Code of practice for design and construction of ring foundation

IS 11089: Scope & Key Specifications Summary

This standard applies to the design and construction of ring foundations, referencing related IS codes for soil and structural parameters.

Key Points & Parameters:

• Definitions: As per IS 2809-1972 (Soil Engineering Glossary).
• Rounding Off: Final test/analysis values must be rounded per IS 2-1960, retaining the same significant digits as specified.
• Parameters for Analysis: Average soil parameters from limited test points critically influence design accuracy.

Necessary Information for Design (Clause 3.1):

References to Related IS Codes:

• IS 2809-1972 (Soil Engineering Glossary)
• IS 2-1960 (Rounding off rules)
• IS 1892-1979 (Subsurface Investigation)
• IS 2950 (Part 1)-1981 (Elastic Modulus & Subgrade Reaction)

flowchart TD
    A[Site Plan] --> D[Design]
    B[Building Plan] --> D
    C[Loading Conditions] --> D
    E[Environmental Factors] --> D
    F[Geotechnical Info] --> D
    G[Elastic Modulus & Subgrade Reaction] --> D
    H[Limiting Distortion & Settlement] --> D
    I[Local Structure Performance] --> D

IS 11089: Definitions & Key Specifications Summary

1. Definitions (Clause 2.1):

• Terms are as per IS 2809-1972 (Soil Engineering Glossary & Symbols).
• Ensures uniformity in soil engineering terminology.

2. Rounding Off Rules (Clause 0.3):

• Follow IS 2-1960 for rounding numerical values.
• Rounded values must retain the same significant figures as the specified values.

3. Parameters for Analysis (Clause 3.2):

• Parameters are averaged from limited soil test points.
• Accuracy of averages critically affects foundation design results.

Important References for Design:

Summary Diagram: Data Flow for Soil Parameter Use

flowchart LR
    A[Soil Test Points] --> B[Parameter Determination]
    B --> C[Average Parameters]
    C --> D[Foundation Design Analysis]
    D --> E[Final Design Decision]

Key takeaway: Use IS 2809 for definitions, IS 2 for rounding, and carefully average soil parameters for reliable foundation design per IS 11089.

IS 11089 - Necessary Information for Ring Foundation Design

Clause 3.1 specifies essential data for reliable design and construction:

• Site Plan: Location of proposed & neighboring structures.
• Building Plan: Height, staging, columns details.
• Loading Conditions: Dead, wind, earthquake loads with design load combinations.
• Environmental Factors: Geological history, seismicity, groundwater conditions, flooding, erosion risks.
• Geotechnical Info: Subsurface stratification (IS 1892-1979), soil properties, shear parameters, compressibility, swelling, field test results (SPT, CPT, pressure meter).
• Elasticity & Subgrade Reaction: Modulus of Elasticity & Modulus of Subgrade Reaction per IS 2950 (Part 1)-1981.
• Structural Limits: Allowable angular distortion and differential settlement.
• Performance Review: Similar local structures.
• Impact Assessment: Effect on nearby structures.
• Site Hazards: Proximity to mines or reservoirs.

Rounding Off (IS 2-1960)

• Final calculated or observed values must be rounded to the same significant figures as specified.

Parameter Averaging (Clause 3.2)

• Soil parameters are averaged from limited test points; accuracy critically affects design results.

Summary Table of Key Parameters

flowchart TD
    A[Necessary Information] --> B[Site Plan]
    A --> C[Building Plan]
    A --> D[Loading Conditions]
    A --> E[Environmental Factors]
    A --> F[Geotechnical Info]
    A --> G[Elasticity & Subgrade Reaction]
    A --> H[Structural Limits]
    A --> I[Performance Review]
    A --> J[Impact Assessment]
    A --> K[Site Hazards]

This structured information ensures a comprehensive foundation design per IS 11089.

IS 11089: Design Considerations - Key Points & Parameters

1. Essential Information for Design (Clause 3.1)

• Site Plan: Location of proposed & neighboring structures.
• Building Plan: Heights, staging, column details.
• Loading Conditions: Dead, wind, earthquake loads with design load combinations.
• Environmental Factors: Geological history, seismicity, groundwater, flooding, erosion.
• Geotechnical Info: Subsurface stratification, soil properties (shear strength, compressibility), field tests (SPT, pressure meter).
• Modulus of Elasticity & Subgrade Reaction: Per IS 2950 (Part 1)-1981.
• Allowable Angular Distortion & Differential Settlement: Limits the superstructure can tolerate.
• Performance Review: Similar local structures.
• Effect on Neighboring Structures: Assessment of impacts.
• Proximity to Mines/Reservoirs: For potential influence.

2. Parameters Averaging (Clause 3.2)

• Average soil parameters from limited bore points.
• Accuracy critical for reliable foundation design.

Typical Parameters for Foundation Design

Design Load Combination (Example)

• Ultimate Load = 1.5 (Dead Load + Live Load) + 1.5 Wind Load
• Or as per IS 1893 for seismic loads

flowchart TD
    A[Site & Building Plans] --> B[Load Calculations]
    B --> C[Soil Investigation & Parameters]
    C --> D[Parameter Averaging]
    D --> E[Foundation Design]
    E --> F[Check Settlement & Distortion Limits]
    F --> G[Finalize

IS 11089: Methods of Analysis — Key Points & Formulas

1. Floatation Check (Clause 4.4.1)

• All foundation parts below groundwater level must be checked for flotation.
• Ensure the buoyant force ≤ weight of structure + soil above foundation base.

2. Parameter Averaging (Clause 3.2)

• Use averaged soil parameters from limited test points for analysis.
• Accuracy of averages is critical for reliable results.

3. Rounding Off (Clause 0.3)

• Final calculated values should be rounded per IS 2:1960 rules.
• Retain the same number of significant digits as specified values.

Typical Formula for Floatation Check

[ \text{Factor of Safety against flotation} = \frac{\text{Weight of structure + soil}}{\text{Buoyant force}} \geq 1.5 ]

Where:

• Buoyant force = Volume of submerged part × Unit weight of water (γ_w)
• Weight includes structure + soil above foundation base

Reference Standards for Analysis Parameters

• IS 2809:1972 — Soil engineering terms
• IS 6403 — Subsurface investigations
• IS 2950 (Part 1) — Raft foundation design

flowchart TD
    A[Soil Parameters] --> B[Average Parameters]
    B --> C[Foundation Analysis]
    C --> D{Check Floatation}
    D -->|Safe| E[Design OK]
    D -->|Unsafe| F[Modify Design]

For detailed soil parameters and design checks, refer to IS 6403 and IS 2950 Part 1 alongside IS 11089.

IS 11089: Structural Design & Construction Key Points

1. Preliminary Data for Design (Clause 3.1)

• Site Plan: Location of proposed & neighboring structures.
• Building Plan: Height, staging, columns details.
• Loading Conditions: Dead, wind, earthquake loads with design load combinations.
• Environmental Factors: Geology, seismicity, groundwater, climate, erosion.
• Geotechnical Info: Soil stratification, shear parameters, compressibility, swelling, field tests (SPT, pressure meter).
• Modulus of Elasticity & Subgrade Reaction: Refer IS 2950 (Part 1)-1981.
• Allowable Angular Distortion & Differential Settlement
• Performance Review: Similar local structures.
• Neighborhood Effects: Impact on adjacent structures.
• Site Risks: Proximity to mines, reservoirs.

2. Foundation Depth (Clause 4.3)

• Follow IS 1904-1978 for minimum foundation depth.

3. Structural Rigidity Condition (Clause 5.1.1)

• Use ring foundation design if relative stiffness factor K > 0.5.
• Calculate K as per Appendix C, IS 2950 (Part 1)-1981.

Key Formula: Relative Stiffness Factor (K)

[ K = \frac{EI}{L} \bigg/ k_s L ]

• (E) = Modulus of Elasticity of superstructure
• (I) = Moment of Inertia
• (L) = Length of foundation
• (k_s) = Modulus of subgrade reaction

Summary Diagram (Ring Foundation Design Process)

flowchart TD
    A[Site & Building Plans] --> B[Loading Conditions]
    B --> C[Environmental & Geotechnical Data]
    C --> D[Calculate Stiffness Factor K]
    D -->|K > 0.5| E[Design Ring Foundation]
    D -->|K ≤ 0.5| F[Use Alternate Foundation]
    E --> G[Check Settlement & Distortion Limits]
    G --> H[Construction & Monitoring]

References:

• IS 11089: Ring Foundation Design
• IS 2950 (Part 1): Elasticity

IS 11089: Choice of Ring Foundations

Key Points from Clause 4.1 & 5.3

• Ring foundations are used mainly for supporting cylindrical structures like silos, tanks, chimneys.
• Choice depends on:
  • Soil bearing capacity
  • Structural load (axial and moment)
  • Flexibility requirements (rigid or flexible ring foundation)
  • Settlement characteristics

Flexible Ring Foundation (Clause 5.3)

• Designed to allow differential settlement without inducing large bending moments.
• Typically used on soft or compressible soils.
• Ring thickness and reinforcement are calculated to resist circumferential tensile stresses.

Key Formulas

• Circumferential stress in ring (σθ):

[ \sigma_\theta = \frac{M}{Z} + \frac{N}{A} ]

Where:

• (M) = bending moment

• (N) = axial force

• (Z) = section modulus

• (A) = cross-sectional area

• Minimum thickness (t):

[ t = \frac{P}{2 \pi r f_c} ]

Where:

• (P) = axial load
• (r) = radius of ring
• (f_c) = permissible compressive stress of concrete

Typical Specifications

Summary

• Use rigid ring foundations on firm soils with minimal settlement.
• Use flexible ring foundations on soft soils to accommodate differential settlements.
• Design thickness and reinforcement based on axial load, bending moment, and soil conditions.

flowchart TD
    A[Structure Load] --> B{Soil Condition}
    B -->|Firm Soil| C[Rigid Ring Foundation]
    B -->|Soft/Compressible Soil| D[Flexible Ring Foundation]
    C --> E[Design for Axial Load & Moment]
    D --> F[Design for Circumferential Stress &

Allowable Bearing Pressure (IS 11089 referencing IS 6403-1981)

• Definition: The maximum pressure that soil can safely bear without risk of shear failure or excessive settlement.

• Determination: As per IS 6403-1981 (Code of Practice for Bearing Capacity of Shallow Foundations).

Key Points from IS 11089 Clause 5.1 (Rigid Foundation - Conventional Method)

• Assumptions:

  • Foundation is rigid relative to soil.
  • Contact pressure varies linearly under the foundation.
  • Soil layer is shallow and compressible soil effects are limited.

• Pressure Distribution:

  • For circular footing under horizontal forces (wind/earthquake), the non-uniform pressure can be approximated as:
    [ p = p_1 + 0.5 p_2 ] where
    • ( p_1 ) = uniform pressure from dead loads
    • ( p_2 ) = pressure due to bending effects

Moment Formulas for Ring Annular Raft (Clause 5.1.2)

[ M_t = -\left[4(1 + 2\nu)(\ln e^2 + 2)\right] \cdot \left(C + \ldots \right) ]

[ M_r = \frac{16}{24}(1-2\nu)(\ln e + 1 - 2) C ]

• ( M_t ) = circumferential moment
• ( M_r ) = radial moment
• ( r ) = radial distance
• ( \nu ) = Poisson’s ratio
• ( C ) = soil cohesion parameter
• ( a, b ) = footing dimensions

Note: The exact terms depend on footing geometry and soil parameters; refer IS 11089 Fig. 1C for ring beam positioning to minimize moments.

Summary Table for Allowable Bearing Pressure

IS 11089 refers to design and construction of ring foundations, with depth of foundation guided by IS 1904-1978.

Key Points on Depth of Foundation (per IS 1904-1978)

• Minimum Depth: To avoid effects of surface moisture changes and frost, minimum depth should be:
  • At least 0.9 m (3 ft) below ground level for non-frost areas.
  • Below frost line in frost-prone regions.
• Safe Bearing Capacity: Depth must ensure foundation rests on soil with adequate bearing capacity.
• Settlement Control: Depth chosen to minimize differential settlement, based on soil investigation.

Typical Depth Determination Formula:

Depth ( d ) depends on soil type, load, and environmental factors (simplified):

[ d \geq \max \left(0.9,m, \text{depth to firm stratum}, \text{frost penetration depth}\right) ]

Parameters from IS 11089 Clause 3.2:

• Use averaged soil parameters from subsurface investigations for design.
• Parameters include soil bearing capacity, moisture content, and compressibility.

Summary Table: Minimum Foundation Depth (IS 1904)

flowchart TD
    A[Start Soil Investigation] --> B[Determine Soil Parameters]
    B --> C{Is Soil Firm?}
    C -- Yes --> D[Set Depth ≥ 0.9 m or frost depth]
    C -- No --> E[Increase Depth to Reach Firm Stratum]
    D --> F[Check Settlement Criteria]
    E --> F
    F --> G[Finalize Foundation Depth]

In brief: Follow IS 1904 for minimum depth, adjust based on soil investigation per IS 11089, ensuring foundation rests on stable soil below moisture/frost influence.

IS 11089: Stability Against Soil Failures for Raft Foundations

Key Points from IS 11089:

• Clause 4.2.2: For rafts on cohesive soils, analyze stability against deep-seated soil failures.
• Clause 3.2: Use averaged soil parameters (from limited bore points) for analysis; accuracy of these averages is critical.
• Clause 5.1.1: Applicable when the structure behaves as rigid with relative stiffness factor ( K > 0.5 ) (see IS 2950 Part 1 Appendix C for ( K ) evaluation).

Important Parameters:

• Shear strength parameters: Cohesion ( c ), angle of internal friction ( \phi ), unit weight ( \gamma ).
• Foundation geometry: Width ( B ), length ( L ), depth ( D_f ).
• Soil profile: Layer thickness, strength variation with depth.

Stability Against Deep-Seated Failure:

• Use limit equilibrium methods or slip circle analysis to check factor of safety ( FS ).
• Typical formula for factor of safety against sliding:

[ FS = \frac{c' A + (W \cos \alpha - U) \tan \phi'}{W \sin \alpha} ]

where:

• ( c' ) = effective cohesion,
• ( A ) = area of failure surface,
• ( W ) = weight of soil mass above failure surface,
• ( U ) = pore water pressure,
• ( \alpha ) = inclination of failure surface.

Reference Tables (from IS Codes & Soil Mechanics):

Summary Diagram: Stability Check Flow

flowchart TD
    A[Start: Soil Parameters] --> B{Is K > 0.5?}
    B -- No --> C[Use flexible foundation methods]
    B -- Yes --> D[Calculate average soil parameters]
    D --> E[Analyze deep-seated failure]
    E --> F{Factor of

IS 11089: Effects of Groundwater and Uplift – Key Points

1. Uplift Due to Groundwater (Clause 4.4 & 4.4.1)

• Uplift force (U) acts on foundations below the groundwater table and must be checked against flotation.
• Check for flotation:
  [ \text{Weight of structure + soil above foundation} \geq \text{Upthrust due to groundwater} ]

2. Uplift Force Calculation

[ U = \gamma_w \times V ]

• ( \gamma_w ) = Unit weight of water (≈ 9.81 kN/m³)
• ( V ) = Volume of soil displaced below groundwater level

3. Design Check for Stability Against Uplift

[ \text{Factor of Safety (FS)} = \frac{\text{Weight of structure + soil}}{\text{Uplift force}} \geq 1.5 \quad \text{(typical)} ]

4. Parameters for Analysis (Clause 3.2)

• Use averaged soil parameters (unit weight, cohesion, angle of internal friction) from subsurface investigations (IS 2809).
• Accurate soil data is critical for precise uplift and flotation analysis.

Summary Table: Uplift Considerations

flowchart LR
    A[Groundwater Table] --> B[Soil below foundation]
    B --> C[Volume V displaced]
    C --> D[Calculate Uplift Force U = γw × V]
    D --> E[Check flotation: Weight ≥ U]
    E --> F{Is FS ≥ 1.5?}
    F -- Yes --> G[Safe against uplift]
    F -- No --> H[Redesign foundation or drainage]

Note:

Pressure Distribution Under Foundation (IS 11089 - Clause 5.1)

Key Assumptions (Rigid Foundation - Conventional Method)

• Foundation is rigid relative to soil.
• Soil compressible layer is shallow.
• Contact pressure varies linearly under the foundation.

Pressure Distribution

• For circular footing under horizontal forces (wind, earthquake), pressure can be approximated as:

[ p = p_1 + 0.5 p_2 ]

where:

• (p_1) = uniform pressure due to dead loads,
• (p_2) = pressure due to bending effects.

Ring Annular Raft Pressure (Clause 5.1.2)

• Pressure distribution transitions from non-uniform to uniform.
• Modified uniform pressure intensity:

[ p = p_1 + 0.5 p_2 ]

Moments in Raft Foundation

• Circumferential moment (M_t):

[ M_t = -\left[4(1+\nu)(\ln \frac{b}{a} + 2)\right] \times \text{(terms involving soil and footing parameters)} ]

• Radial moment (M_r):

[ M_r = \frac{16}{24}(1-\nu^2)(\ln \frac{b}{a} + 1 - \nu^2) \times C ]

where:

• (r) = radial distance,
• (a, b) = footing dimensions,
• (\nu) = Poisson's ratio,
• (C) = soil cohesion.

Design Guidance

• The ratio (b/a) influences bearing area and moment of inertia.
• Fig. 1C (IS 11089) helps locate the ring beam position for minimum max moments.
• Ensure max stress ≤ allowable soil bearing pressure.

Summary Table: Pressure Distribution Types

Assessment of Differential Settlement as per IS 11089

Key Conditions (Clauses 5.1.1 & 5.3.1):

• Rigid Structure Method (K > 0.5)

  • Use when structure + foundation act rigidly.
  • Relative stiffness factor ( K ) evaluated per Appendix C of IS 2950 (Part 1)-1981.

• Flexible Structure Method (K < 0.5)

  • Structure + raft considered flexible.
  • Subgrade modeled as infinite elastic springs with modulus of subgrade reaction ( K ).
  • Adjacent column load variation ≤ 20%.

Important Parameters (Clause 3.2):

• Average soil parameters (modulus, compressibility) from limited bore points.
• Accuracy of averages crucial for settlement prediction.

Design Considerations (Clause 4.2.3):

• Account for long-term settlements in cohesive soils.

Formulas & Concepts:

• Relative stiffness factor ( K ):
  [ K = \frac{E_f I_f}{L^3 K_s} ] Where:

  • ( E_f ) = modulus of elasticity of foundation
  • ( I_f ) = moment of inertia of foundation
  • ( L ) = characteristic length
  • ( K_s ) = modulus of subgrade reaction

• Modulus of subgrade reaction ( K ):
  Contact pressure ( p ) relates linearly to settlement ( s ):
  [ p = K \times s ]

Summary Table:

flowchart LR
    A[Soil Parameters] --> B[Average Values]
    B --> C{Relative Stiffness Factor (K)}
    C -->|K > 0

Impact on Neighboring Structures as per IS 11089 involves evaluating foundation behavior and site conditions to avoid adverse effects.

Key Points & Specifications:

• Rigid Structure Condition (Clause 5.1.1):
  Use this method if relative stiffness factor
  [ K = \frac{EI}{L} \text{ (from IS 2950 Part 1 Appendix C)} > 0.5 ]
  where (E) = Modulus of Elasticity, (I) = Moment of Inertia, (L) = length.

• Information Required (Clause 3.1):

  • Site & building plans including neighboring structures
  • Loading conditions (dead, wind, seismic)
  • Environmental & geotechnical data (see IS 1892-1979)
  • Modulus of Elasticity & subgrade reaction (IS 2950 Part 1)
  • Limiting angular distortion & differential settlement tolerances
  • Historical performance of similar structures
  • Proximity to mines or reservoirs

• Foundation Depth:
  Follow IS 1904-1978 for minimum depth to avoid settlement and bearing failure.

Typical Formula for Relative Stiffness Factor (K):

[ K = \frac{E I}{L} ]

• (E) = Modulus of Elasticity of superstructure + foundation
• (I) = Moment of inertia of the combined structure section
• (L) = Characteristic length of the structure

Summary Table: Key Parameters for Impact Assessment

IS 11089: References and Appendices - Key Points

1. References

• IS 2809-1972: Definitions of soil engineering terms.
• IS 2-1960: Rules for rounding off numerical values.
• IS 1892-1979: Subsurface profile and stratification details.
• IS 2950 (Part 1)-1981: Modulus of Elasticity and Modulus of Subgrade Reaction.
• Additional codes: Subsurface investigations, raft foundation design.

2. Rounding Off Numerical Values (IS 2-1960)

• Final values (test or analysis) rounded to the same significant figures as specified.
• Ensures consistency and accuracy in reporting.

3. Necessary Design Information (Clause 3.1 Highlights)

• Site & Building Plans: Location, height, columns.
• Loading Conditions: Dead, wind, earthquake loads.
• Environmental Factors: Seismicity, groundwater, flooding risk.
• Geotechnical Info: Soil profile, shear parameters, compressibility, field test results.
• Modulus Values: Elasticity and subgrade reaction.
• Structural Limits: Angular distortion, differential settlement.
• Local Experience: Performance of similar structures.
• Neighbourhood Effects: Impact on nearby structures.
• Proximity Hazards: Mines, reservoirs.

Example: Rounding Rule (IS 2-1960)

flowchart TD
  A[Start: Design Input] --> B[Site & Building Plans]
  B --> C[Loading Conditions]
  C --> D[Environmental Factors]
  D --> E[Geotechnical Data]
  E --> F[Modulus of Elasticity & Subgrade Reaction]
  F --> G[Structural Limits & Local Experience]
  G --> H[Final Design & Compliance Check]

This structure ensures comprehensive data collection and adherence to IS standards for ring foundation design per IS 11089.