Code of Practice For Design And Construction Of Shallow Foundations In Soils (Other Than Raft, Ring And Shell)
1985 Edition

IS 1080 outlines the applicable range for shallow foundation design and construction, specifically excluding raft, ring, and shell foundations. These excluded types are governed by separate IS standards such as IS 1904, IS 1081, and IS 1082. The standard promotes a consistent technical methodology that replaces empirical rules, suitable for challenging soil conditions. Foundation sizing should be informed by detailed soil investigation rather than fixed heuristic values. Excavation must ensure safe and practical working dimensions, following IS 3764. Numerical results must be rounded as per IS 2-1960 to maintain precision.

Key References:

This ensures uniform and technically robust foundation designs.

All terminology used in IS 1080 is defined according to IS 2809-1972, which provides a glossary of soil engineering terms. The standard governs shallow foundations, excluding raft, ring, and shell types. A critical requirement (Clause 4.1) mandates reinforcement if the resultant load eccentricity exceeds one-sixth of the footing's least base dimension. Rounding of analytical and test results must conform to IS 2-1960, preserving the specified number of significant digits.

Load Eccentricity Assessment:

Supporting standards include IS 2809-1972 for definitions, IS 2-1960 for rounding, and IS 1904-1985 for general foundation requirements.

This section covers the foundational principles for shallow foundations, excluding raft, ring, and shell types, with overarching requirements detailed in IS 1904-1985. The design approach abandons traditional empirical rules in favor of uniform technical criteria appropriate for complex soil conditions.

Key points include:

• Load eccentricity beyond one-sixth of the footing’s narrowest base dimension necessitates reinforcement to resist bending stresses.
• Numerical results must be rounded following IS 2-1960 standards.
• Other foundation types are addressed by their respective codes: IS 2950 for raft, IS 11089 for ring, and IS 9456 for shell foundations.

Reference Table:

This framework ensures a consistent regulatory environment for foundation design.

IS 1080 stipulates that only shallow foundations (excluding raft, ring, shell types) fall within its scope. The design methodology replaces heuristic rules with scientifically grounded calculations. Load eccentricity exceeding one-sixth of the smallest footing base dimension triggers mandatory reinforcement (Clause 4.1). Footing dimensions must be selected based on soil bearing capacity and applied load (Clause 4.5).

Material design standards include:

• Cement concrete foundations conforming to IS 456-1978.
• Masonry foundation design according to IS 1905-1980.

All final calculated values should be rounded per IS 2-1960.

Important design formulae:

1. Eccentricity limit: e ≤ B/6
2. Allowable bearing pressure: q_allow = P / A
3. Footing width (B) for square/rectangular footings: B = √(P / q_allow)

Supporting references include IS 1904, IS 456, IS 1905, and IS 2.

Construction practices outlined in IS 1080 Clause 5.4 specify:

• Masonry work should commence only after the underlying concrete base has been cured for a minimum of three days to ensure strength and bonding.
• The foundation bed must be compacted thoroughly using manual ramming prior to concrete placement to prevent settlement.
• Cement concrete foundations must adhere to IS 456:1978, while masonry foundations follow IS 1905:1980.

Summary Table:

This approach guarantees durability and performance of shallow foundations.

Clause 5.4 of IS 1080 addresses material requirements:

• The concrete base must cure for at least three days before masonry construction begins.
• Foundation pit beds require thorough manual compaction to provide stable support.
• Masonry must be built atop the cured concrete base to ensure proper bonding and stability.

Referenced IS standards include IS 1904 for general foundation requirements, IS 2809 for terminology in masonry, and other codes governing lime concrete, brickwork, and stone masonry.

Practical considerations involve ensuring moisture content and material quality comply with respective IS codes, employing manual ramming for compaction, and adhering strictly to curing durations to achieve design strength.

Summary:

These measures ensure a stable and durable foundation as prescribed by IS 1080.

IS 1080 specifies load transfer and bearing capacity considerations:

• Load dispersion angles for foundations are:
  • Masonry and cement concrete: 1 vertical to 1 horizontal
  • Lime concrete: 1 vertical to 2/3 horizontal
• Minimum foundation edge thickness is 150 mm.
• When the bearing stratum lies deeper than the foundation base, the base may be elevated with the intervening space filled by concrete of adequate strength or compressible fill materials, extending the foundation width according to a 2 vertical to 1 horizontal dispersion ratio.
• Reinforcement becomes necessary if load eccentricity exceeds one-sixth of the footing base or if the load transfer depth is less than the prescribed dispersion angle.
• Ground beams, when reinforced and structurally integrated with walls, help distribute loads as composite units.

Summary Table – Load Dispersion:

This ensures the safe spread of loads without overloading the soil.

IS 1080 identifies types of shallow foundations:

• Pad or spread footings: isolated masonry or concrete bases transferring loads to soil.
• Strip footings: continuous footings supporting walls or beams longitudinally.

Load eccentricity above one-sixth of the footing's narrowest base dimension requires reinforcement. Footing width should meet or exceed the supported wall's width, maintaining stresses within permissible limits as per IS 1904-1985.

Summary Table:

Design flow involves determining load type, selecting foundation type, verifying bearing pressure, and assessing need for reinforcement based on eccentricity.

This systematic approach ensures foundation stability and soil safety.

Key reinforcement guidelines under IS 1080 include:

• Continuous wall footings require reinforcement especially at points of abrupt load changes or variable ground support to maintain structural integrity.
• Load eccentricity exceeding one-sixth of the footing's least base dimension mandates reinforcement to resist bending moments.
• Construction practice mandates curing the base concrete for a minimum of three days and compacting the foundation bed before concrete placement.

Reinforcement Criteria:

This ensures resistance to bending stresses and prevents structural failures.

IS 1080 does not provide explicit formulas or tables for special site conditions but emphasizes design based on thorough soil investigations rather than empirical rules. Foundation width and depth must reflect site-specific soil parameters, with general requirements outlined in IS 1904. Rounding of values should follow IS 2-1960.

Important considerations include avoiding standard widths without soil data, accounting for soil irregularities such as soft strata, high groundwater, or expansive soils, and ensuring stability against shear failure and settlement. Bearing capacity may be calculated using established equations like Terzaghi’s or Meyerhof’s, adapted for site conditions:

q_u = c' N_c + σ' N_q + 0.5 γ B N_γ

Where c' is effective cohesion, σ' effective overburden pressure, γ unit weight of soil, B footing width, and N_c, N_q, N_γ are bearing capacity factors based on soil friction angle.

For complex sites, consultation with geotechnical experts and detailed site-specific analyses are recommended.

IS 1080 emphasizes inspection and quality control measures:

• Masonry construction should commence only after the base concrete has cured for at least three days.
• Foundation beds must be compacted thoroughly through manual ramming before concrete placement.
• Inspection checklists should verify curing duration, compaction quality, and material compliance with relevant IS standards such as IS 1904 and IS 456.
• Quality control tests should include concrete compressive strength, visual inspection of compaction uniformity, moisture content, and curing conditions.

Inspection Summary:

This procedure ensures the durability and structural soundness of shallow foundations.

IS 1080 references several related standards essential for comprehensive shallow foundation design:

• IS 2809-1972 provides definitions for soil engineering terms used.
• IS 1904-1985 covers general foundation design and construction requirements.
• Separate codes exist for raft foundations (IS 2950 Part 1), ring foundations (IS 11089), and shell foundations (IS 9456).
• IS 2-1960 outlines rules for rounding off numerical data in design calculations.

This interconnected framework ensures consistent and technically rigorous foundation engineering across various soil and structural scenarios.