PILING & FOUNDATIONS · KNOWLEDGE CENTRE
Shallow Foundation vs Deep Foundation: Which Is Right for Your Project?
A shallow foundation spreads building loads into soil close to the surface, while a deep foundation (such as piles) transfers loads to stronger ground far below. Shallow foundations suit sites with firm near-surface soil and moderate loads. Deep foundations suit weak surface soil, heavy loads, high groundwater, or reclaimed land. The right choice depends on soil investigation data, not on cost alone.
Key takeaways
- Shallow foundations rely on soil near the surface; deep foundations reach competent layers at depth.
- Shallow options include pad, strip, and raft foundations.
- Deep options include bored, CFA, and driven piles, and piers.
- Soft soils, high water tables, and heavy loads usually push a project toward deep foundations.
- The cheapest foundation on paper can become the most expensive if it settles or fails.
What is a shallow foundation?
A shallow foundation sits at or near ground level, generally where the foundation depth is not much greater than its width. Common types:
- Pad footings: individual bases under columns
- Strip footings: continuous bases under walls
- Raft (mat) foundations: a large slab spreading load over the whole building footprint
What is a deep foundation?
A deep foundation extends well below the surface to reach stronger soil or rock, or to develop friction along its length. Common types:
- Bored piles
- CFA (continuous flight auger) piles
- Driven piles (precast concrete or steel)
- Piers and caissons
Side-by-side comparison
| Shallow | Deep | |
|---|---|---|
| Depth | Near the surface | Many metres below, depending on soil |
| Best soil conditions | Firm, uniform soil near the surface | Soft, loose, variable, or fill soils near the surface |
| Typical loads | Low to moderate | Moderate to very heavy |
| Groundwater | Can complicate excavation | Piling methods can work through water-bearing ground |
| Settlement control | Depends on soil near the surface | Generally better, since loads reach competent layers |
| Cost | Usually lower if conditions allow | Usually higher, but justified where shallow options are unsafe |
| Speed | Often quicker on suitable sites | Depends on method and pile count |
| Testing | Plate or bearing checks | Pile integrity and load testing |
How to decide
Step 1: Get a soil investigation. Boreholes and in-situ tests show what lies beneath, and how strong it is. Step 2: Define the loads. The building height, use, and structural system set the demand. Step 3: Check groundwater. A high water table affects excavation, dewatering, and buildability. Step 4: Compare options. An engineer compares shallow, raft, and piled solutions on safety, settlement, cost, and programme. Step 5: Consider the surroundings. Adjacent buildings, noise limits, and access influence method choice.
The Lagos context
Many parts of Lagos have soft clays, loose sands, and shallow groundwater, and some areas are on reclaimed land. In these conditions, shallow foundations may settle excessively or lack capacity for taller buildings, so deep foundations are common. That said, some sites do have suitable near-surface soil, and a raft or improved ground may be adequate for lighter structures. Only a site-specific investigation can say.
Common mistakes
- Choosing a foundation before getting soil data
- Copying the foundation of a neighbouring building
- Selecting on first-cost alone and ignoring settlement risk
- Overlooking groundwater and seasonal changes
Frequently asked questions
Is a raft foundation shallow or deep? +
Are deep foundations always better? +
Which is cheaper, shallow or deep? +
Can a building have both? +
Related reading
- What Is a Pile Foundation and When Is It Required?
- How Does Soil Type Affect Foundation Design?
- Building on Soft Soil: Foundation Options
Need piling advice?
Contact our team for a site assessment and a piling proposal.