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IS 2911:2021 Part 3 is the Indian Standard (BIS) for design and construction of pile foundations - under-reamed piles. This standard provides guidelines for the design and construction of bored, cast-in-situ, under-reamed concrete pile foundations. It covers piles with single or multiple bulbs, which are particularly effective in expansive soils for anchoring against uplift and in soft soils for increasing bearing capacity.
Provides guidelines for the design and construction of under-reamed pile foundations, particularly suited for expansive soils.
Quick Reference — Top IS 2911 Part 3:2021 Values
Key design parameters, reinforcement details, construction tolerances, and load test acceptance criteria for under-reamed piles.
✓ Verified 2026-04-27
Reference
Value
Clause
Min. Pile Diameter— For bored cast-in-situ piles with length up to 5 m.
200 mm
Cl. 5.2.1
Bulb to Stem Diameter Ratio (Du/D)— A ratio of 2.5 is commonly adopted.
2.0 to 3.0
Cl. 5.2.2
Spacing between Bulbs (Centre-to-Centre)
1.25 to 1.5 times Du
Cl. 5.2.3
Topmost Bulb Location from Ground Level— In expansive soils, must be below the zone of seasonal moisture variation.
≥ 2 Du
Cl. 5.2.4
Pile Spacing (Multi-Bulb Piles)— Can be reduced to 1.5 Du with justification and for single bulb piles.
2.0 Du
Cl. 5.3.1.1
Pile Spacing (Single Bulb Piles)
1.5 Du
Cl. 5.3.1.1
Min. Concrete Grade
M 25
Cl. 6.1.1
Min. Cement Content (Severe Exposure)— For 20 mm nominal aggregate size. Refers to IS 456 Table 5.
340 kg/m³
Cl. 6.1.2 (Ref: IS 456)
Max. Water-Cement Ratio (Severe Exposure)— Refers to IS 456 Table 5.
0.45
Cl. 6.1.2 (Ref: IS 456)
Min. Clear Cover to Reinforcement
60 mm
Cl. 6.2.3
Min. Longitudinal Reinforcement
0.4 % of c/s area
Cl. 6.2.4.1
Min. Longitudinal Bar Diameter
8 mm
Cl. 6.2.4.1
Lateral Ties - Min. Diameter
8 mm
Cl. 6.2.5
Lateral Ties - Max. Pitch
150 mm
Cl. 6.2.5
Bearing Capacity Factor (Nc) for Bulbs— For bearing capacity calculation in cohesive soils.
9.0
Cl. 7.2.1.1
Adhesion Factor (α) for Stiff Clays— For undrained cohesion (Cu) of 100-150 kN/m².
0.3 - 0.4
Cl. 7.2.1.2 (Table 1)
Tolerance in Verticality (Plumb)
1.5 % of pile length
Cl. 8.4.1
Tolerance in Position at Cut-off Level— For single piles or piles in a row.
75 mm
Cl. 8.4.1
Initial Test Load
2.5 x Safe Load
Cl. 9.1.4.1
Initial Test - Max Settlement— Whichever is less, at test load.
≤ 12 mm or 2% of Du
Cl. 9.1.5.1 (a)
Routine Test Load
1.5 x Safe Load
Cl. 9.1.4.2
Routine Test - Max Settlement— At test load.
≤ 12 mm
Cl. 9.1.5.2 (a)
⚠ Verify against the latest BIS/IRC publication and project specifications. Amendment Slips may modify values.
Report on Design and Construction of Drilled Piers
Directly covers 'belled drilled piers,' which are functionally identical to under-reamed piles.
BS 8004:2015+A1:2020BSI (UK)
MediumCurrent
Code of practice for foundations
Covers design and construction of bored piles, including provisions for piles with enlarged bases.
EN 1997-1:2004CEN (Europe)
MediumCurrent
Eurocode 7: Geotechnical design - Part 1: General rules
Provides the design framework for all geotechnical structures including bored piles, but is less prescriptive on specific pile types.
AS 2159-2009Standards Australia (Australia)
MediumCurrent
Piling - Design and installation
Comprehensive standard on piling that includes bored piles with enlarged bases (bells).
Key Differences
≠Prescriptive Geometry: IS 2911 is highly prescriptive regarding the geometry of under-reams, specifying the under-ream to shaft diameter ratio (typically 2.5) and the spacing between bulbs (1.25 to 1.5 times bulb diameter). In contrast, ACI 336.3R provides general guidelines (e.g., bell slope angle) but leaves the final geometry to engineering design based on soil properties and bearing requirements.
≠Application Focus: IS 2911 has a specific and strong emphasis on using under-reamed piles in expansive soils (like black cotton soil) to anchor foundations below the zone of moisture fluctuation. While ACI 336.3R acknowledges the use of belled piers for uplift, its primary focus is on increasing bearing capacity in various soil types, without the same specific emphasis on expansive soils.
≠Multi-Bulb Design: IS 2911 provides explicit design guidance and formulas for multi-under-reamed piles, a common practice in India. Multi-bell drilled piers are significantly less common in US practice, and ACI 336.3R contains limited specific guidance for designing the interaction and capacity contribution of multiple bells on a single pier.
≠Bearing Capacity Formulation for Cohesive Soils: IS 2911 provides a specific formula for the ultimate capacity of piles in cohesive soils (Qu = Ap * Nc * cp + A'p * Nc * c'p + α * c_avg * As), which accounts for bearing at the base and any upper bulbs separately. The ACI approach is more general, applying standard bearing capacity theory to the lowest bell and skin friction to the shaft, with less explicit formulation for the contribution of upper bells.
Key Similarities
≈Fundamental Concept: Both IS 2911 and its international counterparts like ACI 336.3R are based on the same principle: creating an enlarged base (under-ream or bell) on a cast-in-place bored pile to significantly increase its end-bearing capacity and/or uplift resistance.
≈Requirement for Load Testing: Both the Indian Standard and major international codes (ACI, Eurocode) strongly advocate for and provide detailed procedures for conducting pile load tests (static compression, tension/uplift) to verify design assumptions and confirm performance under load.
≈Emphasis on Geotechnical Investigation: All credible pile design standards, including IS 2911 and its international equivalents, mandate a thorough subsurface investigation as a prerequisite for design. The design methodologies in all codes are fundamentally dependent on reliable soil parameters (strength, stratigraphy, density).
≈Construction Principles: The general construction sequence of drilling the shaft, excavating the enlargement (belling/under-reaming), placing the reinforcement cage, and concreting the pile are broadly similar across all standards. They share common concerns regarding borehole stability, cleanliness of the base, and ensuring concrete integrity.
Parameter Comparison
Parameter
IS Value
International
Source
Under-ream / Bell Diameter Ratio (Du/D)
Typically 2.5. Can be up to 3.0 in special cases with permission.
No prescribed ratio; commonly up to 3.0 times shaft diameter, based on design requirements.
ACI 336.3R-14
Vertical Spacing Between Bulbs
1.25 to 1.5 times the under-ream diameter (Du).
Not specified, as multi-bell piers are uncommon. Design would be based on shear analysis between bells.
ACI 336.3R-14
Factor of Safety on Bearing Capacity (from soil properties)
2.5 to 3.0
Generally 3.0 for end bearing and 2.0 to 3.0 for side resistance.
ACI 336.3R-14
Factor of Safety on Bearing Capacity (from load test)
2.5 for initial test; 2.0 for routine test.
Typically 2.0.
ACI 336.3R-14
Minimum Longitudinal Reinforcement
0.4% of pile stem cross-sectional area.
Typically 0.5% to 1.0% of the gross cross-sectional area (references ACI 318).
ACI 336.3R-14 / ACI 318
Bell Slope Angle (from horizontal)
Not explicitly defined; formed by the cutting tool geometry.
Recommended to be at least 45° to 60° for stability, depending on soil type.
ACI 336.3R-14
Concrete Slump (for Tremie/Underwater Placement)
150 - 200 mm
175 - 225 mm (7 - 9 inches)
ACI 336.1-19
⚠ Verify details from original standards before use
Key Values6
Quick Reference Values
minimum longitudinal reinforcement0.4% of gross area
minimum clear cover50 mm
slump for tremie concreting150 mm to 200 mm
minimum spacing friction piles3 times pile diameter
minimum spacing end bearing piles2.5 times pile diameter
minimum grade of concreteM25
Key Formulas
Qu = Ap * Nc * cp + alpha * c * As — Ultimate bearing capacity of pile in cohesive soil
Qu = Ap * (0.5 * D * gamma * Ngamma + pd * Nq) + sum(K * pdi * tan(delta) * Asi) — Ultimate bearing capacity in cohesionless soil