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IS 8009:1976 Part 1 is the Indian Standard (BIS) for foundation on expansive soils - plain and reinforced concrete foundations. This code provides guidelines for the design and construction of plain and reinforced concrete foundations on expansive soils, like black cotton soil. It covers site investigation, design principles, and precautions to mitigate the effects of soil swelling and shrinking. The code details various foundation types suitable for these conditions, including strip, raft, and under-reamed pile foundations.
Provides guidelines for the design and construction of plain and reinforced concrete foundations in expansive soils.
Quick Reference — IS 8009 Part 1:1976 Expansive Soil Foundation
Foundations on expansive soils — black cotton, Vertisol. Founding depth, swell mitigation, underreamed piles.
✓ Verified 2026-04-28
Reference
Value
Clause
Soil identification — Free Swell Index (FSI)
> 50 % indicates expansive
Cl. 3.2 (IS 2720 Part 40)
Plasticity index PI — expansive
> 35 (high to very high)
Cl. 3.3 (Table 1)
Activity number A — expansive
> 0.85 (medium to high)
Cl. 3.3 (Table 1)
Swelling pressure — high expansive
> 200 kPa
Cl. 3.4 (Table 1)
Active zone depth — typical Indian BC
1.5–3.5 m
Cl. 4.1
Founding depth — strip footing on BC
≥ 1.5 m below NGL or active zone
Cl. 4.2.1
Founding depth — isolated footing
below active zone (> 2 m typical)
Cl. 4.2.2
Cushion / sand replacement depth
1.0–1.5 m of non-swelling fill
Cl. 5.1
Under-reamed pile — typical diameter
300–375 mm shaft, 750–900 mm bulb
Cl. 6.2 (refers IS 2911 Pt 3)
Under-reamed pile — minimum depth
3.5 m below NGL (one bulb)
Cl. 6.3
Under-reamed pile — bulb diameter / shaft
2.5 × shaft diameter
Cl. 6.2
Under-reamed pile — bulb spacing
1.25 to 1.5 × bulb diameter
Cl. 6.2.2
Plinth beam — load-bearing wall on swelling soil
mandatory + tied to under-reamed piles
Cl. 5.2
Slab on grade — separation from soil
polyethylene sheet + 100 mm sand
Cl. 5.4
Drainage — surface
1:50 slope away from building, paved 1.5 m wide
Cl. 5.5
Avoid trees / vegetation near foundation
set back ≥ tree height
Cl. 5.6
Floor finish on slab-on-grade — joint
movement joint at perimeter
Cl. 5.4
Sub-base under floor
moist sand, not gravel (to avoid water entry)
Cl. 5.4
Concrete grade — under-reamed pile
M20 minimum
Cl. 6.2.5
Reinforcement — under-reamed pile
0.4 % minimum (full length)
Cl. 6.2.5
⚠ Reaffirmed. Critical for projects in Maharashtra, MP, Gujarat, Telangana — where black cotton soil dominates. Cross-referenced with IS 2911 Part 3 (under-reamed piles).
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! The primary strategy is to either bypass the zone of moisture fluctuation with deep foundations (like under-reamed piles) or create a rigid foundation system (like a raft) that can withstand differential movement.
! Under-reamed piles, detailed in IS 2911, are a very common and effective solution for these soils, anchoring the structure in a stable stratum.
! Effective surface drainage around the building is non-negotiable to prevent water ingress into the foundation soil, which is the primary trigger for swelling.
Provides detailed design and construction requirements for footings and slabs on reactive (expansive) soil sites.
PTI DC10.1-08Post-Tensioning Institute (US)
HighCurrent
Standard Requirements for Analysis of Shallow Concrete Foundations on Expansive Soils
Focuses on the analysis and design of stiffened raft slabs, particularly post-tensioned, on expansive soils.
BS 8004:2015+A1:2020BSI (UK)
MediumCurrent
Code of practice for foundations
A general foundation code that includes principles for dealing with soils susceptible to volume change, but is less prescriptive.
ACI 360R-10ACI (US)
MediumCurrent
Guide to Design of Slabs-on-Ground
A detailed guide covering slab-on-ground design, with significant discussion on handling expansive soil conditions.
Key Differences
≠IS 8009 extensively details the design and use of single and multi-under-reamed piles as a primary solution for expansive soils, a technique not commonly featured in major US or Australian standards.
≠IS 8009 classifies soil expansiveness using the 'Free Swell Index' (FSI). In contrast, AS 2870 uses 'Characteristic Surface Movement' (ys), and PTI methods use parameters like Thornthwaite Moisture Index and suction to predict potential vertical rise (Ym), which are more direct measures of ground movement.
≠Reinforcement in IS 8009 is often specified using prescriptive minimums (e.g., minimum number and diameter of bars for plinth beams). International codes like PTI DC10.1 and AS 2870 derive reinforcement requirements from structural analysis based on calculated bending moments and shear forces induced by soil heave.
≠IS 8009 is a relatively concise code of practice. AS 2870 and the PTI design procedures provide a more comprehensive framework, including detailed methodologies for calculating soil-structure interaction, slab stiffness, and required strength to resist heave.
Key Similarities
≈All standards share the fundamental objective of designing foundations to mitigate damage from differential soil movement caused by moisture changes in expansive clays.
≈A thorough geotechnical investigation is emphasized by all codes as a prerequisite for design, including soil sampling, laboratory tests (e.g., Atterberg limits, swell tests), and determining the depth of the active soil zone.
≈The use of stiffened raft foundations, which act as a rigid mat to bridge over localized soil movements, is a common design strategy recommended in both IS 8009 and its international counterparts like AS 2870.
≈All standards recognize the critical importance of controlling moisture conditions around the foundation through proper surface grading, drainage, and careful landscaping to maintain soil moisture stability.
Parameter Comparison
Parameter
IS Value
International
Source
Primary Soil Classification Basis
Free Swell Index (FSI), a percentage.
Characteristic Surface Movement (ys), in mm.
AS 2870-2011
Threshold for 'High' Expansiveness
FSI > 35%
ys > 40 mm (Class H1 site)
AS 2870-2011
Minimum Concrete Grade (for RC)
M15 (15 MPa characteristic strength) as per the 1976 code.
20 MPa for normal reinforced, 25 MPa typically for post-tensioned slabs.
AS 2870-2011
Void Under Grade Beams
Suggests a gap of at least 50 mm, fillable with compressible material.
Typically 75 mm to 150 mm (3 to 6 inches) depending on predicted heave.
PTI DC10.1-08
Foundation Design Philosophy
Prescriptive (e.g., under-reamed piles) and simple raft/beam solutions.
Performance-based analysis of stiffened slab on a deformable soil mound (center heave or edge heave conditions).
PTI DC10.1-08
Plinth Beam Min. Reinforcement
Prescriptive, e.g., '3 bars of 12 mm dia at top and 3 at bottom' as a typical minimum.
Calculated based on required moment capacity (M*) to resist bending from soil heave.
AS 2870-2011
Center-to-Center Pile Spacing
2 to 3 times the under-reamed bulb diameter (Du).
Not applicable, as under-reamed piles are not a standard solution in this code.
AS 2870-2011
⚠ Verify details from original standards before use
Key Values4
Quick Reference Values
Degree of expansion for 'High' classification35-50% Differential Free Swell
Typical depth to neglect seasonal moisture variation1.5 m to 3.5 m
Minimum grade of concrete for RCC foundation (by reference to IS 456:1978)M15
Minimum clear cover for foundation reinforcement (by reference to IS 456:1978)40 mm
Tables & Referenced Sections
Key Tables
Table 1 - Classification of Expansive Soils Based on Swell Characteristics
To provide a framework for designing and constructing foundations on soils that swell and shrink with moisture changes, ensuring the stability of the structure.
How is the expansiveness of soil classified?+
Using the 'Differential Free Swell' index, soils are classified as having Low, Medium, High, or Very High potential for expansion (Table 1).
What is a common foundation type recommended for highly expansive soils?+
Under-reamed pile foundations are frequently recommended as they can be anchored in stable soil strata below the zone of moisture fluctuation (Clause 6.2).
What key precaution is mentioned for all foundation types in expansive soils?+
To ensure that the foundation is placed at a depth where volumetric changes in the soil are negligible, or to provide a structural system that can accommodate or resist these changes (Clause 5).