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

IS 1080: Scope Summary

• Applicability: Covers design and construction of shallow foundations (excluding raft, ring, shell).
• Exclusions: Raft, ring, conical, hyperbolic paraboloidal shell foundations have separate IS codes (IS 1904, IS 1081, IS 1082).
• Design Basis: Uniform technical approach replacing empirical thumb rules, suitable for soils with special problems.
• General Requirements: Transferred to IS 1904 (Foundations: General requirements).
• Dimensioning: Foundation width should be based on actual soil investigation, not fixed thumb rules.
• Excavation Dimensions: Must allow safe, efficient working (see IS 3764).
• Rounding Off: Follow IS 2-1960 for rounding test/analysis values.

Key References

This ensures standardized, technically sound design of shallow foundations.

IS 1080 - Definitions and Key Points

• Reference for Definitions:
  All terms in IS 1080 are defined as per IS 2809-1972 (Glossary of terms and symbols relating to soil engineering).

• Scope:
  IS 1080 covers shallow foundations excluding raft, ring, and shell foundations.

• Load Eccentricity Clause (4.1):
  If the load resultant deviates more than 1/6 of the least footing base dimension, reinforcement is mandatory.

• Rounding Off (Clause 0.3):
  Final calculated or observed values should be rounded according to IS 2-1960, retaining the same significant figures as specified.

Summary Table: Load Eccentricity Check

References for Further Details:

• IS 2809-1972 — Definitions in soil engineering
• IS 2-1960 — Rules for rounding numerical values
• IS 1904-1985 — General foundation requirements

flowchart LR
    A[Load on Footing] --> B{Eccentricity e}
    B -->|e ≤ b/6| C[Normal Design]
    B -->|e > b/6| D[Reinforcement Required]

This ensures uniform technical design for shallow foundations under IS 1080.

IS 1080: General Requirements for Shallow Foundations (Summary)

Scope:

• Covers design & construction of shallow foundations (excluding raft, ring, shell foundations).
• General requirements for all foundations are in IS 1904-1985.

Key Points & Specifications:

• Design Basis:
  Earlier empirical thumb rules replaced by uniform technical approach for reliable design, especially in problematic soils.

• Load Resultant Eccentricity:
  If the resultant load deviates more than 1/6 of the least base dimension, reinforcement is mandatory to resist moments.
  [ e \leq \frac{B}{6} ] where ( e ) = eccentricity, ( B ) = least base dimension

• Rounding Off Values:
  Follow IS 2-1960 for rounding numerical results, retaining significant digits as per specified values.

• Separate Codes for Other Foundations:

  • Raft foundations: IS 2950 (Part 1)-1981
  • Ring foundations: IS 11089-1984
  • Shell foundations: IS 9456-1980

Reference Table: Foundation Types & Applicable IS Codes

flowchart LR
    A[General Requirements] --> B[IS 1904-1985]
    A --> C[Shallow Foundations: IS 1080]
    C --> D[Exclude Raft, Ring, Shell]
    D --> E[Raft: IS 2950-1981]
    D --> F[Ring: IS 11089-1984]
    D --> G[Shell: IS 9456-1980]

For detailed structural design, refer to IS 1080 clauses and IS 1904 for general foundation requirements.

IS 1080: Design Considerations for Shallow Foundations

Key Points from IS 1080:

• Scope: Applies only to shallow foundations (excluding raft, ring, shell foundations).
• Design Basis: Uniform technical approach replacing empirical thumb rules.
• Load Eccentricity: If the load resultant deviates > 1/6 of the footing base's least dimension, reinforcement is necessary (Clause 4.1).
• Foundation Dimensions: Refer Clause 4.5 for sizing guidelines (based on soil bearing capacity and load).
• Material Design:
  • Cement concrete foundations: Design per IS 456-1978.
  • Masonry foundations: Design per IS 1905-1980 (Clause 4.11).
• Rounding Off: Final values rounded as per IS 2-1960.

Important Design Formulae and Concepts:

1. Eccentricity Check
   [ e \leq \frac{B}{6} ] Where:

   • ( e ) = eccentricity of load from footing centerline
   • ( B ) = least dimension of footing base

2. Allowable Bearing Pressure
   [ q_{allow} = \frac{P}{A} ] Where:

   • ( P ) = load on footing
   • ( A ) = footing area

3. Footing Size (Square/Rectangular)
   [ B = \sqrt{\frac{P}{q_{allow}}} ]

References for Further Details:

flowchart TD
    A[Load on Foundation] --> B{Is eccentricity > B/6?}
    B -- No --> C[Design footing as per IS 1080]
    B -- Yes --> D[Provide suitable reinforcement]
    C --> E[Check allowable bearing pressure]
    D --> E
    E --> F[Determine footing size

Key Construction Practices from IS 1080 (Clause 5.4)

• Masonry over concrete base:

  • Masonry should be laid only after the base concrete has cured for at least 3 days.
  • Ensures adequate strength and bonding.

• Foundation bed preparation:

  • The bed of the foundation pit/trench must be thoroughly compacted by manual ramming before laying concrete.
  • Prevents settlement and ensures uniform support.

• Design references:

  • Cement concrete foundations (plain/reinforced) per IS 456:1978.
  • Masonry foundations per IS 1905:1980.

Summary Table: Construction Practices

flowchart TD
    A[Excavate foundation pit] --> B[Compact foundation bed by manual ramming]
    B --> C[Pour concrete base]
    C --> D{Cure concrete for 3 days}
    D --> E[Lay masonry over cured concrete]

Note: For detailed mix designs, reinforcement, and workmanship, refer to IS 456 and IS 1905 as per clause 4.11.

IS 1080: Key Specifications & Guidelines for Materials (Clause 5.4)

• Foundation Concrete Curing: Base concrete must cure for minimum 3 days before masonry construction.
• Foundation Pit Preparation: The bed of the foundation pit/trench must be thoroughly compacted by manual ramming before laying concrete.
• Masonry Construction: Masonry is to be constructed over the cured base concrete ensuring proper bonding and stability.

Related IS Codes for Materials and Masonry:

• IS 1904: Design and construction of foundations (general requirements)
• IS 2809: Definitions related to masonry
• IS codes for lime concrete, lime pozzolana mixture, brickwork, and stone masonry provide detailed material specifications.

Practical Notes:

• Ensure moisture content and quality of materials (bricks, stones, mortar) comply with respective IS codes.
• Use manual ramming for soil compaction to achieve adequate bearing capacity.
• Follow curing times strictly to achieve design strength.

Summary Table (Material Preparation & Construction)

flowchart TD
    A[Excavate Foundation Pit] --> B[Compact Soil by Manual Ramming]
    B --> C[Pour Base Concrete]
    C --> D[Cure Concrete for ≥ 3 Days]
    D --> E[Start Masonry Construction]

This ensures a stable, durable masonry foundation as per IS 1080.

IS 1080 Key Points on Load Transfer & Bearing Capacity

1. Load Transfer & Bearing Spread (Clause 4.8, 4.10)

• Load dispersion angles:
  • Masonry foundations: 1 vertical : 1 horizontal
  • Cement concrete: 1 vertical : 1 horizontal
  • Lime concrete: 1 vertical : 2/3 horizontal
• Minimum foundation thickness: 150 mm at edges
• If allowable bearing capacity is deeper, foundation base can be raised and the gap filled by:
  • Concrete with compressive strength ≥ allowable bearing pressure, OR
  • Compressible fill (sand, gravel), with width extended by load dispersion ratio 2 vertical : 1 horizontal beyond foundation width on each side.

2. Reinforcement Requirement (Clause 4.1, 4.8)

• If resultant load deviates > 1/6 of least base dimension → reinforcement mandatory.
• If depth to load transfer < permissible spread angle → foundation must be reinforced.

3. Ground Beams (Clause 4.4)

• Ground beams can transmit loads and act compositely with reinforced walls.
• Must be structurally connected by reinforcement for composite action.

Load Dispersion Illustration

graph LR
    A[Load at Pier/Base] --> B[Foundation Base]
    B --> C[Ground]
    subgraph Load Spread
      direction LR
      B -->|1V:1H (Concrete)| D[Concrete Bearing Area]
      B -->|1V:2/3H (Lime Concrete)| E[Lime Concrete Bearing Area]
      B -->|1V:1H (Masonry)| F[Masonry Bearing Area]
    end

Summary Table: Load Spread Angles

Note: For compressible fill, extend foundation width by 2 vertical : 1 horizontal dispersion on each side.

IS 1080 – Foundation Types and Details: Key Points

1. Types of Foundations (Clause 3.1.1)

• Pad or Spread Footing: Isolated masonry/concrete foundation transferring load safely to soil.
• Strip Footing: Continuous foundation supporting walls or beams longitudinally.

2. Load Eccentricity (Clause 4.1)

• If load resultant deviates > 1/6 of footing’s least base dimension, provide suitable reinforcement to resist moments.

3. Footing Dimensions (Clause 4.5.1)

• Width ( B ) of wall foundation (cm) should satisfy:

  [ B \geq W ]

  Where:
  ( B ) = width at base (cm)
  ( W ) = width of supported wall (cm)

• Ensure maximum stress in concrete/masonry ≤ permissible limits per IS 1904-1985.

4. General Design Notes

• Foundations must be designed to keep soil bearing pressure within safe limits.
• Refer to IS 2950, IS 11089, and IS 9456 for raft, ring, and shell foundations respectively.

Summary Table: Foundation Types

flowchart TD
    A[Load on Structure] --> B{Type of Foundation}
    B -->|Isolated Load| C[Pad/Spread Footing]
    B -->|Continuous Load| D[Strip Footing]
    C --> E[Check Bearing Pressure]
    D --> E
    E --> F{Load Eccentricity > 1/6 base?}
    F -->|Yes| G[Provide Reinforcement]
    F -->|No| H[Normal Design]

Note: Always verify footing width and stresses using IS 1904 and soil bearing capacity for safe design.

IS 1080: Reinforcement and Structural Connections Key Points

1. Reinforcement in Continuous Wall Foundations (Clause 4.2)

   • Provide adequate reinforcement especially at:
     • Abrupt changes in load magnitude.
     • Variations in ground support.
   • Ensures structural integrity under uneven stresses.

2. Reinforcement for Load Eccentricity (Clause 4.1)

   • If load resultant deviates > 1/6 of footing’s least base dimension from centerline:
     • Provide suitable reinforcement to resist bending moments.

3. Construction Practice (Clause 5.4)

   • Base concrete must cure for at least 3 days before masonry.
   • Foundation bed should be compacted by manual ramming before concrete laying.

Typical Reinforcement Check for Eccentric Load

[ e \leq \frac{d}{6} \implies \text{No reinforcement needed} ]

Where:

• ( e ) = eccentricity of load from centerline
• ( d ) = least dimension of footing base

If ( e > \frac{d}{6} ), provide reinforcement to resist bending moments.

Summary Table: Reinforcement Trigger Conditions

flowchart LR
    A[Load on Foundation] --> B{Eccentricity \(e\)}
    B -->|≤ d/6| C[No Reinforcement Needed]
    B -->|> d/6| D[Provide Reinforcement]
    A --> E{Abrupt Load/Ground Changes}
    E -->|Yes| D
    E -->|No| C

References:

• IS 1080 Clause 4.1, 4.2, 5.4
• Good practice: cure base concrete 3 days, compact foundation bed before concrete.

IS 1080 (1985) focuses on shallow foundations excluding raft, ring, and shell types. For Special Site Conditions, the code does not provide explicit formulas or tables but emphasizes:

• Design based on actual soil investigation rather than empirical thumb rules.
• Foundation width and depth should be determined from site-specific soil parameters.
• Refer to IS 1904 (1985) for general foundation requirements.
• Use IS 2-1960 for rounding off calculated or observed values.

Key points for special site conditions:

• Avoid standard width formulas; use soil test data (bearing capacity, settlement).
• Consider soil anomalies like soft layers, high water table, or expansive soils.
• Design must ensure stability against shear failure and excessive settlement.
• Use bearing capacity equations (Terzaghi’s or Meyerhof’s) adapted to site conditions:

[ q_u = c'N_c + \sigma' N_q + 0.5 \gamma B N_\gamma ]

Where:

• (q_u) = ultimate bearing capacity
• (c') = effective cohesion
• (\sigma') = effective overburden pressure
• (\gamma) = unit weight of soil
• (B) = foundation width
• (N_c, N_q, N_\gamma) = bearing capacity factors based on soil friction angle

For complex sites, consult geotechnical experts and apply site-specific tests and analyses beyond IS 1080’s scope.

IS 1080: Inspection and Quality Control – Key Points

1. Base Concrete Curing:

   • Masonry to be constructed only after the base concrete has cured for at least 3 days (Clause 5.4).

2. Foundation Bed Preparation:

   • The bed of the foundation pit/trench must be thoroughly compacted by manual ramming before laying concrete.

3. Inspection Checklist:

   • Verify curing period of base concrete (≥ 3 days).
   • Check compaction quality of foundation bed.
   • Confirm materials comply with relevant IS codes (e.g., IS 1904 for foundations, IS 456 for concrete).

4. Quality Control Tests:

   • Concrete compressive strength tests (cube/cylinder).
   • Visual inspection for compaction and surface uniformity.
   • Check moisture content and curing conditions.

Summary Table: Inspection Points

Quality Control Flowchart

flowchart TD
    A[Start: Foundation Preparation] --> B{Is base concrete cured ≥3 days?}
    B -- No --> C[Delay masonry construction]
    B -- Yes --> D[Compact foundation bed by manual ramming]
    D --> E[Inspect compaction quality]
    E --> F{Compaction satisfactory?}
    F -- No --> D
    F -- Yes --> G[Proceed with masonry construction]
    G --> H[Conduct material & strength tests]
    H --> I[Pass QC?]
    I -- No --> J[Rework or reject]
    I -- Yes --> K[Continue construction]

This ensures structural integrity and durability in shallow foundation construction as per IS 1080 guidelines.

IS 1080: References to Related Standards for Shallow Foundations

• Key Reference Standard:

  • IS 2809-1972 — Definitions of terms used in IS 1080.

• Related IS Codes for Foundations:

  • IS 1904-1985 — General requirements for foundation design and construction (3rd revision).
  • Code of Practice for Raft Foundations (Part 1: Design, 2nd revision).
  • Code of Practice for Ring Foundations.
  • Code of Practice for Shell Foundations (Conical and Hyperbolic Paraboloidal types).

• Rounding Off Values:

  • Follow IS 2-1960 for rounding off results of tests or calculations.
  • Maintain the same number of significant digits as the specified values.

Summary

IS 1080 focuses on shallow foundations (excluding raft, ring, and shell types) and directs users to specialized IS codes for other foundation types and general requirements. It emphasizes uniform design approaches based on technical considerations rather than empirical thumb rules.

flowchart LR
    A[IS 1080: Shallow Foundations] --> B[IS 2809-1972: Definitions]
    A --> C[IS 1904-1985: General Requirements]
    A --> D[Raft Foundations Code]
    A --> E[Ring Foundations Code]
    A --> F[Shell Foundations Code]
    A --> G[IS 2-1960: Rounding Off Rules]

This ensures consistent, technically sound foundation design across various soil conditions.