Which foundation is best for a bungalow in Nigeria?

A strip footing under the load-bearing block walls, on ordinary firm lateritic ground. A bungalow's walls carry perhaps 20 to 30 kN per metre run, so the width the soil actually asks for is often only 300 to 400 mm — less than a person can work in — and the strip ends up 450 to 600 mm wide because that is what can be dug and built rather than because the ground needs it. Where the building is framed, with columns rather than load-bearing walls, pad footings under the columns are the equivalent answer.

When do you need a raft foundation?

Run the pads first and add up their area. When the pads would cover more than roughly half the building's footprint, the concrete saved by leaving gaps costs more in formwork, excavation and edge trimming than it saves, and a raft is the cheaper answer as well as the stiffer one. A raft is also the answer regardless of area where the ground is soft or variable, where there is fill or made ground the pads would sit unevenly on, and where a high water table has to be resisted as one continuous surface.

How deep should a foundation be?

Deep enough to be founded on undisturbed natural ground with a known bearing capacity, which in Nigerian residential practice usually means between 900 mm and 1.5 m. The depth is not chosen from a table: it is whatever it takes to get below the topsoil, below any fill or old excavation, below organic material, below the zone where a shrinkable clay dries out and swells with the seasons, and below the influence of a neighbouring building's foundation. A trial pit tells you; a rule of thumb does not.

Can you mix strip and pad foundations in one building?

It is done constantly — pads under the frame and strips under the infill walls — and it is safe as long as both are founded at the same level on the same material and both are designed at a similar bearing pressure. What causes trouble is mixing foundation types that settle by different amounts: a heavily loaded pad next to a lightly loaded strip on compressible ground will settle more, and the wall spanning between them cracks. Where the settlements would differ, tie them together with a ground beam or use one type throughout.

Which foundation? — strip, pad, raft or piles, and how the choice is made

The choice looks like a matter of judgement and mostly is not. It falls out of three numbers: what the ground can carry, what each column or wall brings down, and how much of the plan the footings would end up covering. The first is a site question, the second comes from a load takedown, and the third is arithmetic you can do in a minute.

What genuinely is judgement is everything at the bottom of this page — water, fill, expansive clay, the neighbour's building — and that is where foundations actually go wrong.

The line that sizes any shallow foundation

A footing is a device for turning a load into a pressure the ground will accept. Everything else is detail:

Area needed A = N / q_allow (service load, not ultimate)

Pad side = sqrt(A)
Strip width = (load per metre run) / q_allow

Service load sizes it; ultimate load reinforces it. Bearing pressure is a settlement question, so it is checked against the unfactored load — and then the steel in the footing is designed for the factored one. Getting these two the wrong way round makes the footing 40% too big or 40% too weak, and it is the commonest error on this page's subject.

Add about 10% to N for the footing's own weight and the earth standing on it. The worked pad adds 16%, because it is founded a metre down and carries that much backfill.

Pad sizes, by column load and ground

Square side in metres, from A = 1.1N/q. Read the column load off a takedown, the bearing pressure off a soil investigation:

Service load	q = 50 kN/m²	75	100	150	200
300 kN	2.57	2.10	1.82	1.48	1.28
500 kN	3.32	2.71	2.35	1.92	1.66
700 kN	3.92	3.20	2.77	2.27	1.96
1000 kN	4.69	3.83	3.32	2.71	2.35

A three-storey interior column on 4 × 4 m bays brings about 410 kN in service — the figure the takedown ends on — which on 150 kN/m² ground is the 1.8 m square the footing example designs.

Sizes here are for concentric load only. An eccentric pad — one on a boundary, or carrying a column at its edge — has a pressure that varies across it, needs more area than this for the same load, and is usually better solved with a combined or balanced base tied to the next column. Structura does not design that case.

The coverage test: when pads become a raft

Pads are cheaper than a raft when they work. The moment they stop working is not when one of them fails — it is when they have grown so large that the gaps between them cost more than the concrete they save:

Bays 4.0 x 4.0 m -> 16.0 m2 of plan per column
Column 500 kN service

q = 150 A = 1.1 x 500 / 150 = 3.67 m2 -> 23% of the bay pads
q = 100 A = 5.50 m2 -> 34% of the bay pads, growing
q = 75 A = 7.33 m2 -> 46% of the bay price a raft
q = 50 A = 11.0 m2 -> 69% of the bay raft

Roughly half the footprint is the usual trigger. Below that, dig separate holes; above it, one slab is less work, less formwork and less edge trimming — and it is stiffer, which is worth having on the sort of ground that produced the low number in the first place.

The five answers, and when each is right

Type	Right when	Wrong when
Strip	The load arrives along a line — block walls carrying the building, as in most bungalows	The building is framed, so the load arrives at points; or the ground is below about 50 kN/m², where the width grows faster than it is worth
Pad	A frame on decent ground: columns bring the load down at points and the pads stay well clear of each other	The pads would cover half the plan, sit partly on fill, or fall on a boundary where they cannot be centred under the column
Combined or balanced base	Two columns close together, or one on a boundary that cannot have a footing centred under it	Anywhere a plain pad works — it is a solution to a geometric problem, not a better footing. Structura does not design it
Raft	Soft or variable ground, fill, a high water table, or pads that have grown into each other	Good ground and light loads, where it is simply more concrete than the building needs
Piles	Competent ground is too deep to reach, or the loads are past what any shallow foundation on this soil will take	Whenever a raft works. Piles need a soil model and a site investigation, not a spreadsheet

Where the raft stops, so does this engine. Structura designs pads, strips and rafts, and when a raft cannot carry the building it says so rather than producing a plausible number. Piles are a different discipline with a different set of site data behind them, and inventing one would be worse than stopping.

How deep, which is a separate question

The width comes from the load. The depth comes from what you are trying to get away from:

Below the topsoil and any organic material. Non-negotiable, and it is why the first trial pit is worth more than any table.
Below fill, old foundations and rubbish. Made ground has no bearing capacity you can rely on, and a plot in a Nigerian city has often been filled at some point by someone who is no longer there to ask.
Below the seasonal moisture zone in shrinkable clay, where the ground swells in the rains and shrinks in the dry season. Founding in that zone is how a building cracks in its second year rather than its first.
Clear of the neighbour. A footing beside an existing building, founded lower, can undermine it; founded higher, it can be undermined by future work next door.

Between 900 mm and 1.5 m covers most Nigerian residential work, and the number is a result rather than an input. The worked pad founds at 1.0 m.

What overrides the arithmetic

Water. A high water table changes the excavation, the concrete, the buoyancy and sometimes the type. A raft can be detailed against it as one surface; pads and a separate ground slab cannot.
Variability across the plot. Uniform poor ground is easier than good ground with a soft corner in it. Differential settlement is what cracks buildings; absolute settlement mostly does not.
Slope. Stepped foundations, and a lateral push nobody put in the calculation.
What the number came from. A presumed bearing value out of a table is a starting point for sizing, not a design value. Above two suspended floors, or on any coastal, filled or reclaimed site, a site investigation stops being optional.
Settlement is not calculated here. Bearing pressure limits are a proxy for it. Structura's foundation sheets say so explicitly, because a footing that passes every bearing and shear check can still settle more than the building will tolerate.

Try the types against your own numbers

Structura designs pad, strip and raft foundations, each as a full calc sheet — bearing pressure on the service load, bending and shear on the ultimate one, punching where it applies — with the bar bending schedule and the quantities beside it. Run the same building on three types and compare them; single members are free to run, as many as you like.

Open Structura in your browser

On Android, get it on Google Play; on iPhone and iPad, get it on the App Store — same account as the browser.