Guide · 7 min read
Groundworks Explained
Groundworks is every operation that turns raw ground into a stable, drained, level formation ready to build on: clearance, excavation, filling, compaction, drainage and the ground floor slab.
What groundworks covers
Groundworks starts the moment the site is handed over and finishes when the formation is proven and the slab is down. It is the least visible and most consequential stage of a build: almost every serious structural or damp problem in Irish housing traces back to something done — or skipped — at groundworks stage.
- —Site strip: vegetation and topsoil removed to expose subsoil
- —Reduced-level dig: excavation down to formation level across the footprint
- —Excavation of foundation trenches or raft dig
- —Import and placement of granular fill in compacted layers
- —Blinding, radon barrier, insulation and reinforcement
- —Ground floor slab pour and finish
Why ground conditions drive everything
Ireland's subsoils vary enormously across short distances — boulder clay in the midlands, limestone close to the surface in parts of the west, deep peat in upland areas, and made ground on infill sites in the cities. The same house design can need a 900mm strip footing on one site and an engineered raft on another.
This is why a rate-card price for groundworks is meaningless without a site inspection or a trial hole. A responsible contractor prices against the ground you actually have, and flags the risk items — rock, running water, soft spots, contamination — before they turn into variations.
Fill and compaction: where corners get cut
Fill should be placed in layers of 200–300mm and compacted with an appropriate roller or plate before the next layer goes down. Dumping 900mm of stone in one go and driving a dumper over it is not compaction, and it is the single most common cause of slab settlement and cracked screeds.
Where the engineer specifies a plate-bearing or in-situ density test, that test should happen before anything is cast over the fill.
Drainage runs before the slab, not after
Foul and storm drainage, radon sump pipework and any under-slab ducting are installed and recorded during the groundworks stage. Retro-fitting a missed duct after a slab is poured is expensive and, in a structural slab, sometimes impossible.
Good practice is a marked-up as-built drawing of every under-slab run before the concrete arrives on site.
Frequently Asked Questions
How long do groundworks take on a one-off house?+
On a straightforward site with reasonable ground, three to five weeks from site strip to a poured ground floor slab is typical. Rock, high water tables or deep fill can add substantially to that.
What is formation level?+
Formation level is the prepared surface, at the correct depth and in undisturbed or properly compacted material, on which foundations or fill are placed. It should be inspected before it is covered.
Can groundworks be done in winter?+
Yes, but wet weather makes formation protection critical. Exposed clay formations soften quickly, so the sequence is usually adjusted to blind and cover formations promptly.