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Foundations are one of the most critical components of any building or civil engineering structure. They transfer structural loads safely to the supporting ground and help prevent excessive settlement, sliding, overturning, bearing failure, and other forms of instability. A properly designed foundation is therefore dependent not only on the structural loads but also on the properties and behavior of the supporting soil.

IS 1904:1986 – Code of Practice for Structural Safety of Buildings: Foundations provides general structural requirements and recommendations related to foundations. The provisions cover shallow foundations, deep foundations, special foundations, site investigation, foundation depth, foundations at different levels, seasonal effects, preliminary construction work, excavation protection, and related construction considerations.

This article summarizes the important foundation-related provisions from the standard in a practical format for civil and structural engineers, students, site engineers, and designers.

1. Scope of IS 1904:1986

IS 1904:1986 covers the general structural requirements for foundations. It deals with different types of foundations, including:

Shallow foundations
Deep foundations
Foundations for special structures

The standard emphasizes that foundation design cannot be separated from proper investigation of the supporting ground.

A foundation must be designed considering:

Nature and properties of soil
Groundwater conditions
Bearing capacity
Settlement characteristics
Seasonal changes
Ground movements
Nearby excavations
Scour
Existing structures
Drainage conditions
Chemical conditions of soil and groundwater
Construction methodology

Therefore, foundation design should be considered as an interaction between the structure, foundation and ground.

2. Types of Foundations

IS 1904 broadly considers three categories of foundations.

2.1 Shallow Foundations

Shallow foundations are foundations where the load is primarily transferred to the bearing strata through the shear resistance of the supporting soil.

The frictional resistance of soil above the bearing stratum is generally not considered for this purpose.

These foundations are normally constructed to relatively shallow depths, generally around 3 m, depending on site conditions and structural requirements.

Common types include:

a) Spread or Pad Foundations

Spread foundations are generally used below individual columns or isolated supports.

The foundation spreads the concentrated structural load over a larger area so that the pressure transmitted to the soil remains within the allowable bearing capacity.

Typical examples include:

Isolated column footing
Square footing
Rectangular footing
Combined footing, where applicable

b) Strip Foundations

Strip foundations are commonly used below walls or closely spaced columns.

The foundation extends continuously along the length of the wall and distributes the load over a wider area of soil.

c) Raft Foundations

A raft foundation consists of a large reinforced concrete slab supporting several columns and/or walls.

Raft foundations are often considered when:

Soil bearing capacity is relatively low
Individual footings would occupy a large percentage of the building area
Differential settlement needs to be controlled
Columns are closely spaced

d) Ring and Shell Foundations

Ring and shell foundations are specialized foundation systems generally used for structures having circular or shell-type geometry.

Examples may include certain:

Tanks
Silos
Chimneys
Circular industrial structures

3. Deep Foundations

Deep foundations are used when adequate bearing strata are not available at shallow depth or when the structural loading and ground conditions require load transfer to deeper strata.

Deep foundations may include:

Piles
Caissons
Diaphragm walls
Combinations of different foundation systems

Load transfer from a deep foundation can occur through:

Skin friction
End bearing
Combination of skin friction and end bearing

For example, a pile may transfer part of its load through friction developed along its shaft and another portion through resistance at its tip.

The selection between shallow and deep foundation systems should be based on geotechnical investigation, structural loading, settlement requirements, construction feasibility and economic considerations.

4. Foundations for Special Structures

Certain structures require foundation systems that cannot be designed using only conventional static-load considerations.

Examples include structures supporting:

Heavy machinery
Vibrating equipment
Industrial installations
Dynamic loads
Impact loads

For these structures, the design should consider the:

Magnitude of dynamic loads
Frequency of vibration
Soil dynamic properties
Foundation stiffness
Natural frequency
Damping
Possible resonance
Interaction between soil and foundation

Thus, foundations for special structures may require specialized analysis and detailing.

5. Importance of Ground Investigation

One of the most important aspects of foundation design is understanding the ground on which the structure will be constructed.

The natural geological deposits extending down to the bedrock should be examined as necessary.

A structural engineer should not assume that the soil profile is uniform across the entire site.

Different soil layers may have significantly different:

Strength
Compressibility
Permeability
Density
Bearing capacity
Settlement characteristics

For important structures, investigation should cover all strata that are likely to be significantly affected by the structural loading.

6. Site Investigation Before Foundation Design

Site investigation is an essential prerequisite for civil engineering construction.

The main purpose of site investigation is to determine:

Suitability of the site
Soil profile
Groundwater conditions
Bearing characteristics
Settlement characteristics
Potential construction problems
Possible environmental effects
Risks to existing structures

The investigation should follow the principles given in the relevant standard for site investigation.

Before detailed exploration begins, available information about the site should be collected.

This may include:

Previous construction records
Geological information
Topographical information
Existing foundation information
Groundwater data
Drainage information
History of the site
Previous land use

7. Groundwater Investigation

Groundwater is an important consideration in foundation engineering.

During site investigation, attention should be given to:

Groundwater level
Underground watercourses
Seasonal groundwater fluctuations
Water-bearing strata
Water pressure
Possible seepage into excavations

A groundwater level observed immediately after drilling may not represent the actual equilibrium groundwater level.

In soils with low permeability, such as certain clayey soils, groundwater may require considerable time to reach equilibrium in a borehole.

Therefore, groundwater observations should be made after allowing sufficient time for stabilization.

This becomes particularly important when:

Deep excavations are planned
Basement construction is involved
Dewatering is required
Water-bearing layers exist
Foundation construction is below groundwater level

8. Existing Drains, Wells and Foundations

Site investigation should identify existing underground features such as:

Old drains
Wells
Pits
Underground watercourses
Existing foundations
Buried services
Filled-up areas

These features can influence foundation stability.

For example, an old underground drain may create a zone of weak or disturbed soil. Similarly, an abandoned foundation or filled pit can create differential ground conditions.

Ignoring these features may result in:

Differential settlement
Local bearing failure
Water ingress
Construction difficulties

9. Ground Movements and Slope Stability

Ground movements may occur independently of the structural load.

Some important causes include:

Mining subsidence
Landslides
Unstable slopes
Clay creep
Excavation
Groundwater changes

These conditions must be identified during site investigation.

Where geological or hydrological conditions are complex, specialist advice may be necessary.

Structures should preferably avoid areas that are clearly affected by unstable slopes or land movement unless appropriate stabilization measures are provided.

10. Mining Subsidence

Mining areas can be subject to ground subsidence.

The magnitude and distribution of subsidence may vary depending on:

Depth of mining
Type of mining
Extent of underground workings
Ground conditions
Future mining activity

Where future subsidence is possible, the structural system and foundation should be designed to accommodate expected ground movements.

Depending on the situation, the structure may need to be:

Strong enough to resist differential movement, or
Flexible enough to accommodate movement

Long continuous buildings may be undesirable in areas with significant anticipated differential settlement.

Large structures may therefore be divided into independent sections with suitable separation and individual foundation systems.

11. Excavations Near Existing Foundations

Excavations, cuttings or sloping ground near foundations can reduce the stability of the supporting soil.

An excavation may remove part of the soil that previously provided lateral support.

This can increase the possibility of:

Shear failure
Sliding
Foundation movement
Settlement
Rotation

Therefore, excavation near existing foundations must be carefully assessed.

Where required, temporary or permanent retaining systems should be provided.

12. Foundation on Sloping Ground

Sloping sites require special attention because the upper layers of soil may have a tendency to move downhill.

The stability depends on factors such as:

Soil type
Slope angle
Groundwater conditions
Rainfall
Climate
Vegetation
Existing drainage

Signs that may indicate ground creep or instability include:

Uneven ground surface
Curved tree trunks
Tilted fence posts
Tilted boundary walls
Cracks in the ground
Previous landslide scars

These indicators should not be ignored during site investigation.

13. Shrinkage and Swelling of Clay Soil

Certain clay soils undergo significant volume changes due to changes in moisture content.

During dry weather:

Loss of moisture → Shrinkage → Cracking

During wet weather:

Increase in moisture → Swelling → Expansion

This behavior can cause foundation movement and differential settlement.

Black cotton soil is a well-known example of expansive soil in India.

The foundation should either be placed below the zone affected by seasonal moisture variation or designed to accommodate the expected movement.

Possible approaches include:

Increasing foundation depth
Providing suitable foundation systems
Using rigid foundations
Using flexible construction where appropriate
Providing measures to control moisture variation
Considering swelling pressure in design

14. Effect of Trees on Foundations

Trees can significantly influence moisture conditions in clayey soil.

Large, water-seeking trees can extract moisture from the surrounding soil and increase drying.

This can cause:

Tree roots → Moisture extraction → Soil drying → Clay shrinkage → Foundation movement

The influence depends on:

Tree size
Root system
Soil type
Distance from foundation
Climate
Seasonal conditions

The standard indicates that large trees should not be planted too close to building foundations and gives a general minimum distance of approximately 8 m.

Existing trees should therefore be considered during foundation planning.

15. Artificial Sources of Heat

Heat generated by installations can also dry clay soils.

Examples include:

Furnaces
Boilers
Kilns
Refrigeration installations
Underground cables
Industrial equipment

Continuous heat can cause substantial drying of soil beneath or around a foundation.

This may result in:

Soil shrinkage
Ground movement
Foundation settlement
Differential movement

Where significant heat transmission is expected, suitable insulation or other protective measures should be considered.

16. Leakage From Water Mains and Sewers

Water leakage can have the opposite effect.

Leakage from:

Water mains
Underground pipelines
Sewers
Drainage systems

may increase the moisture content of surrounding clay.

This can result in:

Swelling
Softening
Loss of soil strength
Differential settlement

Therefore, underground services near foundations should be properly designed, installed and maintained.

17. Effect of Soluble Salts on Foundations

Certain soils and groundwater contain soluble salts that can attack concrete and buried metals.

Sulphates are particularly important because they can react with components of hardened cement paste and cause expansion and deterioration.

Potential consequences include:

Cracking
Expansion
Loss of durability
Concrete deterioration
Reinforcement corrosion

Chemical analysis of soil and groundwater should be carried out where aggressive conditions are suspected.

18. Protection Against Sulphate Attack

Where sulphate exposure is significant, appropriate protective measures should be adopted.

Possible measures include:

18.1 Dense Concrete

Dense cement concrete of suitable grade and low permeability can reduce the penetration of aggressive water.

The supplied provisions refer to M20 concrete or richer as one possible measure under the conditions of the standard.

However, current project specifications and applicable durability provisions should always be checked before selecting the concrete grade.

18.2 Suitable Cement

Cement with improved resistance to sulphate attack may be considered where appropriate.

Examples mentioned in the supplied material include:

Portland pozzolana cement
Sulphate-resistant cement
Other specialized cementitious materials where suitable

18.3 Protective Coatings

A suitable bituminous coating may be applied to exposed foundation surfaces where specified.

18.4 Protective Concrete Layer

A layer of suitable sulphate-resistant cement concrete may be provided below the foundation concrete and protected as required.

The exact protection should be based on the chemical analysis, exposure condition and applicable durability requirements.

19. Methods of Site Exploration

Common methods of site exploration include:

19.1 Trial Pits

Trial pits provide direct visual access to near-surface soil strata.

They are useful for:

Shallow foundations
Identifying soil layers
Examining existing foundations
Inspecting near-surface conditions

19.2 Boring

Boring is commonly used to investigate deeper soil strata.

It allows:

Soil sampling
Groundwater observations
In-situ testing
Identification of deeper bearing layers

19.3 Headings

Headings may be used where access to underground material is required under suitable site conditions.

The appropriate exploration method depends on:

Structure importance
Foundation depth
Soil conditions
Site accessibility
Geological conditions

20. Depth of Foundation

The depth of foundation is not selected merely based on structural requirements.

It depends on several factors.

Important considerations include:

Adequate allowable bearing capacity
Seasonal shrinkage and swelling
Frost effects where relevant
Scour
Ground movement
Nearby excavations
Root holes and cavities
Rainwater erosion
Groundwater
Adjacent structures

21. Minimum Foundation Depth

One of the commonly referenced requirements of IS 1904:1986 is:

Foundations should generally extend to a depth of at least 50 cm below natural ground level.

Therefore:

Minimum foundation depth = 500 mm below natural ground level

However, this should not be interpreted as meaning that every foundation can safely be constructed at exactly 500 mm depth.

The actual foundation depth may need to be considerably greater depending on:

Soil bearing capacity
Settlement
Scour
Expansive soil
Groundwater
Adjacent conditions
Structural loading

On sound rock or other suitable weather-resistant natural ground, removal of topsoil and proper preparation of the bearing surface may be sufficient depending on the design.

22. Foundation Near Excavations, Ponds and Watercourses

Foundations located near:

Excavations
Ditches
Ponds
Watercourses
Filled ground
Other potentially unstable conditions

may be affected by loss of supporting soil.

In such cases, the foundation should either:

Be carried sufficiently below the zone of detrimental influence, or
Be protected by retaining walls or similar structural measures.

This is particularly important where a nearby excavation can intersect the soil mass supporting the foundation.

23. Protection Against Root Holes and Animal Burrows

Foundations should be located below zones that may be weakened by:

Tree roots
Root holes
Animal burrows
Cavities
Loose disturbed soil

Rainwater can also erode soil below footings if drainage is not properly controlled.

Therefore, the foundation should have adequate embedment and the surrounding ground should be protected against erosion.

24. Foundations at Different Levels

Buildings sometimes have foundations at different elevations.

This may occur because of:

Sloping ground
Basement construction
Different structural levels
Adjacent buildings
Different footing sizes

Special precautions are necessary to prevent the upper footing from losing support.

A conceptual way of understanding the requirement is that the soil supporting the higher footing should remain within a safe bearing zone and should not be undermined by the lower excavation.

25. 30-Degree Rule for Foundations

Where the ground surface slopes near a footing, the sloping surface should not intersect the assumed frustum of bearing material beneath the footing.

For soil, the relevant bearing zone is considered using sides inclined at approximately:

30° to the horizontal

For rock, the required horizontal clearance is different.

The purpose of this requirement is to ensure that the soil mass contributing to foundation support is not compromised by the nearby slope.

26. 1V:2H Rule Between Adjacent Footings

Where adjacent foundations are at different levels, the line connecting the relevant edges of the foundations should generally not be steeper than:

1 Vertical : 2 Horizontal

or:

1V : 2H

This requirement is particularly important when checking stepped foundations or adjacent footings.

The exact edge-to-edge points considered depend on whether the soil is granular or clayey, as specified in the relevant provision.

27. When Level-Difference Restrictions May Not Apply

The limitations relating to different foundation levels may not apply in certain situations where adequate protection is provided.

For example:

Adequate lateral support

A retaining wall or similar system can provide lateral support to the soil supporting the higher foundation.

Adequate factor of safety

The requirement may also be treated differently where the foundation soil has an adequate factor of safety against shearing, as specified by the standard.

These situations should be assessed by a competent geotechnical/structural engineer rather than assumed automatically.

28. Seasonal Weather Changes and Foundation Movement

Seasonal weather changes can have a significant impact on foundation performance, especially in expansive clay.

During hot and dry weather:

Evaporation → Moisture reduction → Clay shrinkage

During wet weather:

Rainfall → Moisture increase → Clay swelling

Repeated cycles can cause differential ground movement.

The risk is greater where:

Expansive clay exists
Large trees are nearby
Water leakage occurs
Industrial heat sources are present
Drainage is poor

29. Construction Preparation Before Foundation Work

Proper preparation of the site should be completed before foundation construction.

Important preliminary works include:

Access roads
Main drains
Sewers
Site clearance
Drainage arrangements
Setting out

Where these works are carried out after foundation construction, suitable precautions should be taken to prevent damage to completed foundations.

30. Site Clearance

The construction site should be cleared of obstacles that interfere with foundation work.

This may include:

Tree stumps
Old foundations
Debris
Buried obstructions
Loose material

Holes created by removing:

Old foundations
Trees
Burrowing animals
Other underground obstructions

should be properly backfilled and compacted.

Simply filling such holes with loose soil is not adequate because the resulting weak zones can cause differential settlement.

31. Site Drainage

Surface water should not be allowed to accumulate around foundation excavations.

If the natural ground slopes toward the building, suitable measures should be adopted to divert water away from the foundation area.

Possible measures include:

Surface drains
Interceptor drains
Proper site grading
Temporary construction drainage
Permanent drainage systems

Good drainage is particularly important in clayey and water-sensitive soils.

32. Setting Out of Foundations

Accurate setting out is essential because even a properly designed foundation can become unsafe if constructed at the wrong location.

Foundation setting out should establish:

Centre lines
Grid lines
Column positions
Wall locations
Foundation dimensions
Reference levels

For rectangular or square buildings, diagonals should be checked to verify accuracy.

For important and intricate structures, appropriate surveying equipment such as a theodolite or modern total station may be used.

Permanent reference points should be established so that the accuracy of construction can be checked as the work progresses.

33. Protection of Excavations

Excavations may require protection depending on:

Excavation depth
Soil type
Groundwater
Adjacent structures
Weather conditions
Available working space

Temporary support systems and dewatering arrangements should be provided where required.

Excavation protection is particularly important near existing buildings and infrastructure.

34. Preparation of Excavation Bottom

After excavation, the bottom should be:

Cleared of loose soil
Free from rubbish
Properly levelled
Prepared to receive the foundation

Where appropriate, the soil should be wetted and compacted to provide a suitable and uniform surface.

The objective is to avoid placing foundation concrete over loose or disturbed material.

35. Protection of Exposed Clay

Clay and other atmospheric-sensitive soils may deteriorate when exposed for long periods.

Exposure can cause:

Drying
Cracking
Softening
Loss of strength

Therefore, foundation concrete should preferably be placed soon after excavation.

Where immediate concreting is not possible, the excavation bottom should be protected.

The supplied provisions mention a temporary protective layer of approximately:

80 mm thick cement concrete

of suitable lean mix, such as 1:5:10, before placing the foundation concrete.

Another approach is to leave approximately the final 100 mm of excavation until immediately before concreting so that the bearing surface is not exposed unnecessarily.

The actual construction method should follow the approved project specifications and current applicable standards.

36. Backfilling Around Foundations

After foundation construction, excavation should be refilled carefully.

Backfilling should not disturb:

Foundation concrete
Reinforcement
Waterproofing
Adjacent structures
Services

The supplied provision recommends placing backfill in layers not exceeding approximately:

150 mm thickness

and compacting each layer properly.

Only the minimum water required for effective compaction should be used.

Poorly compacted backfill can result in:

Settlement
Water accumulation
Pavement failure
Damage to services
Foundation exposure

37. Practical Foundation Design Checklist

Before finalizing a foundation design, engineers should consider the following checklist.

Foundation Depth

Adequate bearing capacity available
Settlement requirements satisfied
Minimum embedment requirement satisfied
Scour considered
Root zone considered
Adjacent excavation effects checked

Different Foundation Levels

30° bearing zone checked where applicable
1V:2H condition checked
Adjacent foundation influence assessed
Retaining support provided where necessary

Construction

Site drainage arranged
Foundation setting out checked
Excavation properly protected
Loose soil removed
Excavation bottom prepared
Foundation concrete placed promptly
Backfilling carried out in compacted layers

38. Key Numerical Requirements at a Glance

Requirement	Important Value/Concept
General minimum foundation depth	500 mm below natural ground level
Bearing zone for sloping ground	30° to horizontal for soil
Adjacent foundation level relationship	1V : 2H
Large trees near foundations	Approximately 8 m minimum distance as referenced in the supplied provision
Temporary protective PCC layer	Approximately 80 mm where required
Backfilling layer thickness	Approximately 150 mm maximum per layer
Special soil consideration	Shrinkage/swelling, especially expansive clay
Deep foundation load transfer	Friction, end bearing, or both

These values should be read together with the complete applicable standard, project specifications, geotechnical report and current amendments.

39. Why IS 1904 Is Important for Foundation Engineers

Foundation failures are often associated with problems that are not purely structural.

A footing may have adequate reinforcement and concrete strength but still perform poorly because of:

Insufficient soil investigation
Inadequate bearing capacity
Excessive settlement
Poor drainage
Expansive soil
Scour
Nearby excavation
Groundwater changes
Poor construction practices

This is why foundation engineering requires coordination between:

Structural Engineer + Geotechnical Engineer + Site Engineer + Surveyor + Contractor

The structural engineer designs the foundation to safely resist structural actions, while the geotechnical investigation establishes the ground conditions necessary for selecting and designing an appropriate foundation system.

40. Conclusion

IS 1904:1986 provides important general requirements for safe and reliable foundation construction. Its provisions demonstrate that foundation safety begins long before concrete is placed.

A successful foundation requires:

Proper site investigation → Correct foundation selection → Adequate depth → Safe soil bearing → Settlement control → Proper drainage → Correct detailing → Good construction practice

Particular attention should be given to difficult ground conditions such as expansive clay, slopes, mining areas, groundwater-sensitive soils, filled ground and sites containing soluble salts.

The commonly cited 500 mm minimum depth below natural ground level is an important basic requirement, but it should never be treated as a universal design depth. Actual foundation depth must be determined from the governing site and structural conditions.

Similarly, the 30° bearing-zone concept and 1V:2H limitation are important considerations when foundations are constructed at different levels or near sloping ground.

Ultimately, foundation design should be based on the actual geotechnical conditions of the site, structural loading, applicable design standards and sound engineering judgment.

For practical engineering work, IS 1904 should therefore be used as part of a broader design process involving the relevant geotechnical, structural, concrete, loading and construction standards.

Disclaimer: This article is an explanatory summary based on the material supplied for IS 1904:1986. It should not be used as a substitute for the complete Indian Standard, project-specific geotechnical investigation, approved structural drawings, or the latest applicable codes and amendments. Engineers should always verify the current edition and applicable provisions before using any numerical requirement for design or construction.

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