Overview & Scope
IS 1904 provides the code of practice for the design and construction of foundations for buildings and structures. It covers isolated footings, combined footings, strap footings, raft/mat foundations, and pile caps. It explicitly excludes pile foundations (covered by IS 2911). Cl. 1
- Prevent Shear Failure: Ensure the soil does not fail due to excessive load.
- Limit Settlement: Prevent total or differential settlement that could damage the structure.
- Economy: Design safe foundations without over-conservative sizing.
The design process involves two main checks:
1. Safe Bearing Capacity: Load must be less than the soil's capacity to resist shear failure.
2. Settlement Check: Calculated settlement must be within permissible limits.
Bearing Capacity Concepts
Understanding the difference between Gross, Net, Ultimate, and Safe bearing capacity is crucial. Cl. 3
Ultimate Bearing Capacity ($q_u$)
- The minimum gross pressure intensity at the base of the foundation that causes shear failure in the soil.
- Calculated using Terzaghi, Meyerhof, or Hansen theories.
- Does NOT include safety factors.
Net Ultimate Bearing Capacity ($q_{nu}$)
- The net increase in pressure causing shear failure.
- $q_{nu} = q_u - \gamma D_f$
- (Where $\gamma$ = unit weight, $D_f$ = depth of footing).
Safe Bearing Capacity ($q_s$)
- The ultimate capacity divided by a factor of safety (usually 3).
- $q_s = q_u / FOS$
- The maximum pressure the soil can safely carry.
Allowable Bearing Capacity ($q_a$)
- The lesser of Safe Bearing Capacity or the value derived from Settlement Limit.
- This is the value actually used for design.
- $q_a = \min(q_s, q_{settlement})$
Allowable Pressures for Common Soils
While site-specific tests are mandatory for critical structures, IS 1904 provides approximate values for preliminary design based on soil classification. Table 1
| Type of Soil | Allowable Pressure (kN/m²) | Remarks |
|---|---|---|
| Rock (Hard, Sound) | 300 – 600+ | Depends on rock quality (RQD). |
| Gravel (Coarse/Dense) | 450 – 600 | Well graded, compacted. |
| Sand (Medium Dense) | 150 – 250 | Standard penetration test N > 30. |
| Sand (Loose) | 75 – 150 | N < 10. Avoid if possible. |
| Stiff Clay (Cohesive) | 150 – 300 | Undrained cohesion > 75 kPa. |
| Sandy Clay | 100 – 200 | Mixed properties. |
| Soft Clay/Mud | 50 – 100 | Requires special attention. |
| Burnt Brick Rubble | 100 – 200 | Well-compacted backfill. |
| Black Cotton Soil | 100 – 150 | Consider shrinkage/swelling potential. |
Settlement Limits
Excessive settlement can cause cracks in walls, tilting, or functional failure. IS 1904 sets strict limits on both Total Settlement and Differential Settlement. Cl. 4
Total Settlement
- Isolated Footing on Sand: ≤ 50 mm
- Isolated Footing on Clay: ≤ 75 mm
- Raft Foundation on Sand: ≤ 75 mm
- Raft Foundation on Clay: ≤ 100 mm
Differential Settlement
Difference in settlement between adjacent footings. This is more critical than total settlement.
- Isolated Footings (Sand): ≤ 25 mm (or slope 1/500)
- Isolated Footings (Clay): ≤ 40 mm (or slope 1/300)
- Continuous Footings: Slope ≤ 1/600
Construction Guidelines
Proper execution is as important as design. IS 1904 outlines key requirements for excavation and concreting. Cl. 6–7
Excavation
- Depth: Minimum 1.0 meter from ground level for footings, unless bedrock is encountered at higher level.
- Side Protection: Excavations deeper than 1.5m require sloping sides or shoring to prevent collapse.
- Base Cleaning: The bottom of the excavation must be clean, free from loose soil, organic matter, or water before concreting.
- Water Table: If water is encountered, dewatering must be done carefully to avoid washing out fine particles (piping phenomenon).
Foundation Concreting
- PCC Bed: A lean concrete (PCC) layer (Grade M10, 75mm thick) is recommended to provide a clean working surface and protect reinforcement.
- Concrete Grade: Minimum M20 for footings in mild exposure. M25+ for severe exposure.
- Curing: Minimum 7 days of moist curing is essential for foundation concrete to gain strength.
- Backfill: Use good quality soil in layers (max 300mm) and compact thoroughly. Do not use organic fill or debris.
- Pouring concrete into wet trenches without dewatering.
- Using loose soil for backfill leading to uneven settlement.
- Failing to check levelness of the PCC bed.
- Ignoring the water table effect on soil bearing capacity.
Special Soils
Certain soil types require specific mitigation strategies beyond standard design.
Expansive Soils (Black Cotton Soil)
Highly susceptible to swelling during rain and shrinking in dry seasons. This cyclic movement can crack foundations.
- Deep Foundations: Extend footings below the active zone (depth of moisture variation, typically 1.5–2.0m).
- Under-Reamed Piles: Use under-reamed piles (bulb at bottom) anchored deep in stable strata.
- Soil Treatment: Add lime/cement to stabilize soil near the foundation.
- Moisture Control: Maintain uniform moisture content around the building using drainage and paving.
Collapsible Soils
Loose, low-density soils that suddenly collapse upon wetting (e.g., loess). Often found in arid regions.
- Pre-wetting: Saturate the soil before construction to induce settlement.
- Densification: Use dynamic compaction or vibro-flotation.
- Deep Foundations: Bypass the collapsible layer entirely.
Organic Soils
Peat or humus rich soil. Highly compressible and unstable.
- Remove: Excavate and remove organic material completely.
- Replace: Backfill with granular material.
- Piling: Use piles if removal is impractical.
Related Codes
IS 1904 works alongside these codes for a complete foundation solution: