General Requirements of Shallow and Deep Foundations as per IS:1904-2021

General Requirements of Shallow and Deep Foundations

Foundations are the most important structural elements responsible for safely transferring loads from the superstructure to the underlying soil or rock. Depending upon the depth-to-width ratio, foundations are generally classified into shallow foundations and deep foundations.

Classification Based on Depth/Breadth Ratio:

• Depth/Breadth Ratio ≤ 1 → Shallow Foundation
• Depth/Breadth Ratio > 5 → Deep Foundation
• Intermediate cases → Moderately Deep Foundation

Types of Shallow Foundations

Spread footings are commonly used shallow foundations. The following are the major types:

• Simple square footing
• Strip footing
• Combined footing
• Strap footing
• Raft or mat foundation

Fig: Types of shallow foundations and general requirements.

General Requirements of Foundations

Requirement	Description
Safety Against Shear Failure	Foundation should not fail in bearing capacity.
Settlement Control	Total and differential settlements should remain within permissible limits.
Adequate Depth	Foundation should be below seasonal moisture variation zone.
Stability	Foundation should resist sliding, overturning, and uplift.
Durability	Should withstand environmental and groundwater effects.
Economy	Foundation should be safe and economical.

Depth of Foundations

The minimum depth of foundation recommended by IS 1904 is generally 0.5 m. However, the actual depth depends upon:

• Soil type
• Groundwater conditions
• Seasonal moisture variation
• Adjacent foundations
• Scour depth
• Frost action
• Load intensity

General Recommendations:

• Clay soils: 1.5 m to 3 m
• Sandy soils: 0.5 m to 0.7 m
• Expansive soils: Below active moisture zone

Load Dispersion in Soil

For estimating stress distribution beneath foundations:

• 2 Vertical : 1 Horizontal distribution is commonly assumed.
• Approximate dispersion angle = 26.6°
• In rock, dispersion may be assumed at 60°.

Minimum Foundation Depth Calculator

Type of Soil Sand Clay Expansive Clay Silt Groundwater Condition Normal High Groundwater Table

Recommended foundation depth will appear here

Selection Criteria for Foundation Type

Condition	Recommended Foundation
Good Bearing Capacity Near Surface	Shallow Foundation
Weak Surface Soil	Deep Foundation
Heavy Structural Loads	Pile or Raft Foundation
Adjacent Property Line	Strap Footing
Closely Spaced Columns	Combined Footing
Waterlogged Area	Pile Foundation

Applications of Deep Foundations

• High-rise buildings
• Bridge piers
• Marine structures
• Offshore platforms
• Industrial foundations
• Transmission towers

Engineering Considerations

• Groundwater conditions
• Settlement behavior
• Load intensity
• Soil stratification
• Construction feasibility
• Economic considerations

References

1. Bowles, J. E. Foundation Analysis and Design, McGraw-Hill.
2. Das, B. M. Principles of Foundation Engineering.
3. IS 1904: 1986. Code of Practice for Design and Construction of Foundations in Soils.
4. Terzaghi, Peck & Mesri. Soil Mechanics in Engineering Practice.
5. IS 6403: 1981. Determination of Bearing Capacity of Shallow Foundations.

Author

Prepared By: Mohan Dangi
Civil Engineer & Geotechnical Content Creator

Disclaimer

This article and calculator are intended for educational and preliminary estimation purposes only. Actual foundation depth and type should be finalized based on detailed geotechnical investigation, structural loading, groundwater conditions, and applicable design codes. Users should verify all calculations using professional engineering judgment.