Allowable bearing capacity: The maximum pressure that can be applied to the soil from the foundation so that the two requirements are satisfied:
- Acceptable safety factor against shear failure below the foundation
- Acceptable total and differential settelement
Ultimate bearing capacity: The minimum pressure that would cause the shear failure of the supporting soil immediately below and adjacent to the foundation.
Typical values of soil bearing capacity
For preliminary design purposes, BS 8004 [1] gives typical values of allowable bearing capacity which should result in an adequate factor of safety against shaer failure without accounting for the setllemenet criteria [2].
| Soil type | Bearing value (kPa) | Remarks |
| Dense gravel or dense sand and gravel | > 600 | Width of foundation not less than 1 m. Water table at least at the depth equal to the width of foundation, below base of foundation. |
| Dense dense gravel or medium dense sand and gravel | 200-600 | - |
| Loose gravel or loose sand and gravel | < 200 | - |
| Compact sand | > 300 | - |
| Medium dense sand | 100 - 300 | - |
| Very stiff boulder clays and hard clays | 300 - 600 | Susceptible to long term consolidation settelement |
| Stiff clays | 150 - 300 | - |
| Firm clays | 75 -150 | - |
| Soft clays and silts | < 75 | - |
| Very soft clays and silts | - | - |
Ultimate bearing capacity for shallow foundations according to Terzaghi
The utimate bearing capacity for shallow foundations can be calculated using the relation proposed by Terzaghi [3]:
and the net ultimate bearing capacity:
With the bearing capacity factors:
 |
factor for surcharge |
 |
factor for cohesion |
 |
factor self-weight [4] |
 |
factor self-weight [5] |
and
| B, L, D | Width, Length and Depth of foundation, |
 |
cohesion, effective frinaction angle and effective unit weight |
In case of a rectangular foundation, the terms of the above relations should be multiplied by the respective shape factors as follows [6]:
In case the loads are not applied vertically, additional factors should be also considered for inclined loads.
SpreadSheet for Soil Bearing capacity: Free download
REFERENCES
- BS 8004: (1986): Code of practice for foundations
- Craig, R. F. (1986), Soil Mechanics, 4th ed., ISBN 0-412-38430-2
- Tezaghi, K. (1943): Theoretical soil mechanics, John Wiley and sons, New York
- Hansen, J. B. (1968): A revised extended formula for bearing capacity. Danish Geotechnical Institute Bulletin, No. 28
- Meyerhof, G. G. (1963). Some recent research on the bearing capacity of foundations. Canadian geotechnical journal Vol. 1 No. 1
- Terzaghi, K. and Peck, R. B. (1967): Soil mechanics in engineerin practice (2nd ed). John Wiley and Sons, New York
Citation :
Geotechdata.info, Soil Bearing capacity, http://geotechdata.info/parameter/bearing-capacity.html (as of April 15, 2015).