Code of practice for design and construction of pile foundations, Part 3: Under-reamed piles

IS 2911 Part 3: Scope - Key Formulas, Tables, and Specifications

1. Scope Overview

• Covers design and construction of pile foundations in various soil types.
• Emphasizes safe load determination using Standard Penetration Test (SPT) 'N' values.
• Requires proper design for piles under external moments/lateral loads exceeding Table 1 values.

2. Safe Load Determination (Clause 1.11)

• Use weighted average of SPT 'N' values up to depth = bulb diameter below pile toe.
• For silty soils, take 'N' between sandy and clayey soil values.
• Provide adequate steel for moments/lateral loads beyond Table 1 limits.

3. Modulus of Subgrade Reaction (Appendix C, Clause 5.2.5)

Table 2: nh values for sands

Table 3: K values for preloaded clays

4. Lateral Deflection & Fixity (Appendix D, Clause 6.2)

• Depth of fixity and equivalent cantilever length determined via Fig. 4 using constants K1, K2.
• Constants K1 (kg/cm³) for dry and submerged soils:

| Soil Type | Dry | Submerged |

IS 2911 Part 3: Terminology & Key Specifications

Key Definitions (Clause 2.0)

• Standard definitions apply for piles, soil types, loadings, and related terms.
• "N" value: Weighted average Standard Penetration Test (SPT) value up to bulb diameter below pile toe used for safe load estimation.

Safe Load (Clause 1.11, Table B-1)

• Safe load depends on soil N-value.
• For silty soils, use intermediate N-values between sandy and clayey soils.
• For piles with external moments/larger lateral loads, design steel reinforcement accordingly.

Modulus of Subgrade Reaction (Appendix C, Clause 5.2.5)

Lateral Deflection & Depth of Fixity (Clause 1.2, Table C-1)

• Use constants K1 and K2 (Tables 2 & 3) with pile properties (E, I) to compute lateral deflection and fixity depth.

IS 2911 Part 3: Preliminary Data & Site Investigation Key Points

1. Site Investigation Data (Clause 3.1)

• Follow IS 1892 for subsurface investigation.
• Include:
  • Soil profile from trial borings & penetration tests.
  • Soil strength, compressibility, groundwater levels, chemical tests (sulphate, chloride).
  • Expansiveness via Free Swell Test (IS 2720 Part XL).
  • For bridges: flood level, scour depth, normal water level.
  • For rock: physical & strength description.
  • Structural layout & load details (dead, live, wind, earthquake).
  • Nearby structures info.

2. Degree of Soil Expansiveness (Appendix A)

3. Modulus of Subgrade Reaction (Appendix C)

• For Sands (nh in kg/cm³)

• For Clays (K in kg/cm²)

4. Safe Load from SPT (Clause 1.11)

• Use weighted average N-value up to bulb diameter below pile tip.
• For silty soils, interpolate between sandy and clayey soil values.

5. Lateral

Materials and Concrete Specifications (IS 2911 Part 3)

Concrete:

• Minimum grade: M20 with minimum cement content 400 kg/m³.
• For piles ≤ 6 m depth under favorable conditions: M15 with minimum cement 350 kg/m³.
• Follow IS 456-1978 for concrete, water, and aggregates.
• Use natural rounded shingle as coarse aggregate for better slump and lower water-cement ratio.
• For tremie concreting, max aggregate size ≤ 20 mm.

Steel Reinforcement:

• Conform to:
  • IS 432 (Part I)-1966 (Mild/medium tensile steel bars)
  • IS 1139-1966 (Hard drawn steel wire)
  • IS 1786-1979 (High strength deformed bars)
  • IS 226-1975 (Structural steel)
• Design stresses as per IS 456-1978.

Key Tables:

Modulus of Subgrade Reaction (Appendix C)

Summary:

• Use M20 concrete minimum for piles; M15 allowed under limited conditions.
• Follow IS 456 for mix and materials.
• Steel reinforcement as per IS 1786 or IS 432.
• Aggregate size ≤ 20 mm

IS 2911 Part 3: Design Considerations - Key Formulas & Tables

1. Safe Load on Piles (Clause 1.11, Appendix B)

• Safe load depends on N-value (Standard Penetration Test) averaged over depth = bulb diameter below pile toe.
• For silty soils, take N between sandy and clayey soil values.
• For piles with external moments/larger lateral loads, provide additional steel.

2. Modulus of Subgrade Reaction (Appendix C, Clause 5.2.5)

• Use constants ( n_h ) (sand) and ( K ) (clay) from:

3. Lateral Deflection & Depth of Fixity (Clause 1.2, Appendix C)

• Depth of fixity ( L_f ) and lateral deflection determined via plots (Fig. 4).
• Use constants ( K_1 ) and ( K_2 ) (Tables 2 & 3) with:

[ L_f = f(K_1, K_2, E, I) ]

• Constants ( K_1 ) for soil types:

| Soil Type | Dry (kg/cm³) | Submerged (kg/cm³

IS 2911 Part 3: Equipment and Accessories for Under-Reamed Piles

Key Points from Clauses & Appendices:

• Clause 6.1 & 6.2:

  • Equipment selection depends on pile type, site conditions, soil strata, and economic feasibility.
  • Appendix D lists typical manual construction equipment (e.g., augers, boring rods, tamping tools).

• Clause 5.4.7:

  • The pile should project 40 mm into the pile cap concrete for proper embedment.

Modulus of Subgrade Reaction (Appendix C):

Safe Load Guidance (Clause 1.11):

• Use weighted average N-value from SPT up to depth = bulb diameter below pile toe.
• For silty soils, interpolate N-values between sandy and clayey soils.
• For piles under moments/larger lateral loads, design with adequate steel reinforcement.

Summary Diagram of Equipment Selection Factors:

graph LR
A[Equipment Selection] --> B[Type of Under-Reamed Pile]
A --> C[Site Conditions]
A --> D[Nature of Soil Strata]
A --> E[Economic Considerations]
A --> F[Availability of Manual/Power Equipment]

References:

• IS 2911 (Part 3)

IS 2911 Part 3: Construction Procedures - Key Formulas, Tables, and Specifications

1. Equipment for Manual Construction

• Refer Appendix D (Clause 6.2) for typical equipment list for under-reamed piles, including drop weights, winches, and core assemblies (Clause 6.10).

2. Safe Load Determination

• Use Table 1 (Clause 1.11) for safe loads based on weighted average N-value from SPT up to bulb diameter below pile toe.
• For silty soils, interpolate between sandy and clayey soil values.
• For piles with larger lateral loads or moments, design steel reinforcement accordingly.

3. Modulus of Subgrade Reaction (Appendix C, Clause 5.2.5)

4. Lateral Deflection & Depth of Fixity (Clause 1.2)

• Formula for lateral deflection involves constants K1, K2, Young’s modulus (E), and moment of inertia (I).
• Use Table 2 and 3 for constants K1 (kg/cm³) and K2 (kg/cm²) based on soil type and UCS.

IS 2911 Part 3: Load Testing & Trial Piles - Key Points

1. Safe Load Determination (Clause 1.10, Table B-1)

• Safe loads for under-reamed piles depend on:
  • Pile diameter & length
  • Reinforcement details (No. of bars, bar diameter, ring spacing)
  • Soil type (sandy/clayey including black cotton soil)
• Safe load values given for:
  • Compression
  • Uplift resistance
  • Lateral thrust
• Adjustments:
  • Increase/decrease safe load per 30 cm length as per table columns (11, 12, 15, 16)

2. Soil Parameters for Design (Appendix C, Clause 5.2.5)

3. Lateral Deflection & Depth of Fixity (Clause 1.2, Fig.4 & Tables 2,3)

• Use constants K1 and K2 from Tables 2 and 3 for soil type.
• Calculate lateral deflection & depth of fixity using:

[ \text{Depth of fixity} = f(K_1, K_2, E, I, \text{pile embedment}) ]

Where:

• (E) = Young’s modulus (kg/cm²)
• (I) = Moment of inertia (cm⁴)

4. Key Formula for Safe Load from SPT 'N' Value

• Use weighted average (

IS 2911 Part 3: Pile Grouping and Layout Key Points

Spacing of Piles (Clause 5.2.7)

• Minimum spacing for bored cast in-situ piles: 2D (D = pile diameter).
• Closer spacing reduces individual pile capacity due to group effects.

Group Capacity (Clause 5.2.8.1)

• At 2D spacing, group capacity = sum of individual safe loads.
• At 1.5D spacing, reduce safe load per pile by 10% to account for interaction.
• For under-reamed compaction piles, group capacity may exceed this due to soil compaction effects.

Settlement Considerations (Clause 5.2.8.3)

• Settlement depends on:
  • Soil type (clay vs sand)
  • Pile spacing & group size
  • Installation method
  • Load magnitude & nature
• Clay: Long-term settlements significant.
• Sand: Settlements occur rapidly and stabilize.

Summary Table: Safe Load Reduction Based on Spacing

flowchart LR
    A[Pile Diameter (D)] --> B{Pile Spacing}
    B -->|≥ 2D| C[Group Capacity = n × Load per pile]
    B -->|1.5D| D[Reduce load per pile by 10%]
    D --> E[Group Capacity = n × 0.9 × Load per pile]
    F[Under-reamed Piles] --> G[Group capacity may increase due to compaction]

Note: Always verify group effects through site-specific tests and consider soil-pile interaction for design.

IS 2911 Part 3: Recording and Documentation Key Points

1. Recording Pile Details (Clause 7.5.11)

A competent person should record:

• Date & sequence of pile installation in a group
• Pile details: length, stem & bulb diameter, number of bulbs, type, reinforcement
• Cut-off & working levels
• Boring method
• Groundwater level
• Any other relevant info

2. Degree of Expansiveness (Appendix A)

3. Modulus of Subgrade Reaction (Appendix C)

Table 2: nh for Sands (kg/cm³)

Table 3: K for Preloaded Clays (kg/cm²)

4. Safe Load Considerations (Clause 1.11)

• Use weighted average N-value from SPT up to bulb diameter below pile toe.
• For silty soils, interpolate between sandy and clayey soil values.
• Provide additional steel if moments or lateral loads exceed Table 1 limits.

5. Lateral Deflection & Depth of Fixity (Clause 1.

Degree of Soil Expansiveness (IS 2911 Part 3, Appendix A, Clause 3.1(b))

• Differential Free Swell is the key parameter to classify soil expansiveness.
• Expansive soils cause volume changes affecting pile design and performance.

Key Specifications for Under-Reamed Piles in Expansive Soils

• Safe loads for piles with multiple bulbs:
  • Add 50% of base load (from Table 1, columns 8 or 12) per additional bulb beyond two bulbs in expansive soils.
  • Additional capacity from increased length is obtained from columns 10 and 14.
• Bulb diameter = 2.5 × shaft diameter (Clause 1.1).

Constants for Lateral Deflection and Depth of Fixity (Clause 1.2)

Notes:

• Use Fig. 4 & 5 (IS 2911 Part 3) for depth of fixity and reduction factors.
• Concrete: M15, cover as per Clause 5.2.2.
• Record pile details carefully as per Clause 7.5.11 to monitor soil

Safe Load on Under-Reamed Piles (IS 2911 Part 3)

1. Safe Load Determination (Clause 5.2.3)

Safe load on a pile can be obtained by:

• (a) Calculating ultimate load from soil properties and applying a factor of safety.
• (b) Load test as per IS 2911 Part IV.
• (c) Using Safe Load Tables (Appendix B).

2. Safe Load Table (Appendix B, Table B-1)

• Safe load is based on N-value from SPT (Standard Penetration Test).
• Use a weighted average N-value up to a depth equal to the bulb diameter below pile toe.
• For silty soils, interpolate between sandy and clayey soil values.
• For piles with external moments or larger lateral loads, design with adequate steel reinforcement.

3. Modulus of Subgrade Reaction (Appendix C)

4. Design Note (Clause 5.2.3.4)

• Use the lesser safe load from calculations or tables.
• Higher safe loads allowed only if verified by initial load tests.

Summary Formula for Safe Load:

[ P_{safe} = \frac{P_{ultimate}}{FS} ] where ( FS )

Lateral Load Analysis & Modulus of Subgrade Reaction (IS 2911 Part 3)

1. Lateral Load on Piles (Clause 5.2.5)

• Lateral loads arise from wind, earthquake, water current, earth pressure, vehicles, etc.
• Capacity depends on:
  • Horizontal subgrade modulus of soil (modulus of subgrade reaction).
  • Structural bending strength of pile shaft.
• Axial loads must be considered when checking pile shaft capacity.
• For long piles with bulbs (under-reamed), treat as rigid poles.
• Use Matlock & Reese (sandy soils) or Broms (clayey soils) for flexible piles.
• If lateral loads ≤ Table 1 (Appendix B) and no external moment, detailed analysis may be omitted.

2. Modulus of Subgrade Reaction (Appendix C, Clause 5.2.5)

For clays (preloaded), modulus ( K ) depends on unconfined compressive strength ( q_u ):

3. Key Formulas & Constants

• Depth of Fixity and Equivalent Cantilever Length
  Use constants ( K_1 ) and ( K_2 ) (Tables 2 & 3) and plots (Fig.

IS 2911 Part 3: Manual Construction Equipment for Under-Reamed Piles

Key Equipment List (Appendix D, Clause 6.2 & 1.1)

• Under-reamer: For enlarging the pile base.
• Boring guide: Ensures verticality and alignment during boring.
• Accessories: Spare extensions, cutting tools, concreting funnel, etc.
• Drop weight & winch: For driving and compacting piles vertically (Clause 6.10).

Important Parameters for Design (from related clauses)

Notes on Safe Load (Clause 1.11)

• Use weighted average N-value (SPT) up to bulb diameter below pile toe.
• For silty soils, interpolate between sandy and clayey soil values.
• For piles with external moments or high lateral loads, provide additional steel reinforcement.

flowchart TD
    A[Manual Construction] --> B[Under-reamer]
    A --> C[Boring Guide]
    A --> D[Accessories (extensions, cutting tools, funnel)]
    A --> E[Drop Weight & Winch]
    E --> F[Vertical Driving & Compaction]

This concise summary ensures correct equipment selection and design parameters for manual under-reamed pile construction as per IS 2911 Part 3.

IS 2911 Part 3: Properties & Specifications of Drilling Mud (Bentonite)

Key Properties (Clause E-1.1 & 2.2)

• Bentonite is a montmorillonite clay with sodium cations.
• Forms a jelly-like structure due to electrical bonding between particles.
• In granular soils, bentonite penetrates under pressure, forming a jelly that plaster the borehole sides.
• In impervious clays, bentonite forms a thin film; stability relies on hydrostatic head.

Specifications (Clause E-3.1)

Functional Notes

• Bentonite suspension stabilizes boreholes by forming an impervious layer.
• Jell formation is reversible by agitation (shear thinning behavior).

Summary Diagram: Bentonite Behavior in Soil

flowchart LR
    A[Bentonite Suspension] --> B{Soil Type}
    B -->|Granular Soil| C[Penetrates & Forms Jelly Layer]
    B -->|Impervious Clay| D[Thin Film on Borehole Surface]
    C --> E[Plastering Effect & Impervious Layer]
    D --> F[Stability by Hydrostatic Head]

This ensures borehole stability during piling operations per IS 2911 (Part 3).