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Foundation trenches : how to dig and prepare them

10/04/2026 12 min read

The general earthworks are done, the platform is ready — it is time to dig the foundation trenches. These linear excavations will receive the strip footings, the skeleton of your house. This is a stage where precision matters as much as brute force: a few centimetres too many or too few, a poorly levelled formation level, forgetting the frost depth — and the foundations will be compromised. This guide takes you from setting out to a formation level ready to receive concrete.

Foundation trenches: the key dimensions

Before digging, you must know three fundamental parameters, all specified in your G2 AVP soil survey and the architect’s drawings:

Depth: respecting the frost line

The trench depth must place the bottom of the strip footing below the frost depth for your region. If frost reaches the soil beneath the foundation, the frozen water expands (frost heave) and lifts the footing — causing cracks in the walls.

French frost zone	Frost depth	Regions
Zone 1	50 cm	Atlantic coast, Mediterranean, < 150 m elevation
Zone 2	60-70 cm	Ile-de-France, Centre, plains, 150-400 m
Zone 3	80-90 cm	Alsace, Lorraine, Jura, Burgundy, 400-800 m
Zone 4	100 cm and over	Alps, Pyrenees, Massif Central, > 800 m

Note — These depths are regulatory minimums (DTU 13.12 / NF EN 1997, French standards). Your soil survey may require a greater depth if the bearing stratum is deeper. The soil survey takes precedence over the zone map. In the UK, frost depth is typically 450-600 mm depending on region and exposure. See also our guide on choosing the type of foundations for special cases (piles, raft slab…).

Width: sizing to the footing

The trench width must be greater than the strip footing width to allow for formwork and cover spacers. In practice:

Wall type	Footing width (soil survey)	Recommended trench width
20 cm blockwork load-bearing wall	40-50 cm	50-60 cm
20 cm in-situ concrete load-bearing wall	50-60 cm	60-70 cm
25 cm blockwork load-bearing wall	50-60 cm	60-70 cm
Internal load-bearing wall (cross wall)	30-40 cm	40-50 cm

Footing depth

The standard height of a strip footing is 25 cm for a 20 cm wall, up to 40 cm for heavier loads. The soil survey and the structural engineer’s drawings set this dimension.

Setting out: marking the trenches

Before digging, you must transfer to the ground the exact position of each trench. This is setting out, carried out using profile boards installed by the surveyor.

The profile board method

Profile boards are horizontal planks fixed to stakes, placed 1-2 m back from each corner of the house (outside the excavation zone). They carry nails or notches that define the wall centre lines.

Stretch string lines between the profile boards, along the axes of the load-bearing walls
Check squareness using the 3-4-5 method: measure 3 m on one side, 4 m on the other — the diagonal must be exactly 5 m
Mark the trench edges: from the wall centreline, measure half the width on each side and mark on the ground (marking paint, plaster, lime)
Check the diagonals of the whole layout: opposite diagonals must be equal (tolerance: ± 1 cm)

Tip — Use agricultural lime rather than paint to mark the trenches on the ground. Lime is clearly visible, resists wind and light rain, and is non-polluting. Pour it in a thin stream along your string line, holding it 10 cm above the ground.

Advice

Best practice — Have the surveyor come back a second time just before the foundations are poured to check that the trenches are correctly positioned and at the right level. A check costs 300-500 €; correcting misaligned foundations costs ten times more.

Digging the trenches: method and equipment

Which machine to use?

House footprint	Recommended machine	Hire cost
< 80 m²	1.5-2.5 T mini digger with 40 cm bucket	200-300 €/day
80-150 m²	2.5-5 T mini digger with 50 cm bucket	300-450 €/day
> 150 m² or hard ground	8 T excavator + trenching bucket	500-800 €/day

For a standard 100 m² house, a 2.5 to 3-tonne mini digger with a 50 cm trenching bucket is the ideal tool. You can hire one and operate it yourself (no licence required under 6 T on private land in France).

Digging technique

Start at a corner and work linearly along the marked line
Dig in passes of 20-30 cm depth — do not try to reach the final depth in one go
Store the spoil on one side of the trench only (exterior side preferably), at least 1 m from the edge to prevent collapse
Keep the bottom flat: check regularly with the bucket held flat, or by hand
Check the depth as you go with a tape measure and laser level (the profile board string line serves as the height reference)

Warning — Never over-excavate the trench. If you go too deep, do not backfill with soil to make up the level — the fill will compact under the concrete. Instead, fill the excess with blinding concrete or lean-mix concrete. Over-digging by 10 cm means 10 cm of extra concrete along the whole length — it adds up fast.

Hand digging: when is it needed?

A mechanical excavator cannot reach everywhere. Plan for hand digging (spade, pickaxe, crowbar) for:

Corners: the machine leaves rounded shapes; corners must be finished by hand for a clean formation level
Areas near existing services (water, electricity, gas) — risk of damage
Junctions between perpendicular trenches
Fine levelling of the formation level (the last 5 cm)

The formation level: the base of your foundations

The formation level is the surface on which the strip footing will rest. Its quality determines the service life of the entire house.

Requirements

Horizontal: tolerance of ± 2 cm under a 3 m straight edge
Flat: no humps, no hollows, no disturbed soil
Load-bearing: the ground in place must have the bearing capacity indicated by the soil survey (often > 0.15 MPa for a single-family house)
Dry: no standing water at the bottom (pump out if necessary before pouring)

Blinding concrete

Blinding concrete is a thin layer of lean-mix concrete (dosed at 150-200 kg/m³) poured over the formation level. Its purpose:

Protect the formation level from rain and foot traffic
Provide a clean surface on which to place cover spacers and reinforcement
Prevent direct contact between steel and soil (risk of corrosion)

Thickness: 5 cm minimum. For a 100 m² house with approximately 50 linear metres of footings, this represents around 1.5 m³ of concrete — half a ready-mix truck or 30 x 35 kg bags.

Question

Tip — Pour the blinding concrete the same day the trenches are dug. A formation level left exposed deteriorates quickly: rain saturates it, sun dries and cracks it, frost heaves it. Blinding concrete stabilises it within a few hours. Wait 24 hours before placing the reinforcement.

Formation level pitfalls

Heterogeneous ground

You dig and find a pocket of soft soil, old fill, or a mix of rock and clay side by side. This is heterogeneous ground — the nightmare of foundations, because settlement will be differential.

What to do?

Soft pocket: remove and replace with lean-mix concrete down to hard ground
Outcropping rock: anchor the footing in the rock (over-digging is not necessary)
Significant heterogeneity: call in the geotechnical engineer to adapt the foundation design (widened footing, rigid ground beam, or piles)

Water in the trench

If water rises in the trench (high water table, spring), do not pour concrete into water. Concrete diluted by water loses its strength. Solutions:

Continuous pumping during the pour (sump pump or site pump)
Temporary drainage: deepen the trench at a low point with a pump
Tremie pipe pouring: a professional technique for concreting underwater — rarely needed for a single-family house

flowchart TD A{Water in the trench?} -->|No| B[Pour normally] A -->|Some, seepage| C[Pump + pour quickly] A -->|Heavy, high water table| D{Pumping sufficient?} D -->|Yes| E[Continuous sump pump] D -->|No| F[Call the structural engineer] F --> G[Foundation design adapted] B --> H[Concrete C25/30] C --> H E --> H style A fill:#0F4C81,stroke:#0F4C81,color:#fff style B fill:#56C6A9,stroke:#56C6A9,color:#fff style C fill:#F58220,stroke:#F58220,color:#fff style D fill:#0F4C81,stroke:#0F4C81,color:#fff style E fill:#F58220,stroke:#F58220,color:#fff style F fill:#CD212A,stroke:#CD212A,color:#fff style G fill:#CD212A,stroke:#CD212A,color:#fff style H fill:#56C6A9,stroke:#56C6A9,color:#fff

Underground services

Before digging, you filed a DICT (Declaration of Intent to Start Works — the French equivalent of a utility search / cable and pipe survey) on reseaux-et-canalisations.ineris.fr, didn’t you? If any services cross the excavation zone:

Gas: call the network operator (GRDF in France). No excavation within 50 cm of a gas pipe without authorisation
Electricity: high-voltage cables = exclusion zone. Low-voltage cables: dig by hand near them
Water: locate, protect, and tell the digger operator where they are
Telecoms: less critical but worth avoiding (cutting a cable means no internet during the build)

Warning — The DICT is mandatory in France before any earthworks, even on your own land. If you damage a service without having filed the declaration, you are liable for repairs and face a fine. File the declaration at least 10 working days before starting work — it is free of charge.

Coordination with underground services (VRD)

Foundation trenches are the ideal moment to plan service penetrations under the house. Before pouring, check that you have allowed for:

Conduit ducts for water, electricity and telecom passages through the footings
Pipe sleeves for foul and surface water drainage (PVC pipes cast into the foundation)
The earth bonding cable (looped around the base of the trench)

See our guide VRD: underground services for full coordination guidance.

Best practice — Install the conduit ducts and sleeves before the reinforcement. Once the rebar is in place, it is much harder to thread a 100 mm PVC pipe through the mesh. Also refer to the foundation reinforcement guide for the arrangement of starter bars around the sleeves.

Budget: what do foundation trenches cost?

Item	Indicative cost
2.5 T mini digger + trenching bucket hire (1 day)	250-400 €
Earthworks contractor (50 linear metres of trench)	800-1,500 €
Blinding concrete (1.5 m³ for a 100 m² house)	200-350 €
Spoil removal (5-10 m³)	100-200 €
Surveyor (2nd visit, verification)	300-500 €
DIY total (excluding the footings themselves)	600-1,200 €
Total with earthworks contractor	1,500-3,000 €