IS 1904 : 1986Code of practice for design and construction of foundations in soils (General requirements)

IS 1904:1986 is the Code of Practice for Design and Construction of Foundations in Soils — General Requirements. It is the umbrella foundation code for ordinary buildings, light industrial structures, and infrastructure where deep specialised codes (IS 2911 piles, IS 8009 settlement, IS 1080 shallow) are not invoked.

It governs the general principles: - Site investigation requirements - Allowable bearing pressure determination - Total and differential settlement limits - Choice between shallow vs deep foundation - Eccentric loading rules - Watertable considerations

Most RCC building designers use IS 1904 in combination with IS 6403:1981 (which gives how to calculate ultimate bearing capacity from soil parameters) and IS 8009 Part 1 (settlement of shallow foundations). IS 456:2000 then takes over for the structural design of the footing/raft itself.

Not used for: piles (use IS 2911), well foundations, caissons (IS 3955), tank foundations (IS 11089), or machine foundations (IS 2974).

Two acceptance criteria — BOTH must be satisfied (Clause 5):

1. Safety against shear failure: Net safe bearing capacity (q_ns) = Net ultimate bearing capacity / Factor of Safety (typically FoS = 2.5 to 3.0) 2. Permissible settlement: total settlement ≤ allowable limit; differential settlement ≤ acceptable structural distortion

The allowable bearing pressure (q_a) is the LOWER of the two. In good clay or dense sand, settlement often governs; in soft clay, shear often governs.

Presumptive bearing values (Table 1) — for preliminary design ONLY, when proper soil investigation hasn't been done:

| Soil type | Safe bearing pressure (kN/m²) | |---|---| | Rock (hard, sound) | 3,240 | | Rock (laminated, soft) | 440 | | Compact gravel | 440 | | Dense sand | 245 | | Loose sand | 100 | | Compact clay (stiff) | 245 | | Soft clay | 100 | | Black cotton soil (top 1 m) | 100 |

These values are conservative and presumptive — for final design, always determine bearing capacity from SPT N-value, CPT, plate load test, or laboratory shear parameters per IS 6403.

Allowable settlement limits (Table 2): - Isolated foundations on sand: total ≤ 40 mm, differential ≤ 25 mm - Isolated foundations on clay: total ≤ 65 mm, differential ≤ 40 mm - Raft on sand: total ≤ 65 mm, differential ≤ 40 mm - Raft on clay: total ≤ 100 mm, differential ≤ 65 mm

Angular distortion (δ/L) ≤ 1/500 for buildings with masonry walls; ≤ 1/300 for framed buildings.

Problem: Column DL = 450 kN, LL = 250 kN. Soil: stiff clay, undrained cohesion c_u = 100 kPa from triaxial tests, γ = 18 kN/m³. Foundation depth Df = 1.5 m. Square footing.

Step 1 — Service load: P = DL + LL = 450 + 250 = 700 kN

Step 2 — Net ultimate bearing capacity (Terzaghi for clay, φ = 0, per IS 6403 Eq. 1): q_nu = c_u × Nc + γ × Df × (Nq − 1) = 100 × 5.7 + 18 × 1.5 × 0 = 570 kN/m²

Step 3 — Net safe bearing capacity (FoS = 3, per IS 1904 Clause 4.4): q_ns = 570 / 3 = 190 kN/m²

Step 4 — Settlement check (from IS 8009 or empirical for stiff clay): for stiff clay, settlement at q_ns is typically 30-50 mm — within Table 2 limit of 65 mm ✓

Step 5 — Footing size: A = P / q_ns = 700 / 190 = 3.68 m² Square side B = √3.68 = 1.92 m → provide 2.0 × 2.0 m footing

Step 6 — Verify with full size: Actual pressure = 700 / (2.0 × 2.0) = 175 kN/m² < 190 kN/m² ✓ Gross pressure on soil = 175 + γ × Df = 175 + 27 = 202 kN/m² (for self-weight check on foundation soil — separate calculation)

Step 7 — Hand off to structural design: with q_factored = 1.5 × 175 = 262.5 kN/m² (LSD), proceed to IS 456 Cl. 34 for footing thickness, reinforcement, and one-way + two-way shear checks.

1. Using presumptive Table 1 for final design — Table 1 is for preliminary design only. Doing the structural footing design at 245 kN/m² (presumptive dense sand) without confirming with SPT N-value or plate load test is non-compliant with Clause 4. Always commission a soil investigation per IS 1892 before final design.

2. Ignoring water table — if water table rises above foundation level, submerged unit weight (γ_sub ≈ γ − 9.81) is used in bearing capacity calculation. This typically halves q_ns. Many designers forget the water-table correction; first heavy monsoon settles the structure unevenly.

3. No check on differential settlement — total settlement can be 40 mm and within limit, but if one corner settles 5 mm and another 35 mm, you get 30 mm differential which exceeds the framed-building limit of L/300. Always compute settlement at corner + centre at minimum.

4. Wrong FoS for sandy soils — IS 1904 says FoS = 2.5 to 3.0. For sand where bearing is governed by friction angle, the lower bound (2.5) is acceptable. For clay with c-only strength, 3.0 is safer because cohesive strength has more scatter and creeps under sustained load.

5. Skipping eccentricity check — eccentric column loads from frames produce trapezoidal pressure distributions. Maximum pressure (under heavier side) must still be ≤ q_ns. Use Meyerhof's effective-area method per Clause 5.2 for e > B/6.

6. Black cotton soil — Clause 6.5 specifically calls out: do NOT found shallow footings within the active zone (typically top 1.5-3.0 m) of expansive black cotton soil. Use under-reamed piles per IS 2911 Part 3 or carry foundation below the active zone.

• IS 6403:1981 — code of practice for determination of bearing capacity of shallow foundations (the calculation methods that IS 1904 implicitly invokes)
• IS 8009 Part 1:1976 — calculation of settlement of foundations on cohesionless soils (sand)
• IS 8009 Part 2:1980 — calculation of settlement of foundations on cohesive soils (clay)
• IS 1892:1979 — site investigation for foundations (mandatory before final design)
• IS 2131:1981 — Standard Penetration Test (SPT N-value, the most common bearing-capacity input)
• IS 4968 Parts 1-3 — Dynamic / Static Cone Penetration Test (alternative to SPT)
• IS 1888:1982 — Plate Load Test (direct field bearing-capacity test)
• IS 2911 Part 1 Sec 1:2010 — driven cast-in-situ concrete piles
• IS 2911 Part 3:1980 — under-reamed piles (for expansive soils)
• IS 456:2000 — structural design of the footing/raft after bearing pressure is established

IS 1904:1986 is 40 years old and was last formally revised in 1986. The presumptive Table 1 values reflect 1980s practice and are widely regarded as conservative — modern geotechnical engineering typically gets 20-40% higher safe bearing pressures from proper site investigation + IS 6403 calculations.

A major revision has been on the BIS sectional committee (CED 43) agenda since 2018 but no draft has been circulated. The 2024 IRC-derived 'limit-state foundation design' approach (used in IRC 78:2014 for bridges) has NOT migrated back to IS 1904 yet — IS 1904 remains a permissible-stress / FoS-based code.

For building foundations: IS 1904 remains universally accepted by ULB plan-approval authorities, structural reviewers, and PMC firms. For bridges: use IRC 78:2014 instead — it's limit-state and has updated bearing-capacity provisions. For expansive soil regions (Maharashtra Vidarbha, Karnataka, MP, TN, parts of Gujarat): IS 1904 alone is inadequate — supplement with IS 2911 Part 3 (under-reamed piles) and IS 13094:1992 (foundations on expansive soils).

On liquefaction-prone sites (alluvial Gujarat, coastal Maharashtra): IS 1904 is silent. Supplement with IS 1893 Part 1 Clause 8 (liquefaction assessment via SPT) and consider deep foundations or ground improvement.