation stands above ground level. A combination is common: the lower part is cast against the soil, and the top few tens of centimetres are shuttered so that the top edge is straight and at the right level.

The width, depth and any reinforcement of the strips come from the structural engineer, based on the loads and on how much the soil can carry. The foundation must go below the frost depth, which depends on your climate and local ground conditions. In mild coastal areas it can be fairly shallow, in colder regions it may be a metre or more. Clay soils near trees may also need deeper foundations because they shrink and swell. Local building regulations set the requirements, so check with your local building authority before you start digging. In many places a building inspector wants to see the open trench before the pour.

For a typical family house the pour itself usually takes one day. Whether it goes well, though, is decided in the days before. Once the loaded truck mixer is standing on site, there is no time to fix anything.

The trench and its bottom

The trenches must match the drawings, and the bottom must be at the level set by the design or the surveyor. The bottom should be flat, free of loose soil, mud and standing water. The surface the concrete will sit on is called the formation level. If it turns out to be different from what the design assumed, for example made-up ground, soft clay or water seeping in, the structural engineer or a geotechnical specialist should look at it. In practice it often happens that part of the plot has to be dug deeper until firm ground is reached.

Earthing

An earth electrode is often laid in the foundations, usually galvanised steel strip with tails brought up above ground. The electrical installation and any lightning protection are connected to it later. A qualified electrician designs, installs and checks it. The strip must be fully surrounded by concrete, and it is worth photographing it before the pour.

Ducts for services

Drainage, the water supply, the electricity supply and perhaps pipes for a heat pump all have to pass through or under the foundation somewhere. Before the pour, sleeves are set in these places: pipes of a larger diameter through which the service pipe or cable is pulled later. Agree their positions with whoever designs the plumbing, drainage and electrics. Drilling or breaking through a new foundation afterwards is costly and weakens it.

Reinforcement, if the design calls for it

On simple buildings on good ground the strips are often plain, unreinforced concrete. On weaker ground, under heavier loads or on a slope, the engineer specifies reinforcement cages. These must sit on spacers, not directly on the soil, so the concrete can surround them on all sides.

Access and ordering

A truck mixer is a heavy vehicle. It needs firm access as close to the trench as possible and room to turn. If it cannot get close enough, a concrete pump is ordered. The volume is calculated from the trench dimensions plus a margin, because the real trench is almost always a little wider than on the drawing.

Strength class and consistency

The concrete class comes from the design. For house foundations, classes such as C16/20 or C20/25 are common, higher in more demanding conditions. If the ground or groundwater is aggressive, for example containing sulfates, the engineer also specifies an exposure class, which changes the mix. When you order from the plant, give the strength class, exposure class, maximum aggregate size and consistency.

Consistency describes how easily the concrete flows. For trenches a softer mix is usually ordered, for example S3, and S4 for pumping or dense reinforcement. What matters is that the concrete arrives with the right flow from the plant. Adding water on site to make it pour more easily lowers its strength.

Plain or reinforced

Plain concrete handles compression well but copes poorly with tension. If the ground is not equally firm everywhere, the strip can bend slightly and plain concrete will crack. Reinforcement takes up these tensile forces. So the decision belongs to the engineer, not to the contractor or a neighbour who has "always done it this way".

Getting the concrete into the trench

• Straight from the truck mixer chute: simplest and quickest, if the truck can reach every trench with its chute.
• With a concrete pump: when the trench is far from the access, behind the house or on a slope. The boom reaches distant spots and places the concrete evenly.
• By wheelbarrow or site dumper: for small volumes, such as strips for a garden wall. It is slower and needs more people.

Level and stepped strips

On a flat plot the trench bottom is at one level. On a slope the bottom is stepped. The height and length of the steps come from the design. The pour starts at the lowest point and works upwards, taking care that soil from the steps does not mix into the concrete.

The sequence is similar on most sites. An experienced crew knows it well, but every step has a reason.

1. Check the trench: dimensions, bottom level, cleanliness, standing water and fallen soil.
2. Have the formation level inspected and recorded, if the building inspector or site supervisor requires it.
3. Check the position of the earth strip, sleeves and reinforcement, and photograph everything.
4. Mark the top of the foundation, for example with levelled pegs or battens, so everyone knows how high to pour.
5. Take the delivery ticket from the first truck and check the concrete class, consistency and batching time.
6. Place the concrete in layers, usually up to roughly 0.3 to 0.5 m deep, and do not let it drop from a great height onto one spot.
7. Compact each layer with a poker vibrator until air bubbles stop rising.
8. Keep going without long breaks, so each new layer joins a still-fresh one and no unplanned joint forms.
9. Bring the top surface to the marked level, strike it off with a straightedge and finish it with a float.
10. If blockwork or hollow formwork blocks go straight onto the strip, set any starter bars into the fresh concrete as the design shows.
11. Cover and cure the concrete to suit the weather.

Continuity is the key. If the pour stops for several hours, the lower layer starts to set and the new one will not bond fully with it. If a break cannot be avoided, for example on very long strips, place the construction joint where the engineer allows, not wherever the concrete happened to run out.

The rate per cubic metre varies with site conditions. Knowing what drives it helps you compare quotes and prepare the site.

Makes it easier

• Good access for the truck mixer right up to the trench, with no pump needed.
• Cohesive soil that keeps the trench walls straight.
• A dry trench with no groundwater.
• A larger continuous volume poured in one go, without breaks.
• Plain concrete strips or simple reinforcement.
• Mild weather, without frost or great heat.

Makes it harder

• Narrow or soft access that calls for a pump or manual transport.
• Loose soil, so the top has to be shuttered or the trench walls cleaned repeatedly.
• Water in the trench that has to be pumped out before and during the pour.
• A slope and stepped foundations.
• Dense reinforcement that slows down placing and compaction.
• Small volumes spread over several places, such as strips for a garden wall and an extension done separately.
• Winter pours that need concrete with admixtures, covering and longer curing.

Keep in mind that what you pay for is the concrete actually placed. If the excavator digs wider, or the walls keep falling in, the volume grows compared with the calculation from the drawing.

The foundations sit in the middle of a chain of trades. Whoever coordinates it saves repairs and waiting.

• The surveyor sets out the building before digging and can check the levels of the trench bottom and the top of the strips.
• Excavation should finish shortly before the pour. A trench left open for days in rain or frost loses the quality of its bottom.
• The structural engineer or geotechnical specialist assesses the formation if it differs from expectations.
• The electrician handles the earth electrode and its tails.
• The plumber sets the position and diameter of sleeves for drainage and water.
• Bricklayers build the foundation wall after the pour, often from blockwork or hollow formwork blocks, followed by the ground floor slab.
• Waterproofing comes later, and its quality depends partly on how flat and clean the foundation surface is.

Where there is a site manager or building inspector, inspection of the formation and reinforcement belongs in the site records. On a self-build project it is worth having at least clear photos with visible measurements.

Foundation faults are hard to put right. Most only show years later, when the walls are plastered and the floors are finished.

• Pouring into water and mud. The concrete mixes with soil, loses strength and leaves a weak layer at the bottom. This can later cause uneven settlement and cracks in the walls.
• Loose soil on the bottom. Loosened soil compresses under load. The house settles more in one place than another.
• Foundations too shallow. A strip above the frost line can be lifted by frost in the ground. Typical signs are diagonal cracks at wall corners and around openings.
• Reinforcement lying on the soil. Without concrete cover the steel corrodes and stops working.
• Water added to the mix. The concrete flows better but is weaker and shrinks more.
• Poor compaction. Voids and honeycombing remain, especially around the reinforcement and against the trench walls.
• Forgotten sleeves and earthing. Core drilling later weakens the foundation, and earthing has to be solved another way.
• An uneven top surface. The first course of blocks then needs thick mortar beds, and the whole foundation wall often ends up out of true.
• No curing. In hot weather the surface dries out, cracks and stays dusty and soft.

A crew can usually pour the strips for an ordinary family house in one working day, and on smaller jobs in a few hours. Preparing reinforcement, sleeves and earthing usually takes from one to several days, depending on the scope.

Concrete starts to set a few hours after mixing, depending on temperature and mix. Once it arrives it should be placed as soon as possible. The time on the delivery ticket and the plant's advice are what count.

You can usually walk carefully on the strips the next day. In summer, laying the first course of blocks normally starts after a few days, later in cold weather. Concrete reaches its design strength after roughly 28 days, but this does not usually hold up the blockwork if the site manager or engineer agrees.

How to cure the concrete

Why keep concrete wet once it is poured? Cement needs water to harden. If the top dries too fast, it stays weak and cracks. In summer the surface is covered with polythene sheeting or geotextile and kept damp for several days, usually around a week. Concrete in the ground is easier, because its sides are in damp soil.

At around +5 °C and below, the concrete must be protected from frost. The plant can supply a mix with cold-weather admixtures, and the fresh surface is covered with insulating blankets or sheeting with insulation. Never pour into a frozen trench or onto a snow-covered bottom.

Safety around the trench

Do not climb into deeper trenches with unstable walls. They can collapse even when they look solid, especially after rain or next to a heavy vehicle. When a pump is used, watch for overhead power lines, and nobody should stand under the boom.

Checking pays off during the pour, not only afterwards. You can see most faults on the day, while they can still be corrected. At handover, ask to see the paperwork and walk the whole length of the foundations.

• Delivery tickets for every truck with the correct concrete class, consistency and quantity
• Total delivered concrete compared with the calculation and the invoiced volume
• Record of the formation and reinforcement inspection, if site records are kept
• Photos of the trench, reinforcement, earth strip and sleeves before the pour
• Earth strip tails in the places shown on the electrical design
• Service sleeves in the right places, open and not blocked with concrete
• Top surface of the strips at design level, flat, without major unevenness
• Width and line of the strips as drawn, with no narrowing at corners and junctions
• Surface free of honeycombing, holes and exposed reinforcement
• No cracks and no crumbly, dusty surface after the first few days
• Record of curing, especially in hot or cold weather
• Confirmation of when the foundation wall can be started