Every building you have ever walked into is standing on a decision somebody made about the ground beneath it. Get that decision right, and the structure sits quietly for a century. Get it wrong, and you get cracked walls, jammed doors, tilted floors — the classic symptoms of a foundation that was never matched to its soil.
This article classifies every major foundation type, explains when each one is used, and gives you a selection framework plus a worked sizing example you can actually follow. Foundations divide into two great families — shallow and deep — and the dividing line is simply how far down the load has to travel to find ground strong enough to carry it.
What a Foundation Does
A foundation has three jobs:
- Transfer the building’s loads (dead load, live load, wind, seismic) safely into the ground.
- Limit settlement to values the structure can tolerate — both total settlement and, more critically, differential settlement between different parts of the building.
- Provide stability against overturning, sliding, and uplift.
The required soil information comes from a geotechnical site investigation — boreholes, SPT N-values, and laboratory tests. If you want to understand how that data is collected, read our guide to the Standard Penetration Test (SPT).
Shallow Foundations (D/B ≤ 1)
A foundation is classified as shallow when its depth (D) is less than or about equal to its width (B). Shallow foundations spread the load over a wide area of soil near the surface. They are cheaper and faster to build — and they are the right choice whenever the near-surface soil is strong enough.
1. Strip (Wall) Footing
A continuous strip of concrete (or stepped brick masonry in traditional construction) running under load-bearing walls. The width spreads the wall’s line load so the pressure on the soil stays within the allowable bearing capacity.
- Used for: load-bearing masonry walls in houses and low-rise buildings.
- Typical form in Pakistan: stepped brick footing — each masonry course is offset outward as it goes down, widening the base (see the featured photo above).
2. Spread / Pad (Isolated) Footing
An individual square or rectangular pad under each column. The simplest and most common foundation for framed (RCC column-and-beam) structures on decent soil.
- Used for: single- and multi-storey framed buildings where columns are spaced apart and soil is reasonably uniform.
3. Combined Footing
One footing carrying two or more columns — used when columns are so close together that their individual pads would overlap, or when a column sits right at a property boundary and its footing can’t extend past the line.
4. Strap (Cantilever) Footing
Two isolated footings connected by a stiff strap beam, used when one column is at the property line and its footing must be kept eccentric — the strap balances the overturning.
5. Raft (Mat) Foundation
A single thick reinforced slab covering the entire building footprint, sometimes stiffened with beams. The building “floats” on it.
- Used for: heavy loads on weak or variable soil, basements, waterlogged sites, and where individual footings would cover more than about half the plan area — at which point a raft is usually more economical.
- Advantage: bridges over soft pockets and greatly reduces differential settlement.
Deep Foundations (D/B > 1, typically much greater)
When competent soil lies deep, or surface soils are too weak, compressible, or variable, the load must be carried down to a stronger stratum. Deep foundations work by a combination of end bearing (the toe resting on hard ground) and skin friction (resistance along the shaft).
6. Pile Foundations
Slender structural elements driven or cast deep into the ground. Main types:
- Driven piles: precast concrete, steel H-piles or pipes, and timber piles — hammered or vibrated into the ground. Fast in suitable soils; noisy, and driving can disturb neighbours.
- Bored (cast-in-place) piles: a hole is drilled, a reinforcement cage lowered in, and concrete poured — quiet, no vibration, and adaptable to variable ground. The most common deep foundation for buildings and bridges in Pakistan.
- Auger-cast (continuous flight auger) piles: drilled with a hollow-stem auger while concrete is pumped through the stem as the auger withdraws — fast and clean, excellent in urban sites. See our detailed article on auger cast piles.
7. Drilled Piers (Drilled Shafts)
Large-diameter cylindrical excavations filled with reinforced concrete — essentially very large bored piles (typically 0.75 m diameter and above). Used for bridges, towers, and heavy structures where a single pier can replace a group of piles.
8. Caissons
Large watertight box or cylindrical structures sunk into the ground, then filled with concrete. Types:
- Open caissons (wells): open at top and bottom, sunk by excavating soil from inside as the caisson sinks under its own weight. The traditional foundation for bridge piers in rivers.
- Box caissons: closed-bottom boxes floated into position and sunk onto a prepared bed — used for waterfront structures.
- Pneumatic caissons: pressurized working chamber at the bottom lets workers excavate below the water table in dry conditions. Specialized and expensive.
For a detailed side-by-side of these three, see piles vs piers vs caissons.
Foundation Selection Criteria
No foundation type is “best” in the abstract. The choice follows the ground, the load, and the water. Here’s the decision framework used in practice:
| Site condition | Indicated foundation |
|---|---|
| Strong soil at shallow depth, light loads (houses) | Strip or pad footings |
| Framed building, moderate loads, uniform soil | Isolated pad footings |
| Columns close together / boundary column | Combined or strap footing |
| Weak, variable, or waterlogged surface soil; heavy building | Raft (mat) foundation |
| Competent stratum at depth (10–40 m), heavy loads | Bored piles or driven piles |
| Very heavy concentrated loads (bridges, towers) | Drilled piers or pile groups |
| Bridge pier in a river / deep water | Open caisson (well foundation) |
| Expansive or collapsible surface soils | Deep foundations bypassing the problem layer |
| High water table with basements | Raft with waterproofing, or piles + basement slab |
Practical note: The cheapest foundation that safely does the job is the right foundation. I have seen projects where piles were specified out of caution on ground that would have carried simple pads — and the extra cost bought nothing but delay. A proper soil investigation (even a few boreholes with SPT) costs a fraction of one unnecessary pile, and it is the only honest basis for the decision.
Worked Example: Sizing a Pad Footing
Suppose a column carries a total service load of 600 kN, and the soil investigation gives an allowable bearing capacity of 150 kN/m².
Step 1 — Required base area:
A = Load ÷ Allowable bearing pressure = 600 ÷ 150 = 4.0 m²
Step 2 — Footing size:
For a square pad: B = √4.0 = 2.0 m → provide a 2.0 m × 2.0 m pad.
Step 3 — Check: 600 kN ÷ (2.0 × 2.0) = 150 kN/m² ≤ 150 kN/m² ✓
The structural design (thickness, reinforcement) then follows from punching shear and bending checks per the applicable concrete code — but this bearing check is the step that connects the structure to the soil, and it’s the step most often skipped on small projects.
Common Foundation Failures (and What Causes Them)
- Differential settlement — different parts of the building settle by different amounts: the number one cause of cracked masonry walls. Usually traceable to variable soil that wasn’t investigated, or footings founded at different depths.
- Bearing capacity failure — the soil shears under overload; rare in buildings but catastrophic. Prevented by honest allowable pressures with proper factors of safety.
- Founding on fill or topsoil — footings must bear on natural, undisturbed soil (or properly compacted engineered fill), never on loose backfill, topsoil, or rubbish.
- Water damage — a rising water table or broken drain softening the bearing soil. Always check the water table depth in the investigation.
- Eccentric loading — the column load not centered on the footing, causing tilt. Caught by checking the layout before concreting — a five-minute check that prevents a permanent lean.
Frequently Asked Questions
What is the difference between shallow and deep foundations?
Shallow foundations (strip, pad, raft) transfer loads to soil near the surface by spreading them over a wide area. Deep foundations (piles, piers, caissons) carry loads down to stronger strata at depth through end bearing and skin friction.
What is the minimum depth of foundation?
There is no single universal number — depth must reach below topsoil, fill, frost/seasonal moisture variation, and any weak surface layer, onto competent bearing soil. In practice, building codes and local practice set minimums (commonly around 0.9–1.0 m for small buildings in stable climates), but the soil investigation governs.
Raft foundation or piles — which is better?
Neither is universally better. Rafts suit weak but reasonably uniform shallow soils and basements; piles suit situations where a strong stratum exists at depth or surface soils are unacceptable (very soft, expansive, or filled). Cost, water table, and programme also decide.
What is a stepped brick footing?
A traditional strip footing where each course of brickwork is offset outward as it goes down, widening the base to spread the wall load — still common in low-rise masonry construction in Pakistan (see the featured photo).
How many boreholes are needed for a soil investigation?
It depends on the building size and soil variability — small buildings may need only a few boreholes, large or irregular sites need a proper grid. The rule of thumb: never design foundations on zero investigation. Even basic SPT boreholes are cheap insurance.
Final Thoughts
Foundation classification isn’t academic trivia — it’s the vocabulary of the most consequential decision on any project. Learn to match the foundation to the ground: investigate first, spread the load where the soil is good, go deep where it isn’t, and never found a building on soil you haven’t checked. Everything above ground depends on it — literally.
Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.
Mohsin Raza Adil
BSc Civil Engineering Technologist (NTC-registered), Pakistan. I write practical, field-tested guides on construction materials, soil testing and site engineering — the way it is actually done on site.
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