The safe bearing capacity of soil is the pressure the ground can carry from a foundation with an adequate margin against shear failure. It is one of the first numbers a structural engineer asks for.
The terms explained
- Ultimate bearing capacity: the pressure at which the soil fails in shear.
- Net ultimate bearing capacity: the ultimate value minus the existing overburden pressure at foundation level.
- Safe bearing capacity: net ultimate divided by a factor of safety, often 2.5 to 3, plus the overburden.
- Allowable bearing pressure: the pressure that is safe in shear and also keeps settlement within limits. This is what the design normally uses.
Indicative values
Presumptive values like these are often quoted for preliminary thinking only. They are not a substitute for testing.
| Ground type | Indicative value (kN/m²) |
|---|---|
| Hard, sound rock | 3240 |
| Soft rock | 440 |
| Compact gravel or coarse sand | 440 |
| Compact, dry medium sand | 245 |
| Medium clay | 245 |
| Loose medium or fine sand | 100 |
| Soft clay | 100 |
| Very soft clay | 50 |
| Black cotton soil | Only after investigation |
How it is determined
- Boreholes and SPT to establish layers and strength.
- Laboratory tests for shear strength and consolidation.
- Plate load test where a direct field check is useful.
- Analysis under codes such as IS 6403 for bearing capacity and IS 8009 for settlement.
What changes the value
Foundation size and depth, groundwater level, nearby slopes and soil type all matter. The same soil can give different values for different footings, so one number should not be reused across sites.
FAQ
How do I find the bearing capacity of soil on my plot?
Through a soil investigation: boreholes, field tests and laboratory tests, followed by an engineering analysis for your foundation.
Why is black cotton soil treated differently?
It swells and shrinks with moisture, so movement often governs the design more than strength does.
Is safe bearing capacity the same as allowable bearing pressure?
No. Allowable bearing pressure also accounts for settlement, so it can be lower.
Talk to our team
Need bearing capacity for your site? Call +91 98718 04288, email contact@geowingseng.com or see our soil investigation service.
