Code of Practice for Design and Construction of Foundations in Soils —
General Requirements
The foundational standard for shallow foundation design. It covers spread footings, combined footings, and raft foundations — prescribing minimum depths, bearing capacity factors of safety, settlement limits, and construction requirements for all soil types.
📋 Note: IS 1904:1986 provides general requirements for shallow foundations. For detailed bearing capacity calculations, refer to IS 6403. For pile foundations, refer to IS 2911. This code is used in conjunction with site investigation data per IS 2131 and IS 2720.
IS 1904:1986 covers the design and construction of shallow foundations in soils for buildings and civil engineering structures. It applies to:
Bearing Capacity Calculations IS 1904 does not provide bearing capacity formulae. For ultimate bearing capacity calculations (Terzaghi, Meyerhof, Hansen methods), refer to IS 6403:1981 — Code of Practice for Determination of Bearing Capacity of Shallow Foundations.
Adequate site investigation is mandatory before foundation design. The extent depends on the type and size of structure and variability of the soil profile.
| Structure Type | Min. Borings / Trial Pits | Spacing |
|---|---|---|
| Small buildings (area < 500 m²) | Minimum 4 — one at each corner | As required |
| Medium buildings (500–2000 m²) | One per 250–500 m² of plan area | 15–30 m grid |
| Large buildings / industrial | One per 500 m² minimum | 30–50 m grid |
| Bridges / heavy structures | At each pier / abutment location | Per structure |
The depth of investigation shall extend to:
| Test | Standard | Purpose |
|---|---|---|
| Standard Penetration Test (SPT) | IS 2131 | N-value for bearing capacity and settlement in sands |
| Static Cone Penetration Test (CPT) | IS 4968 | Continuous soil profile; qc for bearing capacity |
| Plate Load Test | IS 1888 | Direct measurement of bearing capacity and settlement |
| Laboratory tests (Atterberg, shear) | IS 2720 | Soil classification, c and φ for clays |
The minimum depth of foundation must satisfy several independent criteria — the governing (deepest) requirement controls:
| Criterion | Minimum Depth | Remark |
|---|---|---|
| General minimum | 0.5 m below NGL | Protects against surface disturbance |
| Frost depth | Below frost line | Applicable in hilly / cold regions |
| Shrinkage zone (expansive soils) | Below active zone (typically 1.5–2.0 m) | Black cotton soil, montmorillonite clays |
| Scour depth (bridges, river structures) | Below maximum scour level | Per IS 3955 / IRC 78 |
| Adjacent structures | Below 45° line from base of adjacent footing | Prevents stress overlap |
Expansive Soils In black cotton soil areas, the minimum foundation depth is typically 1.5 m to 2.0 m to get below the active zone of seasonal moisture variation. Shallow foundations on expansive soils without adequate depth will experience heave and cracking.
IS 1904 defines the relationship between ultimate and safe bearing capacities and provides presumptive values for preliminary design.
Presumptive bearing values from IS 1904 Table 1 (for preliminary design only — must be verified by investigation):
| Soil / Rock Type | Presumptive Safe Bearing Capacity (kN/m²) |
|---|---|
| Hard rock (granite, trap, diorite) | 3,240 |
| Laminated rock (sandstone, limestone) | 1,620 |
| Residual deposits of shattered rock | 880 |
| Soft rock | 440 |
| Gravel, sand and gravel (compact) | 440 |
| Coarse sand (compact, dry) | 440 |
| Medium sand (compact, dry) | 245 |
| Fine sand (loose) | 100 |
| Stiff clay | 440 |
| Medium clay | 245 |
| Moist clay / soft clay | 100 |
| Black cotton soil (dry) | 150–200 |
Presumptive Values Are Conservative Estimates These values are for preliminary sizing only. Final design must be based on actual site investigation data and bearing capacity calculations per IS 6403. Presumptive values should never be used for important structures without verification.
IS 1904 prescribes maximum permissible total and differential settlements to prevent structural damage and maintain serviceability.
| Foundation Type | Soil Type | Max. Total Settlement |
|---|---|---|
| Isolated footing | Sand | 25 mm |
| Isolated footing | Clay | 40 mm |
| Raft foundation | Sand | 65 mm |
| Raft foundation | Clay | 65–100 mm |
| Parameter | Limit | Remark |
|---|---|---|
| Differential settlement | ≤ 0.75 × total settlement | Between any two adjacent columns |
| Angular distortion (framed buildings) | ≤ 1/300 | Δ/L between adjacent columns |
| Angular distortion (load-bearing walls) | ≤ 1/500 | More sensitive to differential movement |
| Tilt of rigid structures | ≤ 1/500 | Chimneys, silos, towers |
Angular Distortion vs. Differential Settlement Angular distortion (δ/L) is the ratio of differential settlement between two points to the distance between them. It is a better indicator of structural damage than total settlement. Framed structures can tolerate more total settlement but are sensitive to differential movement.
| Type | When to Use | Typical Span |
|---|---|---|
| Isolated Spread Footing | Individual columns; good bearing soil; columns well-spaced | — |
| Combined Footing | Two columns close together; edge column near property line | Up to 5–6 m |
| Strap Footing | Edge column near boundary; strap beam transfers eccentricity to interior footing | Up to 6–8 m |
| Raft / Mat Foundation | Weak soil; heavy loads; individual footings would cover >50% of plan area; differential settlement control needed | Full building |
Rule of Thumb for Raft If the sum of individual footing areas exceeds 50% of the total building plan area, a raft foundation is generally more economical and provides better settlement control. Raft foundations also reduce differential settlement by redistributing loads across the entire footprint.
IS 1904 specifies several construction requirements to ensure the foundation performs as designed:
| Requirement | Specification |
|---|---|
| Dewatering | Excavation must be kept dry; pumping or well-point dewatering as required; no concrete in standing water |
| Bearing surface preparation | Remove all loose, disturbed, or softened soil; trim to firm undisturbed stratum; place lean concrete (PCC) immediately |
| Backfill compaction | Backfill in layers ≤ 200 mm; compact to 95% of Proctor density; use approved material free of organic matter |
| Frost protection | In cold regions, insulate or heat formwork; do not place concrete on frozen ground |
| Shrinkage soil protection | Protect exposed foundation trenches from drying; place concrete promptly after excavation in expansive soils |
| Scour protection | Provide apron, cut-off walls, or riprap around foundations near water bodies |
Never Cast on Disturbed Soil The bearing stratum must be undisturbed and at the design bearing level. If the excavation is over-dug or the soil is softened by rain or seepage, the disturbed material must be removed and replaced with lean concrete (PCC 1:4:8) before placing the structural footing.