Every foundation calculation starts with one number — the allowable bearing pressure — and it is the number in the whole design least likely to have been measured. It gets inherited from the last job on the same street, or taken as "150" because that is what everybody uses.
This page is about where that number should come from, what Nigerian ground is actually like, and how the value you settle on decides the foundation you end up drawing.
What the number means
Soil fails in two different ways, and the allowable pressure has to keep both away:
- Bearing failure — the soil shears and the footing punches into the ground. This is a strength problem, and the ultimate capacity is divided by a factor of safety of about 3 to get an allowable value.
- Settlement — the soil does not fail, it just squashes, and the building cracks. In practice this is what governs on most sites, and it is why an allowable value that looks conservative for strength may still be the right one.
Two footings can pass the same bearing check and still crack the building between them: what damages a structure is differential settlement, one column dropping more than its neighbour. That is a reason to keep pressures uniform across a building, not just below a limit.
Presumed values, for preliminary sizing
BS 8004 gives presumed bearing values by soil description. They are explicitly for preliminary work — a starting point to get a size on paper, to be confirmed by test.
| Soil | Presumed value kN/m² |
|---|---|
| Dense sand and gravel | > 600 |
| Medium dense sand and gravel | 200–600 |
| Loose sand and gravel | < 200 |
| Compact sand | > 300 |
| Medium dense sand | 100–300 |
| Loose sand | < 100 |
| Very stiff or hard clay | 300–600 |
| Stiff clay | 150–300 |
| Firm clay | 75–150 |
| Soft clay and silt | < 75 |
| Very soft clay, peat, made ground | not to be relied on |
The 150 kN/m² used throughout these worked examples sits at the top of "firm clay" and is a reasonable presumed value for the firm lateritic soil much of Nigeria is built on, a metre down. It is not a value to design a block of flats on without a test.
What the ground is like, by region
Broad strokes — every site is its own site, and the point of the list is to know what you are looking for:
- Lateritic soils cover most of the country. Reddish, well drained, firm to stiff when they are undisturbed and below the topsoil. Generally kind to shallow foundations, which is why so much Nigerian housing sits on pads at a metre deep. They soften considerably when saturated, so drainage around the building is part of the foundation design.
- Lagos and the coastal belt — loose to medium dense sands over soft marine clay, a water table often within a metre or two of the surface, and on the newer developments, sand fill over what used to be swamp. Filled land is the hazard: it looks like a firm sandy site and the fill is only as good as its compaction, with the soft material still underneath it. Anything more than a bungalow here needs a borehole, not an assumption.
- The Niger Delta and riverine areas — soft clays, silts and organic material to depth. This is piled territory for anything substantial, and the reason is not strength alone but settlement over time.
- Abuja, Jos and the higher ground — firmer residual soils over weathered rock, often good bearing at modest depth, with the caveat that rockhead can be shallow and irregular and one column can land on it while its neighbour does not.
- Parts of the North East carry expansive "black cotton" clays that swell wet and shrink dry. Bearing capacity is not the problem — movement is, and the fix is founding below the active zone, not making the pad bigger.
Two things override all of it. Made ground and old borrow pits can put a metre of rubbish under one corner of a plot and nothing under the rest. And a high water table cuts the bearing capacity of a granular soil to roughly half what the same soil gives when dry, as well as making the excavation a pumping job.
When a test stops being optional
A site investigation is cheap next to a foundation, and absurdly cheap next to a foundation that has to be repaired. Ordinary practice, and worth insisting on:
| Situation | What is normally enough |
|---|---|
| Bungalow, familiar lateritic ground | Trial pits at the corners; check the founding stratum is what you assumed and is continuous |
| Duplex or two suspended floors | Trial pits plus a CPT sounding or a light borehole |
| Three storeys and up, or any coastal or filled site | Boreholes with SPT to depth, reported by a geotechnical engineer, with a stated allowable pressure and a settlement estimate |
| Soft clay, peat, deep fill, or a neighbour with cracks | Full investigation before the foundation type is chosen, not after |
When an SPT report does arrive, the descriptions map onto the presumed-value table like this:
| SPT N | Sands | Clays |
|---|---|---|
| 0–4 | Very loose | Very soft to soft |
| 4–10 | Loose | Soft to firm |
| 10–30 | Medium dense | Firm to stiff |
| 30–50 | Dense | Very stiff |
| > 50 | Very dense | Hard |
Correlations from N to an allowable pressure exist, and they belong to the geotechnical engineer who saw the borehole log, the water table and the soil descriptions. Reading a bearing pressure off an N value at a desk is how a footing gets designed on a number nobody is accountable for.
What the number does to the foundation
The same column — 410 kN in service, from the worked takedown — on a 4 m × 4 m grid, founded on different ground:
| Allowable kN/m² | Pad needed | Share of the bay | Sensible answer |
|---|---|---|---|
| 200 | 1.6 m sq | 16% | Pads, comfortably |
| 150 | 1.8 m sq | 20% | Pads — the worked example |
| 100 | 2.2 m sq | 30% | Pads, but check they clear each other |
| 75 | 2.5 m sq | 39% | Compare a raft on cost |
| 50 | 3.1 m sq | 59% | Raft |
| < 40 | — | — | Raft if settlement allows it, otherwise piles |
Sized as A = 1.15N / qallow, the 15% covering the footing's own weight and the backfill over it — the same start as the pad footing example.
The useful rule is in the third column. Once the pads are taking much more than about half the footprint, the ground between them is doing nothing, the excavation and formwork of many separate pits costs more than one slab, and a raft is both cheaper and stiffer — stiffer being the part that matters, because a raft spreads differential settlement instead of letting each column find its own level.
Below that, it is piles, and piles need a soil model that only comes out of a proper investigation — end bearing, shaft friction, the depth of the competent stratum. That is where Structura stops: it designs pads, strips and rafts, and when a raft will not do it says so rather than producing something plausible.
Try the sizes on your own building
Enter your allowable bearing pressure and Structura designs the footing to BS 8110 — sizing on the service load, then bending, vertical shear and punching shear at ultimate, with the bar schedule and quantities. Pads, strips and rafts are all in, and single members are free to run, as many as you like.
What it will not do is calculate settlement, and every foundation sheet says so plainly. A PASS means the pressure and the concrete check out — not that the building will not move.
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