Intermediate

Groundworks : site stripping, levelling and compaction

13 min read

Groundworks are the first physical act of building your house. Before pouring a single foundation, you need to transform your raw plot — with its topsoil, bumps, roots and stones — into a flat, stable and compacted platform, ready to receive strip footings or a ground slab. This is a stage many self-builders underestimate, yet it conditions everything that follows: a poorly prepared platform leads to settling foundations, a cracking slab, and 30 years of problems. This guide explains how to strip, level and compact your plot properly.

GROUNDWORKS CROSS-SECTION : STRIPPING + LEVELLING Natural ground to finished formation level ready for foundations BEFORE : NATURAL GROUND Topsoil Subsoil AFTER : STRIPPED FORMATION LEVEL Footprint + 1 m each side 20-40 cm 1 Topsoil stripping 2 Levelling + compaction Level 0 Slight 2-3% slope for surface water drainage

Site stripping: removing the topsoil

Site stripping (or “topsoil removal”) consists of removing the layer of topsoil — that brown, organically rich earth that makes up the top 20 to 40 centimetres of the ground. Why remove it? Because topsoil is:

  • Compressible: it contains humus, roots and air — it settles unpredictably
  • Unstable: under load it deforms, swells with water and shrinks in dry conditions
  • Biodegradable: organic matter decomposes, creating voids beneath the structure

Warning — Never build on topsoil. Even a simple garage slab must rest on ground fully stripped of organic material. This is rule number one of groundworks, and it is non-negotiable. The G2 AVP soil survey will tell you exactly how deep to strip.

How deep to strip?

The depth varies depending on the plot:

Ground type Topsoil depth Volume for 150 m²
Cultivated land (garden, field) 30-40 cm 45-60 m³
Natural grassland 20-30 cm 30-45 m³
Wooded plot 15-25 cm (+ stumps) 22-38 m³
Previously filled ground Variable — soil survey required To be determined

What to do with the topsoil?

Don’t waste it! Topsoil has value:

  • Store it on site: heap it into a bund (elongated mound) in a corner, no more than 2 m high so it stays alive. You will reuse it for the garden, borders and final grading
  • Give it away or sell it: a neighbour, market gardener or landscaping company will be glad to take it
  • Skip hire: as a last resort, budget 10 to 20 €/m³ for removal and tipping (inert waste facility — equivalent to a UK ISDI/inert skip)

Tip — Before stripping, have your house set out by a surveyor. They will pin the corner stakes and profile boards that define the exact footprint. Strip over a zone wider than the footprint: add 1 m on each side for plant access and future foundation trenches.

Levelling: achieving a flat platform

Once the topsoil has been removed, the exposed ground is rarely level. Levelling means flattening the surface to the required datum, cutting down the high spots and filling the hollows.

The reference datum

The platform level is set by:

  • The site plan from the planning permission (which gives the finished ground-floor level relative to existing ground — often called “±0.00” or NGF in France)
  • The soil survey which indicates the depth of bearing strata (foundation seating horizon)
  • Surface water drainage requirements: the platform should be slightly above the surrounding natural ground, or graded at 2–3% away from the building

Tip

Best practice — Mark out the reference level with timber stakes every 5 m, linked by a string line. Use a rotating laser level (hire ~50 €/day) to transfer the level to ±5 mm accuracy. It is the essential tool for levelling.

Levelling tolerances

The French building standard DTU 13.3 (ground slabs) requires:

  • Flatness tolerance: ± 3 cm under a 3 m straight-edge for a formation level
  • Level tolerance: ± 2 cm relative to the reference datum

In practice, aim for ± 2 cm flatness. This is achievable with a well-guided machine and laser control.

Cut and fill

  • Cut: you remove earth to lower the level (high spot or raised area)
  • Fill: you add material to raise the level (hollow or low-lying area)

Warning — Never backfill with topsoil or with uncontrolled mixed material. Fill must be a structural material: natural gravel 0/31.5, recycled aggregate, Type 1 sub-base (MOT aggregate)… Fill must be compacted in layers of 30 cm maximum. Poorly compacted fill guarantees differential settlement.

Compaction: the key to stability

Compaction is the most underestimated step in groundworks. Yet it is what transforms loose ground into a load-bearing formation level. Without compaction, the ground will settle under the weight of the structure — slowly, unevenly and destructively.

Why compact?

Compaction reduces the voids between soil particles by expelling air. The result:

  • Density increases (the target is 95–98% of MDD — Maximum Dry Density, established by the Proctor test)
  • Bearing capacity increases (the ground supports more load without deforming)
  • Permeability decreases (less direct infiltration beneath the house)

Compaction plant

Plant Use Weight Hire cost/day
Wacker plate Trenches, small areas, fill in layers 60-150 kg 50-80 €
Trench rammer (jumping jack) Narrow trenches, cohesive soil (clay) 60-80 kg 40-70 €
Roller compactor Large areas, formation level, parking 1-4 T 150-300 €
Pneumatic tyred roller Very large areas, thick layers 8-15 T With operator

For a single house on a level plot, the 100–150 kg wacker plate is the standard tool. Allow 4 to 6 passes per layer of fill.

The 30 cm rule

Compaction is done in layers of 30 cm maximum:

  1. Spread a 30 cm layer of material
  2. Lightly moisten if the ground is dry (moisture content affects compaction)
  3. Run the wacker plate in parallel strips with 20 cm overlap
  4. Make 4 to 6 passes per strip
  5. Check density if required (dynamic cone penetrometer)
  6. Next layer

Tip — How can you tell if it is compacted enough? A simple test: walk on the compacted surface. If you leave no visible footprint with your site boots, that is a good indicator. For a more rigorous check, request a plate bearing test — your soil survey will specify the minimum required bearing capacity.

Which plant for your groundworks?

WHICH PLANT FOR WHICH GROUNDWORKS? Choosing the right machine by area and volume MINI DIGGER 1.5-3 T Area: < 200 m² Narrow access (< 2 m) Trenches, excavations ~200-350 EUR/day No special licence needed (if under 6 T, own land) Ideal for self-builders EXCAVATOR 8-14 T Area: 200-1000 m² Medium-scale stripping Foundations, pool ~500-800 EUR/day Qualified operator required (CACES category B1) Standard for single houses BULLDOZER / LOADER Area: > 1000 m² Large soil volumes Levelling, cut and fill ~800-1200 EUR/day Qualified operator required (CACES category C1) Large plots / developments AVERAGE TOTAL GROUNDWORKS COST 100 m² house 2,500 - 5,000 EUR Flat plot, easy access Gentle slope 5,000 - 10,000 EUR Cut + spoil removal Difficult plot 10,000 - 25,000 EUR+ Rock, steep slope, tight access

Question

Can you do the groundworks yourself?

Yes, within limits:

  • Mini digger under 6 tonnes: no special licence required to operate it on your own land (not on the public highway). Delivery hire costs 200–350 €/day. Allow a day to get familiar if it is your first time.
  • Excavator over 6 tonnes: requires a French CACES category B1 licence or a qualified operator. Compulsory for large-scale stripping.
flowchart TD A{Area to excavate?} -->|Less than 100 m2| B[Mini digger 1.5-3 T] A -->|100 to 500 m2| C[Excavator 8-14 T] A -->|More than 500 m2| D[Bulldozer + excavator] B --> E{Doing it yourself?} C --> F[With professional operator] D --> F E -->|Yes| G[Hire 200-350 EUR/day] E -->|No| H[Groundworks contractor all-in] F --> I[500-1200 EUR/day] H --> I 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:#6B5876,stroke:#6B5876,color:#fff style E fill:#0F4C81,stroke:#0F4C81,color:#fff style F fill:#F58220,stroke:#F58220,color:#fff style G fill:#56C6A9,stroke:#56C6A9,color:#fff style H fill:#F58220,stroke:#F58220,color:#fff style I fill:#F58220,stroke:#F58220,color:#fff

Groundworks costs

Groundworks are often the first major budget item in the shell. Here are realistic price ranges:

Item Indicative cost
Topsoil stripping (20-30 cm) 5-10 €/m²
Levelling and shaping 3-8 €/m²
Spoil removal (off site) 10-20 €/m³
Type 1 sub-base / 0/31.5 gravel fill 25-35 €/m³ supplied and laid
Compaction Included in the above
Total for 100 m² footprint, flat plot 2,500-5,000 €
Total with slope or difficult access 5,000-10,000 €+

Warning — The spoil removal item can blow the budget. Stripping 30 cm over 200 m² = 60 m³ of earth. At 15 €/m³ for removal, that is 900 € just to get the soil off site. If you can store the topsoil on your plot for future landscaping, do so — you save that entire line.

Classic mistakes to avoid

  1. Not stripping deep enough — you leave a layer of topsoil under the foundations. Settlement is guaranteed.
  2. Backfilling with topsoil instead of aggregate — the fill settles, the slab cracks.
  3. Not compacting in layers — you dump 80 cm in one go and run the plate over it. Only the top 30 cm get compacted.
  4. Forgetting surface water drainage — without a slope away from the building, water ponds beneath the future house.
  5. Underestimating soil volumes — stripping 30 cm over 150 m² = 45 m³ of earth ≈ 5 lorry loads of 8 m³. Where do they go?
  6. Working in wet conditions — waterlogged soil cannot be compacted; it “pumps” under the wacker plate. Wait for 48 hours of dry weather.

Best practice — Get the groundworks contractor on site the same day as the surveyor for setting out. The surveyor drives the stakes, the contractor strips immediately. You save a day and avoid the pegs being disturbed between the two visits. Also coordinate with foundation trenches and below-ground services so all excavation is done in a single plant mobilisation.

Temporary site drainage

During groundworks, rainwater has nowhere to go — you have removed the topsoil that was absorbing it. Plan for:

  • A peripheral ditch around the platform (30 cm deep, falling to a low point)
  • A temporary soakaway at the low point (hole filled with stone) or a submersible pump
  • Geotextile membranes on earthwork faces if the soil is fine (clay, silt) to prevent erosion

When to move on to foundations?

Ideally, foundations should be poured within days of completing the groundworks. An exposed formation level deteriorates quickly:

  • Rain softens the ground and reduces its bearing capacity
  • Frost heaves the surface layers
  • Sun dries out clays which crack

If you cannot pour within a week, protect the formation level with a geotextile membrane and a 5–10 cm layer of gravel.

Checklist: site groundworks

  • G2 AVP soil survey obtained (topsoil depth, bearing capacity)
  • Setting out by surveyor (corner stakes, profile boards, ±0.00 datum)
  • Stripping zone defined (building footprint + 1 m each side)
  • Topsoil stripped to full depth (20-40 cm)
  • Topsoil stored in a bund or removed from site
  • Formation level laser-levelled (tolerance ± 2 cm)
  • Surface water drainage slope (2-3% away from building)
  • Type 1 sub-base / 0/31.5 gravel used for fill if required (no topsoil!)
  • Compaction in 30 cm max layers (4-6 passes per layer)
  • Bearing capacity check (plate bearing test if required by soil survey)
  • Peripheral ditch and temporary drainage in place
  • Formation level protected if delay before foundations > 48 h
  • Foundation trenches and below-ground services coordinated