Foundation design for slabs and footings is the process of determining the size, depth, and reinforcement configuration of building foundations to safely transfer structural loads to the ground. In Australia, residential foundation design is governed by AS 2870-2011, while commercial and industrial foundations follow AS 2159 (piling) and AS 3600 (concrete structures).
Types of Foundations
Shallow Foundations
| Type | Description | Typical Use |
|---|---|---|
| Strip footing | Continuous footing under load-bearing walls | Residential, light commercial |
| Pad footing | Isolated footing for columns | Columns, posts, pillars |
| Stiffened raft slab | Concrete slab with integral perimeter and internal beams | Standard residential (more stable) |
| Waffle raft slab | Grid of ribs (beams) with void formers between them | Residential (moderately reactive sites) |
| Combined footing | Multiple columns on one footing | Close-spaced columns |
| Shoe / edge beam footing | Edge beam thickening on a slab | Low-reactive sites |
Deep Foundations
| Type | Description | Typical Use |
|---|---|---|
| Driven piles | Precast concrete or steel piles driven to bearing stratum | All soil types, high loads |
| Bored piles (CFA) | Continuous flight auger piles cast in place | Variable soil conditions |
| Screw piles | Steel helical piles screwed into ground | Reactive soils, limited access |
| Pier and beam | Concrete piers at depth with suspended floor beams | Highly reactive sites, sloping sites |
| Micro-piles | Small-diameter drilled and grouted piles | Restricted access, underpinning |
Foundation Design Process
Step 1: Geotechnical Investigation
A site classification or geotechnical report provides:
- Soil profile and classification
- Bearing capacity
- Reactivity (shrink/swell index)
- Groundwater conditions
- Slope stability assessment (if required)
Step 2: Determine Site Class (AS 2870)
The site class determines the foundation type and design parameters:
| Site Class | Typical Foundation | Depth of Beam |
|---|---|---|
| A (sand/rock) | Slab on ground, edge beam | 300 mm |
| S (slightly reactive) | Waffle raft or stiffened raft | 450 mm |
| M (moderately reactive) | Stiffened raft | 600 mm |
| H1 (highly reactive) | Stiffened raft | 750 mm |
| H2 (highly reactive) | Stiffened raft or pier and beam | 900 mm |
| E (extremely reactive) | Pier and beam | Piers to > 2 m |
| P (problem site) | Engineered solution | Site-specific |
Step 3: Structural Design
The structural engineer calculates:
- Bearing pressure — load divided by footing area, must be ≤ allowable bearing capacity
- Settlement — total and differential settlement within acceptable limits
- Punching shear — resistance of slab/beam around column or wall
- Bending moment — reinforcement required in beams and slab
- Crack control — reinforcement to limit cracking from ground movement
- Reactive soil effects — design for soil heave/shrinkage
Step 4: Foundation Detailing
| Detail | Purpose |
|---|---|
| Concrete strength (f'c) | 20–32 MPa typical for residential; higher for aggressive soils |
| Cover to reinforcement | 20–50 mm depending on exposure class |
| Waterproofing | For below-ground walls and slabs |
| Articulation joints | For long walls to control cracking |
| Damp-proof membrane | Under slab on ground floors |
Bearing Capacity
Allowable Bearing Capacity (Typical Values)
| Material | Allowable Bearing Capacity (kPa) |
|---|---|
| Hard rock | > 1,000 |
| Soft rock | 500–1,000 |
| Very dense sand / gravel | 300–500 |
| Dense sand | 200–300 |
| Stiff clay | 150–250 |
| Firm clay | 75–150 |
| Loose sand | 50–100 |
| Soft clay | < 50 |
Bearing Capacity from SPT N-Values
For sands: q_all (kPa) ≈ N × 20 (Terzaghi rule of thumb)
For clays: q_all (kPa) ≈ N × 6.25 × 1/FoS
Settlement Considerations
| Foundation Type | Maximum Total Settlement | Maximum Differential Settlement |
|---|---|---|
| Residential slab (AS 2870) | 30–40 mm | 20 mm |
| Commercial footing | 25 mm | 15 mm |
| Mat foundation | 50 mm | 25 mm |
| Piled foundation | 10–25 mm | 5–10 mm |
Ground Improvement for Foundations
When in-situ soils are inadequate, ground improvement may be required:
| Issue | Improvement Method |
|---|---|
| Low bearing capacity | Remove and replace, stone columns, deep soil mixing |
| Reactive clay | Lime stabilisation, controlled filling |
| Loose sand / liquefaction | Vibro-compaction, stone columns, compaction grouting |
| Soft clay / high settlement | Preloading + PVD, deep soil mixing |
| Filled ground | Dynamic compaction, preloading, piles through fill |
Australian Standards
| Standard | Title |
|---|---|
| AS 2870-2011 | Residential Slabs and Footings |
| AS 2159-2009 | Piling — Design and Installation |
| AS 3600-2018 | Concrete Structures |
| AS 1726-2017 | Geotechnical Site Investigations |
| NCC (National Construction Code) | Building Code Requirements |
Frequently Asked Questions
What is the difference between a stiffened raft and a waffle raft?
A stiffened raft has beams that are formed by excavating trenches in the ground. A waffle raft uses polystyrene void formers on a flat ground surface with concrete ribs cast between them. Waffle rafts are typically used on moderately reactive sites where excavation is more difficult.
Do I always need a geotechnical investigation?
Yes. The NCC requires a site-specific classification for all new residential construction. Without it, no structural engineer can certify the foundation design.
Can a geotechnical investigation identify groundwater problems?
Yes — groundwater depth, flow characteristics, and potential construction implications are part of a standard geotechnical investigation.
What happens if unexpected soil conditions are found during excavation?
If the conditions exposed in the excavation differ from the geotechnical report assumptions, a geotechnical engineer should inspect the excavation and may recommend foundation modifications.