Classification of Foundations in Civil Engineering

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. If you are looking for a clear classification of foundation types used in civil engineering — all the types of foundation from a simple strip footing to a pneumatic caisson — this is the complete reference. 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:

1. Transfer the building’s loads (dead load, live load, wind, seismic) safely into the ground.
2. Limit settlement to values the structure can tolerate — both total settlement and, more critically, differential settlement between different parts of the building.
3. 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.

Types of Foundation and Their Typical Bearing Capacity Values (Approximate)

Every foundation type in the classification above ultimately answers one question: how much pressure can the soil take? The table below gives typical safe bearing capacity (SBC) values used for preliminary sizing. These are approximate, textbook-typical values — the actual SBC for your site comes only from the geotechnical investigation, and the structural engineer fixes the design value.

Soil / rock type	Typical safe bearing capacity (kN/m²)
Hard sound rock	2000+
Laminated / weathered rock	1000
Dense compact gravel	500–600
Compact coarse sand	400
Medium dense sand	200–300
Stiff clay	150–300
Medium clay	100–150
Soft clay / silt	75–100
Made ground, loose fill, topsoil	Not suitable — must be removed or bypassed

Practical note: On small house projects in Pakistan, engineers commonly work with 100–150 kN/m² for ordinary soils unless the investigation says otherwise. But “commonly” is not “always” — I have seen stiff-looking clay test at under 100 kN/m² because of a high water table nobody checked. The table guides your first guess; the borehole gives the answer.

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.

Worked Example 2: Sizing a Strip Footing for a Load-Bearing Wall

Strip footings carry a line load (kN per metre run), not a point load. Suppose a 9-inch load-bearing wall for a two-storey house imposes 60 kN per metre run (including the wall’s own weight and the floors it carries), and the soil’s allowable bearing capacity is 120 kN/m².

Step 1 — Required width per metre run:

B = Line load ÷ Allowable bearing pressure = 60 ÷ 120 = 0.50 m

Step 2 — Provide a practical width:

Provide a 600 mm wide strip footing (round up to a workable size — bricklayers and formwork don’t deal in 500 mm niceties when the excavation is hand-dug).

Step 3 — Check: 60 kN/m ÷ 0.60 m = 100 kN/m² ≤ 120 kN/m² ✓

Practical note: In traditional stepped brick footings, that 600 mm base is built up in offsets — for example, three courses each stepping out 75 mm per side from a 340 mm wall. The arithmetic is the same; only the construction is brick instead of concrete.

Classification of Foundation by Depth: Practical Depth Rules

The shallow-vs-deep classification is about ratios, but on site you also need actual depths. Practical rules:

• Always found below topsoil, fill, and made ground. Footings must bear on natural, undisturbed soil (or properly compacted engineered fill) — never on loose backfill, topsoil, or rubbish. If the top 1.5 m is fill, your “shallow” footing starts at 1.5 m.
• Minimum ~0.9–1.0 m for small buildings in stable climates — deep enough to get below seasonal moisture variation that makes clay swell and shrink.
• Below frost depth in cold regions (northern Pakistan) — frost heave can lift a shallow footing.
• Bridge foundations below scour depth — flood water erodes riverbed soil around piers; the foundation must sit below the lowest scoured level, which is why river bridges so often end up on wells or piles.
• On sloping ground, step the footings — each step founded on firm soil, with the steps keyed into each other so the foundation acts as one.

Practical note: When the excavation is open, look at the bottom with your own eyes before anyone pours PCC. Loose pockets, old root holes, a soft patch that the excavator smeared over — the bearing surface you accept is the bearing surface you get. Five minutes with a shovel and your boots saves five years of settlement disputes.

Types of Foundation on Site: Field Tips That Prevent Failures

• PCC leveling course first. A 75–100 mm plain concrete (1:4:8) blinding under every footing gives you a clean, level surface for the steel and stops the footing concrete mixing with soil. Skipping it to “save money” is the most expensive saving on site.
• Inspect the steel before the pour. Cover blocks in place, bars as per drawing, no mud on the reinforcement. Once concrete is in, you can’t check anything.
• Dewater before concreting, not during. Standing water in the excavation dilutes the concrete and ruins the bearing surface. Pump it dry, then pour.
• Backfill in layers. Backfill around footings in 150–225 mm layers with compaction — dumping a metre of loose soil back in one go guarantees settlement of the plinth and cracked floors later.
• Protect open excavations from rain. A night of heavy rain can turn a perfect founding level into soup. If rain is coming, cover the excavation or keep the pump ready.
• Mark founding levels on a ranging rod. When the excavator operator can see the exact depth on a painted rod, you get fewer “oops, 200 mm too deep” moments — and every extra 200 mm of excavation is concrete you pay for.

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.

What is the safe bearing capacity of clay soil?
It depends on consistency: stiff clay typically 150–300 kN/m², medium clay 100–150 kN/m², and soft clay only 75–100 kN/m² (approximate, typical values). Soft, saturated clay is the classic problem soil — always confirm with a proper investigation, because the water table can halve the number you assumed.

How deep should the foundation be for a house in Pakistan?
For ordinary low-rise houses on decent soil, foundation depth is commonly 0.9–1.2 m — but the real rule is: below all topsoil, fill, and loose material, onto natural undisturbed soil. Among the types of foundation used for houses, strip footings for load-bearing walls and pad footings for framed structures both follow this rule.

What is negative skin friction in pile foundations?
When soft surrounding soil settles more than the pile (for example, a new fill placed around existing piles), the settling soil drags the pile downward instead of supporting it — adding load rather than carrying it. Designers account for it as an extra downward force on the pile. It’s a classic exam question and a real site issue on reclaimed or filled land.

Can we build a house on filled-up ground?
Only with great caution. Ordinary loose fill is not a founding stratum in any classification of foundation practice. Options: remove the fill and found on natural soil below, use properly compacted engineered fill (placed and tested in layers), or bypass it entirely with piles or a raft designed for the conditions. Never place a footing directly on uncompacted fill.

Pile foundation or raft foundation — which is cheaper?
There is no fixed answer — it depends on depth to competent soil, water table, and building footprint. As a rule of thumb: if individual footings would cover more than about half the plan area, a raft is usually cheaper than separate pads; piles become economical only when shallow options can’t work, because piling rigs, testing, and pile caps cost real money. Get both options priced — assumptions are expensive.

What factor of safety is used in foundation design?
Typically 2.5–3.0 against bearing capacity failure for shallow foundations (the allowable pressure is the ultimate pressure divided by this factor), with settlement — not just shear failure — often governing the design. For piles, partial factors on shaft friction and end bearing are applied per the applicable code.

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.

The classification of foundation types — shallow versus deep, strip versus pad versus raft versus piles — exists so that every engineer on a project speaks the same language about the ground. Master these types of foundation, size them honestly against real soil data, and build them the way the site tips above describe, and your structures will sit quietly for their full design life.

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Written by Mohsin Raza Adil — BSc Civil Engineering Technologist (NTC-registered), Pakistan.