Footings transfer structural loads from buildings to the soil. The type of foundation used depends on ground and soil conditions. Isolated footings support individual columns spaced far apart on uniform bearing soil. Combined footings support multiple close columns on variable soil. Methods to improve poor soil bearing capacity include increasing depth, drainage, compaction, replacing soil, grouting, and chemical stabilization. Foundation depth depends on factors like bearing capacity, seasonal soil movement, frost depth, excavation, groundwater, and practical minimum depth. Footing width varies by structure and load but is typically based on structural design.

1. FOUNDATIONS
BUILDING MATERIALS AND CONSTRUCTION TECHNOLOGIES
DAP, INDIRA GANDHI DELHI TECHNICAL UNIVERSITY FOR WOMEN

2. Footings are structural elements, which transfer live and Dead loads to the soil
from Superstructure.
Choosing a kind of foundation depends on: • The ground conditions
• The groundwater conditions
• The site, the environment
• The structure the building

4. ISOLATED FOOTING
Used where there is a good uniform
bearing stratum at a relatively shallow
depth, columns may be supported on
independent pad footings provided the
columns are spaced sufficiently far apart
from each other.

5. ISOLATED FOOTING

6. ISOLATED FOOTING

7. COMBINED FOOTING

8. COMBINED FOOTING
Combined footing support two or more
columns and may be used if
- Close spacing of column would lead to
overlap of individual pad footing.
- Bearing capacity of the soil is variable.

9. CANTILEVERED FOOTING

10. RAFT FOUNDATION

12. BEARING CAPACITY OF SOILS
The ultimate bearing capacity of the soil, is the maximum bearing pressure
that the soil can withstand.
Dry coarse and well graded dense sand have maximum shear resistance and
maximum bearing capacity. In general submerged soil and clay have less bearing
capacity.

13. TO IMPROVE BEARING CAPACITY OF SOIL
•Increasing depth of foundation
- At deeper depths the over burden on soil is higher, soil is more compacted at deeper depth. As a
result it shows higher bearing capacity.
-This is applicable only for soils such as sandy and gravelly soils.
-This method is not applicable if the subsoil material grows wetter as depth increase.
-This method has a limited use because with increase in depth, the weight and cost of foundation also
increases.
•Draining the soil
-With increase in percentage of water content in soil, the bearing capacity decreases.
-Soil can be drained by laying the porous pipes to a gentle slope, over a bed of sand and filling the trenches
above the pipes with loose boulders. These trenches subsequently should lead to the nearest well or any
water body.
The ultimate bearing capacity of the soil, is the maximum bearing pressure
that the soil can withstand.

14. TO IMPROVE BEARING CAPACITY OF SOIL
•Compacting the soil
- By spreading broken stones, gravel or sand and thereafter ramming well in the bed of trenches.
- Using an appropriate roller as per the soil type to move at a specified speed.
•Replacing the poor soil
- Poor soil is first removed and then the gap is filled up by superior material such as sand, stone, gravel or
any other hard material.
•Using grouting material
-This method is applicable for soils where there is presence of pores, fissures or cracks etc underneath the
foundation. In this method, poor soil bearing strata is hardened by injecting the cement grout under pressure,
because it scales off any cracks or pores or fissures etc.
•Stabilizing the soil with chemicals
-This method of improving bearing capacity of soil is costly and applied in exceptional cases. In this method,
chemical solutions, like silicates of soda and calcium chloride is injected with pressure into the soil. These
chemical along with the soil particles form a gel like structure and develop a compact mass. This is called
chemical stabilization of soil and used to give additional strength to soft soils at deeper depths.

15. DEPTH OF FOUNDATION
Depth of foundation depends on following factors:
1.Availability of adequate bearing capacity
2.Depth of shrinkage and swelling in case of clayey soils, due to seasonal changes which may cause
appreciable movements.
3.Depth of frost penetration in case of fine sand and silt.
4.Possibility of excavation nearby
5.Depth of ground water table
6.Practical minimum depth of foundation should not be less than 50 cm. to allow removal of top soil
and variations in ground level.
Width of Foundation / Footings
The width of footings should be laid according to structural design. For light loaded
buildings such as houses, flats, school buildings etc. having not more than two storeys, the
width of foundation is given below: