Code of practice for design and construction of foundations in soils: General requirements

IS 1904: Scope & Key Specifications Summary

IS 1904 covers the design and construction of foundations in soils, emphasizing safe bearing capacity and permissible settlements.

1. Safe Bearing Capacity

• Calculated as per IS 6403-1981 for shallow foundations.
• For deep foundations, refer to IS code for deep foundations under symmetrical static vertical loading.
• Use soil test data for accurate design; otherwise, local empirical values may be used for preliminary design.

2. Permissible Differential Settlements & Angular Distortion

(Table 1 from Clause 16.3.4)

• L = length of deflected part or distance between columns (mm)
• For Raft foundations, permissible settlements and angular distortions are higher (e.g., max settlement up to 125 mm for multistoreyed buildings).

3. Rounding Off Results

• Final values from tests or calculations must be rounded as per IS 2-1960.
• Maintain the same number of significant figures as specified in the standard.

Visualization: Permissible Settlement Concept

graph LR
A[Structure Type] --> B[Isolated Foundation]
A --> C[Raft Foundation]
B --> D[Max Settlement & Diff. Settlement Limits]
C --> E[Higher Settlement Limits]
D --> F[Angular Distortion Limits (Tilt)]
E --> F

References:

• IS 1904: 1986
• IS 6403: 1981
• IS 2: 1960 (Rounding rules)

This scope

IS 1904: Types of Foundations - Key Specifications & References

1. Foundation Types Covered

• Shallow Foundations
• Deep Foundations
• Special Foundations (Machine, Tower, Combined)

2. Machine Foundations (IS 2974 Series)

• Reciprocating type: IS 2974 (Part 1) - 1982
• Impact type (Hammer): IS 2974 (Part 2) - 1980
• Rotary type (medium & high frequency): IS 2974 (Part 3) - 1975
• Rotary type (low frequency): IS 2974 (Part 4) - 1975
• Other impact types: IS 2974 (Part 5) - 1987

3. Tower Foundations

• Transmission line towers & poles: IS 4091 - 1979
• Radar, microwave, TV towers: IS 11233 - 1985

4. Deep Foundations (IS 2911 Series)

• Pile Foundations:

  • Driven cast in-situ: IS 2911 (Part 1/Sec 1) - 1979
  • Bored cast in-situ: IS 2911 (Part 1/Sec 2) - 1979
  • Driven precast: IS 2911 (Part 1/Sec 3) - 1979
  • Bored precast: IS 2911 (Part 1/Sec 4) - 1984
  • Timber piles: IS 2911 (Part 2) - 1980
  • Under-reamed piles: IS 2911 (Part 3) - 1980

• Caissons: IS 9527 (Part 1) - 1981

• Diaphragm walls: IS 9556 - 1980

• Well foundations: Refer IS 1904 Clause 3.2.1(d)

• Combined foundations: Combination of above types

5. Basic Formula for Bearing Capacity (Shallow Foundations)

[ q_{ult} = c N_c + \sigma' N_q + 0.5 \gamma B N_\gamma ]

Where:

IS 1904 - Site Investigation: Key Points & Specifications

1. Site Investigation Principles (Clause 5.2)

• Follow IS 1892-1979 for site exploration methods.
• Collect preliminary info: groundwater level, old drains, pits, wells, previous foundations, presence of sulphates or harmful compounds.
• Explore all strata affected by structural loads; determine soil type, thickness, sequence, and dip.

2. Safe Bearing Capacity (Clause 18.1)

• Calculate safe bearing capacity as per IS 6403-1981 for shallow foundations.
• For deep foundations, refer to relevant IS codes for deep foundation design.

3. Permissible Differential Settlement & Angular Distortion (Table 1, Clause 16.3.4)

L = length of deflected wall or distance between columns

4. Common Site Exploration Methods (Clause 6.1)

• Soil borings
• Test pits
• Standard Penetration Test (SPT)
• Plate load test
• Groundwater observation

Summary Diagram: Site Investigation Process

flowchart TD
    A[Preliminary Data Collection] --> B[Site Reconnaissance]
    B --> C[Subsurface Exploration]
    C --> D[Sampling & Testing]
    D --> E[Soil Profile & Groundwater Assessment]
    E --> F[Safe Bearing Capacity Calculation]
    F --> G[Foundation Design]

References:

• IS 1892-1979 (Site Exploration

IS 1904 - Soil Characteristics and Effects: Key Points

1. Safe Bearing Capacity (Clause 18.1)

• Calculated as per IS 6403:1981 for shallow foundations.
• For deep foundations, refer to the respective IS code part on deep foundations under symmetrical static vertical loading.
• Safe bearing capacity should be based on soil test data; local values can be used for preliminary design.

2. Permissible Differential Settlements and Angular Distortion (Clause 16.3.4, Table 1)

• L = length of deflected wall/raft or center-to-center distance between columns (mm).
• H = height of wall from foundation footing (mm).
• For intermediate L/H ratios, interpolate values.

Notes:

• Values are guidelines; final permissible settlements and tilts depend on design requirements.
• Raft foundations have higher permissible settlements (up to 125 mm) and angular distortions (up to 1/300).

Summary Formula for Differential Settlement Limit:

[ \text{Differential Settlement} \leq k \times L ] where (k) varies by structure type (e.g., 0.003 for steel structures).

flowchart TD
    A[Soil Characteristics] --> B[Safe Bearing Capacity]
    B --> C[IS 6403:1981 for Shallow Foundations]
    B --> D[Deep Foundations: Relevant IS Code]
    A --> E[Permissible Settlements]
    E -->

IS 1904: Foundation Depth and Protection – Key Points

1. Foundation Depth Criteria (Clause 7.1, 7.2, 7.4)

• Minimum depth: At least 50 cm below natural ground level.
• Soil considerations:
  • Below the zone of seasonal shrinkage/swelling in clayey soils.
  • Below the frost-affected zone in fine sands and silts.
  • Below the maximum scour depth in water-affected areas.
  • Avoid zones weakened by root holes, cavities, or burrowing animals.
• On rock or weather-resistant ground, remove topsoil and prepare surface (stepped if needed) for good bearing.

2. Protection Measures

• Foundations must extend below any scour or erosion depth.
• Consider heat transmission effects and ground movements.
• Maintain minimum horizontal spacing between new and existing footings equal to the width of the wider footing to minimize damage.

3. Recommended Checks

• Perform bearing capacity and settlement analysis when new foundations are close to existing ones.

Summary Table: Foundation Depth Factors

flowchart TD
    A[Foundation Depth] --> B[Minimum 50 cm below ground]
    A --> C[Below shrink/swell zone (Clay)]
    A --> D[Below frost zone (Sand/Silt)]
    A --> E[Below scour depth]
    A --> F[Avoid root holes/cavities]
    A --> G[On rock: remove topsoil & prepare]
    H[Protection] --> I[Spacing ≥ width of wider footing]
    H --> J[Consider heat & ground movement]
    H --> K[Analyze bearing capacity & settlement]

This ensures safe, durable foundations per IS 1904 guidelines.

Effects of Soil Volume Changes (IS 1904 Highlights)

Key Points:

• Clay soils shrink during hot, dry weather causing cracks and swell in wet weather closing cracks (Clause 9.1).
• Trees and artificial heat sources increase drying and volume changes; maintain minimum 8 m distance from large trees to foundations.
• Leakage from underground pipes can cause swelling or shrinkage; prevent leakages (Clause 9.1).
• Seasonal swelling/shrinkage and vegetation effects must be considered in foundation design (Clause 5.3.3).

Permissible Differential Settlements and Angular Distortion (Clause 16.3.4)

• L = length of deflected part or column spacing (mm)
• Angular distortion = differential settlement / length

Settlement Causes to Consider (Clause 16.1)

• Elastic compression
• Consolidation (primary and secondary)
• Groundwater fluctuations
• Seasonal swelling/shrinkage of clays
• Ground movement (erosion, creep, landslides)
• Vegetation effects

Summary Diagram: Soil Volume Change Effects on Foundations

flowchart TD
    A[Hot, Dry Weather] --> B[Clay Shrinkage & Cracks]
    C[Wet Weather] --> D[Clay Swelling & Crack Closure]
    E[Trees & Heat Sources] --> B
    E --> D
    F[Leakage from Pipes] --> D
    F --> B
    B --> G[Foundation Differential Settlement]
    D

IS 1904 Key Points on Mass Movements and Unstable Areas

• Clause 5.3 & 10.0: Mass movements include mining subsidence, landslips, unstable slopes, and clay creep. These must be carefully investigated during site assessment. Expert geological/hydrological advice is recommended.

• Slope Stability in Footings:

  • Footing slope joining line should not be steeper than 1 vertical : 2 horizontal (see Fig. 1).
  • For footings on granular or clayey soil, ensure proper design to prevent sliding (see Fig. 2).

• Settlement Limits (Clause 16.4.1):

  • Settlement calculations per IS 8009 (Part 2)-1980.
  • Permissible total settlement, differential settlement, and tilt (angular distortion) are specified in related IS codes.

• Design Checks:

  • Stability against overturning and sliding must be ensured (Clause 17).
  • Avoid foundation surfaces intersecting unstable slip surfaces.

Typical Formula for Slope Stability Factor of Safety (FOS):

[ FOS = \frac{\text{Resisting forces or moments}}{\text{Driving forces or moments}} \geq 1.5 \text{ (recommended)} ]

Recommended Actions:

• Conduct detailed site investigation focusing on geological and hydrogeological conditions.
• Avoid foundation placement on or near slip surfaces.
• Use retaining structures or ground improvement if unstable slopes are unavoidable.

flowchart TD
    A[Site Investigation] --> B{Mass Movement Risk?}
    B -- Yes --> C[Geotechnical & Hydrogeological Study]
    C --> D[Design Foundations with Slope Stability]
    D --> E[Check Settlement per IS 8009]
    E --> F[Check Overturning & Sliding Stability]
    B -- No --> G[Proceed with Standard Design]

Summary: IS 1904 emphasizes thorough site investigation for mass movements, adherence to slope gradients (1:2), settlement limits per IS 8009, and stability checks to ensure safe foundation design in unstable areas.

IS 1904 Guidelines on Layered Soil Conditions:

• Bearing Capacity:
  Safe bearing capacity for shallow foundations should be calculated as per IS 6403-1981. For layered soils, detailed soil investigation is crucial to identify hard and soft layers, as soft layers may change properties under load (Clause 13.1).

• Settlement Criteria (Clause 16.3 & Table 1):
  Permissible settlements and differential settlements depend on soil type and structure type. Key values for isolated foundations on layered soils (sand, hard clay, plastic clay) are:

• Notation:

  • L: Length of deflected wall/raft or spacing between columns (mm)
  • H: Height of wall from footing (mm)

• Important:

  • Layered soils require careful investigation for soft layers (Clause 13.1).
  • Settlement values should be used as guidelines; final design must consider soil test data and structural requirements.

Summary Diagram of Layered Soil Impact on Foundation

flowchart TD
    A[Layered Soil Profile] --> B[Hard Layer]
    A --> C[Soft Layer]
    C --> D[Compression & Settlement]
    B --> E[High Bearing Capacity]
    D --> F[Increased Settlement Risk]
    E --> G[Stable Foundation Support]
    F --> H[Foundation Design Adjustments]
    G --> I[Safe Bearing Capacity per IS 6403]
    H --> I

References:

• IS 1904: Clauses 13.1, 16.3, 18.1
• IS 6403-1981 for bearing capacity

IS 1904 Key Guidelines for Spacing Between Existing and New Foundations

Minimum Horizontal Spacing (Clause 14.1)

• Spacing ≥ Width of the wider footing
• This minimizes damage due to interaction effects.
• If new foundation is deeper and closer, risk of damage increases.

Additional Recommendations

• Perform bearing capacity and settlement analysis for adjacent foundations.
• Consider soil stratification and groundwater changes (Clause 13.1).
• Avoid shear failure zones near cuttings/excavations (Clause 10.2.5).
• On slopes, use stepped foundations with laps ≥ foundation thickness or twice step height (Clause 10.2.4).

Summary Table: Spacing Between Foundations

Conceptual Diagram: Spacing Between Foundations

graph LR
A[Existing Foundation] -- Spacing ≥ Width --> B[New Foundation]
B --> C{Depth of New Foundation}
C -->|Deeper| D[Higher Risk - Perform Analysis]
C -->|Same/Shallower| E[Standard Spacing Adequate]

Always conduct detailed soil and structural analysis to confirm spacing adequacy and prevent settlement or shear failures.

IS 1904: Loads on Foundations - Key Points

Types of Loads (Clause 15.1)

• Vertical loads: Upwards or downwards (dead + live loads)
• Horizontal/lateral loads: Wind, seismic forces
• Moments or couples

Load Combinations for Design (Clause 15.1.3)

• (a) Dead load + Live load
• (b) Dead load + Live load + Wind load or Seismic load

Safety Factors Against Overturning (Clause 17.2)

Bearing Capacity & Settlement (Clause 16.3.1)

• For coarse-grained soils, settlements are estimated for load combination 15.1.3(b).
• Settlements occur rapidly; design must ensure allowable settlement limits.

Typical Bearing Capacity Formula (Terzaghi’s for Shallow Foundations):

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

• (q_u): Ultimate bearing capacity
• (c'): Cohesion
• (\sigma'): Effective overburden pressure
• (\gamma): Unit weight of soil
• (B): Width of foundation
• (N_c, N_q, N_\gamma): Bearing capacity factors (from soil friction angle)

flowchart TD
    A[Loads on Foundation]
    A --> B[Vertical Loads (Dead + Live)]
    A --> C[Horizontal Loads (Wind/Seismic)]
    A --> D[Moments/Couples]
    B --> E[Load Combinations]
    E --> F[Dead + Live]
    E --> G[Dead + Live + Wind/Seismic]
    A --> H[Design Checks]
    H --> I[Overturning FoS]
    H --> J[Bearing Capacity & Settlement]

Summary: Design foundations considering vertical, lateral, and moment loads with appropriate load combinations. Use FoS ≥ 2 for overturning without wind/seismic, and ≥1.5 with them. Estimate settlements especially for coarse

IS 1904: Settlement Considerations - Key Points

1. Types of Settlement (Clause 16.1)

• Elastic compression of foundation and soil.
• Consolidation settlement including secondary compression.
• Effects from groundwater fluctuations.
• Seasonal swelling/shrinkage of expansive clays.
• Ground movements (erosion, landslides).
• External factors: adjacent excavation, mining, vegetation effects.

2. Permissible Settlements (Clause 16.3.4, Table 1)

3. Settlement Analysis Criteria

• Shallow Foundations (Clause 16.3): Settlement should be within permissible limits; consider soil compressibility and load distribution.
• Deep Foundations (Clause 16.4): Analyze pile settlement considering pile-soil interaction and group effects.

4. Important Notes

• Settlement in clays and silts may continue for years (consolidation).
• Settlement in sands and gravels is mostly immediate.
• Account for slow consolidation and secondary compression in design.

Summary Formula for Consolidation Settlement (One-Dimensional):

[ S_c = \frac{H}{1 + e_0} \cdot C_c \cdot \log \frac{\sigma'_0 + \Delta \sigma'}{\sigma'_0} ]

Where:

• ( S_c ) = consolidation settlement
• ( H ) = thickness of compressible layer
• ( e_0 ) = initial void ratio
• ( C_c ) = compression index
• ( \sigma'_0 ) = initial effective stress
• ( \Delta \sigma' ) = increase in effective stress due to foundation load

flowchart TD
    A[Foundation Load Applied] --> B[Elastic Compression]
    A --> C[Consolidation Settlement]
    C --> D[Primary Consolidation]
    C --> E[Secondary Compression]
    A --> F[Groundwater Fluctuation Effects]
    A --> G[Seasonal Swelling/Shr

IS 1904: Stability Against Sliding and Overturning

1. Factor of Safety (FOS) Requirements

2. Stability Against Sliding

• FOS against sliding = (Resisting forces) / (Driving forces)
• Resisting forces mainly from soil friction under the base and any anchorage.
• For slippery soils (low friction), improve sliding resistance by:
  • Anchor-type cut-off walls
  • Piles or inclined base underside

3. Stability Against Overturning

• FOS against overturning = (Restoring moment due to vertical loads) / (Overturning moment due to lateral loads)
• Ensure moments about the toe of the foundation satisfy the FOS above.

4. Reference for Settlement Calculation

• Settlement per IS 8009 (Part 2)-1980.

graph LR
A[Loads on Foundation] --> B{Check Stability}
B --> C[Sliding Stability]
B --> D[Overturning Stability]
C --> E[Calculate FOS Sliding]
D --> F[Calculate FOS Overturning]
E --> G{FOS ≥ 1.5 or 1.75?}
F --> H{FOS ≥ 1.5 or 2.0?}
G --> I[Safe / Modify Design]
H --> I

Summary: Always verify sliding and overturning with the specified FOS values. Use soil friction and anchorage methods to enhance sliding resistance. Calculate moments for overturning checks. Settlement limits per IS 8009.

IS 1904 - Bearing Capacity: Key Points

1. Safe Bearing Capacity Calculation

   • For shallow foundations, use IS 6403:1981.
   • For deep foundations, refer to IS code for deep foundations under symmetrical static vertical loading.

2. Permissible Differential Settlements & Angular Distortions (Clause 16.3.4, Table 1)

   • Depends on soil type (Sand, Hard Clay, Plastic Clay) and structure type (Steel, RCC, Multistoreyed).
   • Maximum settlement and differential settlement limits are given in mm or as a fraction of length (L) or height (H).
   • Angular distortion (tilt) limits typically range from 1/300 to 1/500 for isolated foundations.

3. Factor of Safety Against Overturning (Clause 17.2)

   • Minimum 1.5 when wind/seismic loads included.
   • Minimum 2.0 with dead, live, and earth pressures only.

4. Load Combinations and Bearing Capacity (Clause 15.1.7)

   • If wind/seismic load > 25% of dead+live load, allowable bearing pressure can increase by 25%.
   • Seismic increases per IS 1893:1984; liquefaction and settlement checks needed for non-cohesive soils.

Typical Bearing Capacity Formula (from IS 6403):

[ q_{safe} = \frac{q_u}{FS} ]

Where:

• (q_u) = Ultimate bearing capacity (from soil tests or Terzaghi’s formula)
• (FS) = Factor of safety (usually 3)

Terzaghi’s Ultimate Bearing Capacity for Shallow Foundations:

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

• (c) = soil cohesion
• (q) = overburden pressure at foundation base
• (B) = width of footing
• (\gamma) = unit weight of soil
• (N_c, N_q, N_\gamma) = bearing capacity factors (from soil friction angle (\phi))

Permissible Differential Settlement Example (Isolated Foundations in

IS 1904: Setting Out and Site Preparation - Key Points

1. Site Leveling & Layout (Clause 19.4 & 19.5):

• Site must be leveled before foundation layout.
• For sloping terrain, set out dimensions in one or more horizontal planes.
• Use steel tapes for layout; use theodolites for angles if length > 16 m or complex structures.
• Check rectangular layouts by verifying diagonals.
• Establish permanent rows of pillars (≥ 25 cm wide, bedded deep) beyond building periphery for wall alignment.
• Pillars’ tops should be at the same level (preferably plinth/floor level).
• Mark center lines of walls on pillar tops and datum lines parallel to external walls for accuracy checks.

2. Accuracy Checks:

• Use diagonal measurements to verify rectangular layouts.
• Datum lines on pillars serve as permanent references.

3. Pillar Specifications:

• Minimum width: 25 cm
• Depth: Sufficient to avoid disturbance
• Height: At plinth or floor level for consistent reference

Summary Table: Setting Out Tools & Methods

flowchart TD
    A[Site Leveling] --> B[Mark Foundation Layout]
    B --> C{Length > 16m or Complex?}
    C -- Yes --> D[Use Theodolite for Angles]
    C -- No --> E[Use Steel Tape & Side Measurements]
    E --> F[Check Diagonals]
    D --> F
    F --> G[Set Permanent Pillars]
    G --> H[Mark Center Lines & Datum Lines]
    H --> I[Proceed with Excavation & Construction]

For detailed permissible settlement and bearing capacity, refer to IS 1904 Table 1 and IS 6403 for safe bearing capacity calculations.

Protection of Excavation - IS 1904 Key Points & Specifications

Clause 20.4: Refilling and Compaction

• Refilling must avoid disturbing the foundation.
• Compact in layers ≤ 15 cm thick.
• Use minimum water for proper compaction.

Clause 20.1: Protection During Construction

• Timbering and dewatering as per IS 3764-1966.
• Ensure excavation stability and safety.

Clause 20.3: Excavation in Clay/Soils Susceptible to Atmospheric Effects

• Concreting immediately after excavation is preferred.
• Alternatively:
  • Provide 8 cm thick cement concrete layer (mix 1:5:10) at excavation bottom.
  • Remove last 10 cm of excavation just before concreting for a dry, hard bottom.

Summary Table: Excavation Protection Measures

Practical Notes:

• Proper compaction avoids settlement and foundation damage.
• Immediate concreting prevents soil weakening due to weather.
• Use timbering and dewatering to maintain excavation safety and stability.

flowchart TD
    A[Excavation] --> B{Soil Type?}
    B -->|Clay/Weak Soil| C[Concreting Immediately]
    B -->|Other Soils| D[Timbering & Dewatering (IS 3764)]
    C --> E[8 cm Concrete Layer (1:5:10)]
    E --> F[Foundation Concrete]
    D --> F
    F --> G[Refilling in 15 cm Layers]
    G --> H[Compaction with Minimum Water]

This ensures excavation protection and foundation integrity as per IS 1904.