Foundations explained: from strip footings to piles

What a foundation does

It spreads the weight of the building over enough ground to stay stable, resists movement from soil changes, and keeps the building level over time. It must also deal with water in the ground and, in some places, with frost, earthquakes or flooding.

Common types

• Strip foundations are continuous strips of concrete under walls. They are widely used for houses on good ground.
• Trench-fill foundations are strips cast in narrow trenches filled with concrete up to near the ground level, used where the soil needs a deeper foundation.
• Pad foundations are individual concrete pads under columns, used for framed buildings.
• Raft foundations are a thick concrete slab under the whole building, spreading the load, often used on weaker ground or where there is a risk of uneven settlement.
• Piled foundations transfer loads through weak soil to stronger layers below, using long columns of concrete, steel or timber. They are used for heavy buildings or poor ground.
• Ground screws and other lightweight systems suit small, light structures in some conditions.
• Basements create their own foundation and retaining structure, with waterproofing as a key issue.

It spreads the weight of the building over enough ground to stay stable, resists movement from soil changes, and keeps the building level over time.

What decides the choice

1. The ground. The type of soil, its strength, groundwater and whether it shrinks and swells. A ground investigation tells us.
2. The building. Its weight, its shape and how sensitive it is to movement.
3. Trees and vegetation. Roots can dry out clay soils, making them shrink, and the removal of a tree can make them swell. Foundations near trees need extra care.
4. Contamination and previous uses. Old buildings, fill and contamination may affect the design.
5. Climate and hazards. Frost, flooding, earthquakes and sea-level rise may influence the type and depth.
6. Neighbours. Digging near other buildings can affect them, and in the UK the Party Wall etc. Act 1996 may apply.
7. Cost and programme. Piles can be more expensive but faster in some cases. Compare options with the engineer.

Why a ground investigation matters

Without knowing what is under a site, a foundation is a guess. A ground investigation, by boreholes, trial pits and tests, reveals the soil layers, water levels and any problems. It is a small cost compared to the risk of getting it wrong.

What can go wrong

• Settlement and heave, where the ground moves under or around the building.
• Cracking in walls and finishes.
• Water entering basements and floors.
• Damage from tree roots or removed trees.
• Poor workmanship, such as shallow digging, wrong concrete or missing reinforcement.

Who does what

The structural engineer designs the foundations using the ground investigation. The building control body or authority checks the design and inspects the work. The builder must follow the design and call for inspections before the foundations are covered. You should never cover a foundation until it has been inspected.

In short

Foundations are chosen to suit the ground and the building, and are designed by an engineer after a ground investigation. Never guess, and never cover them before inspection.

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