EXPERT Subsidence · Further Knowledge · E1 of 6
When does substructure work get used?
Only when mitigation hasn't stopped continuing irreversible movement. If tree management or drain repair returns the building to monitoring stability, substructure work is unjustified. Get this triage right — the FOS will scrutinise it, and so will the recovery defendant if you go after a tree owner.
If you're going through soils desiccated by trees that have been removed, the clay will re-wet and swell. Sleeves, slip membranes and compressible barriers between the new foundation and the soil are mandatory.
Method 1 — Traditional underpinning
Mass concrete foundation poured beneath the existing footing in 1.0–1.5 m hit-and-miss bays.
| Step | Detail |
|---|---|
| 1 | Excavate to agreed depth (below tree roots / to firm strata) |
| 2 | Clean foundation underside; install anti-heave precautions |
| 3 | Pour mass concrete to within 75–100 mm of the existing footing |
| 4 | Wait 24 hours for the concrete to cure and shrink |
| 5 | Ram in dry pack (semi-dry sand-cement mix) into the 75–100 mm gap |
| 6 | Wait 3 days before commencing the adjacent bay |
Practical depth limit: ~2 m. Beyond that, Health & Safety shoring becomes uneconomic. Absolute limit ~4 m.
Method 2 — Pad & beam
Where firm strata is at depth and continuous mass concrete would be uneconomic.
- Intermittent concrete pads at 2.5–3 m centres
- Continuous beam spans the pads to support the wall
- Higher bearing capacity required than continuous underpinning (loads concentrated at pads)
- Largely superseded by mini-piling as piling tech has improved
Method 3 — Mini piles
| Type | Suitable for | NOT suitable for |
|---|---|---|
| Driven piles | Loose soils, fill | Desiccated clays |
| Augered piles | Firm soils, deep stable strata | Loose soils (collapse into hole) |
Piles are only one part of the system — they support beams and slabs that transfer the building load. When passing through desiccated layers, an anti-heave sleeve is required.
Resin injection
An increasingly used alternative — geopolymer resin injected into the ground to raise, re-level and re-support the structure. Avoids excavation. Works best on granular soils with localised loss of bearing.
Source: Subsidence Handbook 4th Edition, Chapter 6 — Substructure Repairs.