Factors affecting foundation design, improving bearing capacity

Factors affecting the design of foundation

1. Bearing capacity of soil and the type of soil (clay, sand, rock).
2. Loads on the structure: Dead, live, wind, seismic and their eccentricity.
3. Depth of foundation: Depends on bearing strata, frost, shrinkage zone, scour, water table, and depth of nearby foundations. Rankine's formula gives minimum depth $D = \frac{q}{\gamma}\left(\frac{1-\sin\phi}{1+\sin\phi}\right)^2$.
4. Settlement: Total and differential settlement must be within limits.
5. Ground water table: Affects bearing capacity, excavation and uplift.
6. Type of structure: Sensitivity to settlement, e.g. framed or load-bearing.
7. Seismic zone and liquefaction risk.
8. Adjacent structures and underground services.
9. Soil problems: Expansive, collapsible or filled soils; sulphate in soil and ground water.
10. Site topography: Slope, drainage and erosion.
11. Construction methods, materials and economy.
12. Building codes: Nepal National Building Code, IS 1904, IS 6403.

Improving the bearing capacity of soil

1. Compaction by roller, rammer or vibration.
2. Drainage of water by sumps, well points, sub-soil drains.
3. Replacement of weak soil by a sand or gravel cushion.
4. Increasing foundation depth and width.
5. Sand piles or stone columns.
6. Grouting or chemical injection.
7. Stabilisation with cement, lime or fly ash.
8. Preloading with surcharge, and geotextile reinforcement.
9. Confining soil with sheet piles.
10. Using pile foundation.