Safe and ultimate bearing capacity, improvement

Ultimate bearing capacity

Ultimate bearing capacity ($q_u$) is the maximum pressure per unit area that the soil can bear at the foundation level, at which it fails in shear.

Safe bearing capacity

Safe bearing capacity ($q_s$) is the maximum pressure that soil can carry safely without shear failure and with settlement within the permissible limit.

$$q_s = \frac{q_u - \gamma D_f}{FS} + \gamma D_f$$

or simply $q_s = q_u/FS$ with a factor of safety of 2 to 3. The net safe bearing capacity is the net ultimate capacity divided by FS.

Methods to improve bearing capacity

1. Compaction: The soil is compacted in layers using rollers, rammers and vibrators to raise density; good for granular soils.
2. Drainage: Lowering the water table by sumps, well points and sub-soil drains gives higher effective stress.
3. Replacement: Removing weak soil and replacing it by sand, gravel or murrum compacted in layers (sand cushion).
4. Confining soil: Using sheet piles or retaining walls to prevent sideways flow.
5. Increasing depth and width of footing.
6. Sand piles and stone columns.
7. Chemical treatment and grouting: Cement, lime or silicate grout filling pores.
8. Soil stabilisation: Mixing cement, lime, fly ash or bitumen.
9. Preloading with surcharge to consolidate soft clay, and geotextiles for reinforcement.
10. Pile foundation if the above are not enough.

 footing
 [=======]
 |sand cushion|   <- replacement
 weak clay